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# A Cool and Inflated Progenitor Candidate for the Type Ib Supernova 2019yvr
at 2.6 Years Before Explosion

Charles D. Kilpatrick1, Maria R. Drout2,3, Katie Auchettl4,5,6,7, Georgios
Dimitriadis6, Ryan J. Foley6, David O. Jones6, Lindsay DeMarchi1, K. Decker
French8, Christa Gall7, Jens Hjorth7, Wynn V. Jacobson-Galán1, Raffaella
Margutti1, Anthony L. Piro3, Enrico Ramirez-Ruiz7,6, Armin Rest9,10, César
Rojas-Bravo6
1Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)
and Department of Physics and Astronomy,
Northwestern University, Evanston, IL 60208, USA
2David A. Dunlap Department of Astronomy and Astrophysics, University of
Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4 Canada
3The Observatories of the Carnegie Institution for Science, 813 Santa Barbara
St., Pasadena, CA 91101, USA
4School of Physics, The University of Melbourne, Parkville, VIC 3010,
Australia
5ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)
6Department of Astronomy and Astrophysics, University of California, Santa
Cruz, CA 95064, USA
7DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200
Copenhagen, Denmark
8Department of Astronomy, University of Illinois, 1002 West Green St., Urbana,
IL 61801, USA
9Department of Physics and Astronomy, Johns Hopkins University, 3400 North
Charles Street, Baltimore, MD 21218, USA
10Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, USA
Email<EMAIL_ADDRESS>
(Accepted 0000, Received 0000, in original form 0000)

###### Abstract

We present Hubble Space Telescope imaging of a pre-explosion counterpart to SN
2019yvr obtained 2.6 years before its explosion as a type Ib supernova (SN
Ib). Aligning to a post-explosion Gemini-S/GSAOI image, we demonstrate that
there is a single source consistent with being the SN 2019yvr progenitor
system, the second SN Ib progenitor candidate after iPTF13bvn. We also
analyzed pre-explosion Spitzer/IRAC imaging, but we do not detect any
counterparts at the SN location. SN 2019yvr was highly reddened, and comparing
its spectra and photometry to those of other, less extinguished SNe Ib we
derive $E(B-V)=0.51\begin{subarray}{c}+0.27\\\ -0.16\end{subarray}$ mag for SN
2019yvr. Correcting photometry of the pre-explosion source for dust reddening,
we determine that this source is consistent with a $\log(L/L_{\odot})=5.3\pm
0.2$ and $T_{\mathrm{eff}}=6800\begin{subarray}{c}+400\\\ -200\end{subarray}$
K star. This relatively cool photospheric temperature implies a radius of
320$\begin{subarray}{c}+30\\\ -50\end{subarray}~{}R_{\odot}$, much larger than
expectations for SN Ib progenitor stars with trace amounts of hydrogen but in
agreement with previously identified SN IIb progenitor systems. The photometry
of the system is also consistent with binary star models that undergo common
envelope evolution, leading to a primary star hydrogen envelope mass that is
mostly depleted but still seemingly in conflict with the SN Ib classification
of SN 2019yvr. SN 2019yvr had signatures of strong circumstellar interaction
in late-time ($>$150 day) spectra and imaging, and so we consider eruptive
mass loss and common envelope evolution scenarios that explain the SN Ib
spectroscopic class, pre-explosion counterpart, and dense circumstellar
material. We also hypothesize that the apparent inflation could be caused by a
quasi-photosphere formed in an extended, low-density envelope or circumstellar
matter around the primary star.

###### keywords:

stars: evolution — supernovae: general — supernovae: individual (SN 2019yvr)

††pagerange: A Cool and Inflated Progenitor Candidate for the Type Ib
Supernova 2019yvr at 2.6 Years Before Explosion–LABEL:LastPage††pubyear: 2020

##  1 INTRODUCTION

Core-collapse supernovae (SNe) are the terminal explosions of stars with
initial mass $>$$8~{}M_{\odot}$ (Burrows et al., 1995). This aspect of massive
star evolution was empirically confirmed by the discovery of the blue
supergiant progenitor of SN 1987A (Podsiadlowski, 1993) and subsequent
discovery of over two dozen SN progenitors in nearby galaxies (Smartt et al.,
2015, and references therein, with more discovered since). The majority of
these stars are red supergiant (RSG) progenitors of hydrogen-rich type II SNe
(SNe II), although several hydrogen-poor SN IIb progenitor stars, all of which
are A–K supergiants, have also been explored in the literature (notably for
SNe 1993J, 2008ax, 2011dh, 2013df, and 2016gkg; Aldering et al., 1994;
Crockett et al., 2008; Maund et al., 2011; Van Dyk et al., 2014; Kilpatrick et
al., 2017).

To date, there is only one confirmed example of a progenitor star to a
hydrogen-stripped SN Ib; the progenitor of iPTF13bvn in NGC 5608 was initially
identified as a compact Wolf-Rayet (WR) star in pre-explosion Hubble Space
Telescope (HST) imaging (Cao et al., 2013) and confirmed as the progenitor by
its disappearance (Eldridge & Maund, 2016; Folatelli et al., 2016). There are
numerous upper limits on the progenitor systems of other SNe Ib in the
literature (Eldridge et al., 2013), implying they tend to have low optical
luminosities. Overall, this relative lack of pre-explosion detections has
contributed to an ongoing debate on the nature of stripped-envelope SN
progenitors.

The transition from hydrogen-rich type II to hydrogen-poor type IIb to
hydrogen-free type Ib SNe, and finally to helium-free type Ic SNe is commonly
understood as a continuum in final hydrogen (or helium) mass in the envelopes
of their progenitor stars (Filippenko, 1997; Dessart et al., 2011, 2012; Yoon
et al., 2012; Yoon, 2015; Dessart et al., 2015; Maund & Ramirez-Ruiz, 2016).
Possible mechanisms that can deplete stellar envelope mass include radiative
mass loss (Heger et al., 2003; Crowther, 2007; Smith, 2014), eruptive mass
loss (Langer et al., 1994; Maeder & Meynet, 2000; Ramirez-Ruiz et al., 2005;
Dessart et al., 2010), and mass transfer in binary systems (Woosley et al.,
1994; Izzard et al., 2004; Fryer et al., 2007; Yoon, 2017). Stars with higher
initial masses or metallicities are predicted to be more stripped at the time
of core collapse due to their strong radiative winds (Heger et al., 2003).
However, extremely high mass stars that are predicted to efficiently deplete
their envelopes have more compact and thus less explodable cores, which is
thought to lead to a significant fraction of failed SNe, that is, direct
collapse to a black hole with no luminous transient (Burrows et al., 2007;
Sukhbold et al., 2016; Murguia-Berthier et al., 2020). In addition, the large
relative fraction of stripped-envelope SNe in volume-limited surveys (i.e.,
SNe Ib and Ic; Li et al., 2011; Shivvers et al., 2017a; Graur et al., 2017a;
Graur et al., 2017b) suggests they come from a progenitor channel including
stars with initial masses $<$30 $M_{\odot}$ (Smith et al., 2011; Eldridge et
al., 2013).

Mass transfer in a binary system is therefore an appealing alternative
mechanism to strip massive star envelopes as the majority of massive stars are
observed to evolve in binaries (Sana et al., 2012; Kiminki & Kobulnicky,
2012), and binary interactions can lead to a wide variety of outcomes based on
mass ratio, orbital period, and the characteristics of each stellar component
(e.g., Wu et al., 2020). In particular, Case B (during helium core
contraction; Kippenhahn & Weigert, 1967) or Case BB (after core helium
exhaustion for a star with previous Case B mass transfer; Delgado & Thomas,
1981) mass transfer can remove nearly all of a star’s hydrogen envelope,
although this process typically stops before hydrogen is completely depleted
(Yoon et al., 2010; Yoon, 2015, 2017). Stars with a small amount of hydrogen
remaining might also swell up in the latest stages of evolution (Divine, 1965;
Habets, 1986; Götberg et al., 2017; Laplace et al., 2020) and fill their Roche
lobes to restart mass transfer. If the mass transfer is non-conservative, that
is some of the material is not accreted by the companion star, this scenario
can lead to dense circumstellar material (CSM) in their local environments.
Thus, when the primary star explodes the SN ejecta might encounter and shock
this material, producing strong thermal continuum and hydrogen and helium line
emission at optical wavelengths (i.e., SN IIn and Ibn features; Vanbeveren et
al., 1979; Claeys et al., 2011; Maund et al., 2016; Yoon et al., 2017; Smith,
2017; Götberg et al., 2019). Thus, the final envelope mass, radius, and
composition of the star can result in SNe with diverse photometric and
spectroscopic properties (James & Baron, 2010) ranging from type II to type
IIn to type Ic-like evolution.

One prediction from this model of binary mass transfer is that there may be a
continuum between SNe with type IIb and Ib-like behaviour, depending on their
final hydrogen mass. Dessart et al. (2012) find that progenitor stars with as
little as $10^{-3}~{}M_{\odot}$ hydrogen envelope mass would produce a SN
whose spectra exhibit broad H$\alpha$ line emission up to 10 days after
maximum light (although other studies find the envelope mass can be as large
as 0.02–0.03 $M_{\odot}$ with no H$\alpha$ signature; Hachinger et al., 2012).
Stars on either edge of this dividing line are expected to vary not only in
the spectroscopic evolution of their resulting SN but also their appearance in
pre-explosion imaging. Above this threshold, spectroscopic evolution should be
similar to archetypal SNe IIb such as SN 1993J (Filippenko et al., 1993;
Richmond et al., 1994; Woosley et al., 1994), and the progenitor star can
inflate to radii $>$400 R⊙ (Yoon, 2017; Laplace et al., 2020). Indeed, the
progenitor of SN 1993J was a K-type supergiant (Nomoto et al., 1993; Aldering
et al., 1994; Fox et al., 2014). In contrast, stars with final hydrogen-
envelope masses low enough that they would be classified as a type Ib SN prior
to maximum light are only expected to inflate to radii of at most $\sim$100 R⊙
(Yoon et al., 2012; Yoon, 2015, 2017; Kleiser et al., 2018; Laplace et al.,
2020) and in many cases remain significantly smaller. This should result in
hotter progenitor stars for a given luminosity.

Intriguingly, some SNe Ib exhibit signatures of circumstellar interaction with
hydrogen-rich gas weeks to months after explosion, which suggests their
progenitor stars (or binary companions) recently released this material. The
best-studied example to date is SN 2014C (Milisavljevic et al., 2015; Margutti
et al., 2017; Tinyanont et al., 2016, 2019), which was discovered in NGC 7331
at $\approx$15 Mpc, but several other stripped-envelope SNe with similar
evolution have been presented in the literature (e.g., SNe 2001em, 2003gk,
2004dk, 2018ijp, 2019tsf, 2019oys; Chugai & Chevalier, 2006; Bietenholz et
al., 2014; Chandra, 2018; Mauerhan et al., 2018; Pooley et al., 2019;
Sollerman et al., 2020; Tartaglia et al., 2020) as well as the initially
hydrogen-free superluminous SN iPTF13ehe (Yan et al., 2017). Although non-
conservative mass transfer or common envelope ejections have been proposed as
the source of this material (Sun et al., 2020), it is still unclear what
evolutionary pathways lead to these apparently hydrogen-stripped stars or what
exact mechanism causes an ejection timed only years before explosion (up to
$1~{}M_{\odot}$ of hydrogen-rich CSM for SN 2014C in Margutti et al., 2017).

Understanding how common stripped-envelope SNe with circumstellar interactions
are might aid in ruling out less likely mechanisms, but constraining the exact
rate is difficult as few SNe are close and bright enough to follow to late
times and stripped-envelope SNe tend to be further extinguished in their host
galaxies (Stritzinger et al., 2018). Some SN Ib exhibit clear signatures of
circumstellar interaction with helium-rich material at early times (so-called
SNe Ibn, with narrow emission lines of helium indicative of interaction
between SN ejecta and slow-moving, circumstellar helium; Pastorello et al.,
2008; Shivvers et al., 2017b), potentially from massive, helium-rich WR stars
undergoing extreme mass loss immediately before explosion (Smith et al.,
2017). However, events from this class are rare and there with significant
photometric and spectroscopic diversity (Hosseinzadeh et al., 2017). Margutti
et al. (2017) analyzed 183 SNe Ib and Ic with late-time radio observations and
found that 10% exhibit evidence for rebrightening consistent with SN
2014C-like evolution, implying this phenomenon may be relatively common.
However, volume-limited samples with light curves beyond $100$ days of
discovery (when most of these interactions occur; Sollerman et al., 2020) are
small (e.g., in Li et al., 2011; Shivvers et al., 2017a), and so there may be
an observational bias preventing precise constraints on the intrinsic rate of
these interactions in SNe Ib/c.

In this paper we discuss a progenitor candidate for the SN Ib 2019yvr
discovered in NGC 4666 on UTC 2019 December 27 12:30:14 (MJD 58844.521) by the
Asteroid Terrestrial impact Last Alert System (ATLAS; Smith et al., 2019)111SN
2019yvr is also called ATLAS19benc. We use SN 2019yvr throughout this paper
for consistency with follow-up reports.. We present early-time light curves
and spectra of SN 2019yvr demonstrating that it resembles several other SNe Ib
and is spectroscopically most similar to iPTF13bvn, albeit with much more
line-of-sight extinction than most known SNe Ib. Although we do not present
any observations beyond 35 days from discovery, we note that SN 2019yvr
exhibited signatures of circumstellar interaction at $>$150 days from
discovery, with evidence for relatively narrow H$\alpha$, X-ray, and radio
emission at these times (Auchettl et al. in prep.). From this information, we
infer that SN 2019yvr is similar to SN 2014C, with early-time type Ib-like
evolution but transitioning around 150 days to a light curve powered by shock
interaction with CSM at all wavelengths.

NGC 4666 has deep Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3)
imaging in F438W, F555W, F625W, and F814W bands (roughly $BV\\!RI$,
respectively) that covers the site of SN 2019yvr 2.6 yr before its explosion
(Shappee et al., 2016; Foley et al., 2016; Graur et al., 2018). Compared with
limits on the progenitor stars of other SNe Ib in the literature (Eldridge et
al., 2013) as well as the detection of the progenitor star of iPTF13bvn (Cao
et al., 2013), these data are among the deepest pre-explosion imaging for any
SN Ib. We compare follow-up adaptive optics-fed imaging to the pre-explosion
HST images and identify a single progenitor candidate with ${\rm
F555W}=25.35\pm 0.03$ mag and ${\rm F555W}-{\rm F814W}=1.10\pm 0.04$ mag (AB
mag; Table 2).

Using constraints on the interstellar host extinction to SN 2019yvr inferred
from photometry and spectra of the SN itself, we characterize the progenitor
candidate’s intrinsic spectral shape and find it is consistent with a star
with $\log(L/L_{\odot})=5.3\pm 0.2$ and
$T_{\mathrm{eff}}=6800\begin{subarray}{c}+400\\\ -200\end{subarray}$ K. This
is much cooler than most SN Ib progenitor stars are generally thought to be
(including the progenitor star of iPTF13bvn; Cao et al., 2013; Bersten et al.,
2014; Folatelli et al., 2016), implying that if the counterpart is dominated
by the SN 2019yvr progenitor star it must be significantly inflated compared
with expectations for SN Ib progenitor systems. Analyzing Binary Population
and Spectral Synthesis (BPASS; Eldridge et al., 2017) stellar evolution
models, we find that the HST photometry could be consistent with a
$19~{}M_{\odot}$ progenitor star with a relatively low-mass
($\approx$1.9$~{}M_{\odot}$), close companion star that undergoes common
envelope evolution and sheds most of its hydrogen envelope. However, all of
these models predict a significant residual hydrogen envelope mass and are
thus in conflict with the observed type Ib spectral class of SN 2019yvr.
Therefore, we hypothesize that the progenitor star may have shed its remaining
hydrogen envelope through pre-SN eruptive mass ejection in the last 2.6 yr
before explosion. Otherwise, the apparently inflated radius may be caused by a
much lower mass of hydrogen forming a compact quasi-photosphere in the
progenitor star’s circumstellar environment soon before explosion.

Throughout this paper, we assume a distance to NGC 4666 of $m-M=30.8\pm 0.2$
mag ($14.4\pm 1.3$ Mpc) derived from the light curve of the type Ia SN
ASASSN-14lp also observed in this galaxy (Shappee et al., 2016). We assume a
redshift to NGC 4666 of $z=0.005080$ (Allison et al., 2014) and Milky Way
reddening $E(B-V)=0.02$ mag (Schlafly & Finkbeiner, 2011).

##  2 OBSERVATIONS

###  2.1 High-Resolution Pre-Explosion Images of the SN 2019yvr Explosion Site

We analyzed HST/WFC3 imaging of NGC 4666 obtained from the Mikulski Archive
for Space Telescopes222https://archive.stsci.edu/hst/. These data were
observed over five epochs from 2017 April 21 to August 7 (Cycle 24, GO-14611,
PI Graur; see Table 2), corresponding to 980 to 872 days (2.68 to 2.39 years)
before discovery of SN 2019yvr. Using our analysis code
hst123333https://github.com/charliekilpatrick/hst123, we downloaded every HST
image covering the explosion site of SN 2019yvr. These comprised WFC3/UVIS flc
frames calibrated with the latest reference files, including corrections for
bias, dark current, flat-fielding, bad pixels, and geometric distortion. We
optimally aligned each image using TweakReg with 1000–2000 sources per frame
and resulting in frame-to-frame alignment with 0.1–0.2 pix
(0.005–0.010${}^{\,\prime\prime}$) root-mean-square dispersion. We then
drizzled all images in each band and epoch with astrodrizzle. With the
drizzled F555W frame as a reference, we obtained photometry in the flc frames
of every source on the same chip as the SN 2019yvr explosion site using
dolphot (Dolphin, 2016). Our dolphot parameters followed the recommended
settings for
WFC3/UVIS444http://americano.dolphinsim.com/dolphot/dolphotWFC3.pdf as
described in hst123. We show a colour image constructed from the F814W, F555W,
and F438W frames obtained on 2017 June 13 in Figure 1.

In addition, multiple epochs of Spitzer/Infrared Array Camera (IRAC) imaging
of NGC 4666 were obtained from 2005 January 4 to 2014 September 25, or roughly
15.0 to 5.3 yr before discovery of SN 2019yvr. There was a single epoch of
Channel 4 (7.9$\mu$m) imaging that observed NGC 4666 (AOR 21999872; PI Rieke),
but no Spitzer/IRAC observations cover NGC 4666 in Channel 3 (5.7$\mu$m). We
downloaded the basic calibrated data (cbcd) frames and stacked them using our
custom Spitzer/IRAC pipeline based on the photpipe imaging and reduction
pipeline (Rest et al., 2005; Kilpatrick et al., 2018a). The IRAC frames were
stacked and regridded to a pixel scale of 0.6 arcsec pixel-1 using SWarp
(Bertin, 2010). We performed photometry on the stacked frames using DoPhot
(Schechter et al., 1993) and calibrated our data with Spitzer/IRAC
instrumental response (for the cold and warm missions where appropriate; Hora
et al., 2012) in the stacked frames. Based on the PSF width and average sky
background, the average depth of the Spitzer/IRAC images is approximately
(3$\sigma$; AB mag) 24.3 mag, 24.6 mag, and 23.0 mag at 3.6, 4.5, and 7.9
$\mu$m, respectively.

###  2.2 Adaptive Optics Imaging of SN 2019yvr

We observed SN 2019yvr in $H$-band on 2020 March 8, or 72 days after
discovery, with the Gemini-South telescope from Cerro Pachón, Chile and the
Gemini South Adaptive Optics Imager (GSAOI; McGregor et al., 2004). We used
the Gemini Multi-conjugate Adaptive Optics System (GeMS; Rigaut et al., 2014)
with the Gemini South laser guide star system to perform adaptive optics
corrections over the GSAOI field of view
(85${}^{\,\prime\prime}$$\times$85${}^{\,\prime\prime}$) and using SN 2019yvr
itself to perform tip-tilt corrections. We alternated observations between a
field covering SN 2019yvr and a relatively empty patch of sky 4${}^{\,\prime}$
to the south in an on-off-on-off pattern, totaling 1005 seconds of on-source
exposure time over 39 frames. Using the GSAOI reduction tools in IRAF555IRAF
is distributed by the National Optical Astronomy Observatory, which is
operated by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with the National Science Foundation., we
flattened the images with a flat-field frame constructed from observations of
a uniformly-illuminated screen in the same filter and instrumental setup with
unilluminated frames of the same exposure time to account for bias and dark
current. We then subtracted the sky frames from our on-source frames.

GSAOI has a well-understood geometric distortion pattern (Neichel et al.,
2014). We used this distortion pattern to resample each on-source frame to a
corrected grid, aligned the individual exposures, and constructed a mosaic
from each amplifier in the on-source frames with the GSAOI tool disco-
stu666http://www.gemini.edu/sciops/data/software/disco_stu.pdf. Finally, we
stacked the individual frames with SWarp using an inverse-variance weighted
median algorithm and scaling each image to the flux of isolated point sources
observed in every on-source exposure. The final stacked frame is shown in the
upper-left inset of Figure 1 centered on SN 2019yvr.

Figure 1: (Right) Hubble Space Telescope imaging of the SN 2019yvr explosion
site from 2.5 years before discovery consisting of F814W (red), F555W (green),
and F438W (blue). All images are oriented with north up and east to the left.
The color image on the right is
165${}^{\,\prime\prime}$$\times$165${}^{\,\prime\prime}$, while the left-upper
and left-middle images are
38.8${}^{\,\prime\prime}$$\times$38.8${}^{\,\prime\prime}$, and the left-lower
image is 2.4${}^{\,\prime\prime}$$\times$2.4${}^{\,\prime\prime}$. The blue
box denotes the approximate location of SN 2019yvr. (Upper left):
Gemini-S/GSAOI $H$-band image of SN 2019yvr obtained 67 days after discovery
of the transient. The image is centered on the location of SN 2019yvr. (Middle
left): Pre-explosion F555W imaging of NGC 4666 showing the same location as
the upper left. (Lower left): Pre-explosion F555W imaging zoomed into the blue
box from the middle left. The location of the SN 2019yvr progenitor candidate
derived from our Gemini-S/GSAOI imaging is shown as red lines, which agrees
with the location of a single point source as discussed in Section 4.1.

###  2.3 Photometry of SN 2019yvr

We observed SN 2019yvr with the Swope 1.0-m telescope and Direct/4K$\times$4K
imager at Las Campanas Observatory, Chile from 2020 January 1 to 28 in
$uBV\\!gri$. Following reduction procedures described in Kilpatrick et al.
(2018a), we performed all image processing and photometry on the Swope data
using photpipe (Rest et al., 2005). The final $BV\\!gri$ photometry of SN
2019yvr were calibrated using PS1 standard sources (Flewelling et al., 2020)
observed in the same field as SN 2019yvr and transformed into the Swope
natural system following the Supercal method (Scolnic et al., 2015). In
$u$-band, we calibrated our images using SkyMapper standards (Onken et al.,
2020) in the same frame as SN 2019yvr.

We also observed SN 2019yvr with the Las Cumbres Observatory (LCO) Global
Telescope Network 1-m telescopes from 2019 December 29 to 2020 February 3 with
the Sinistro imagers and in $g^{\prime}r^{\prime}i^{\prime}$. We obtained the
processed images (from the BANZAI pipeline; McCully et al., 2018) from the LCO
archive and processed them in photpipe, registering each image to a corrected
grid with SWarp (Bertin, 2010) and performing photometry on the individual
frames with DoPhot (Schechter et al., 1993). We then calibrated the
$g^{\prime}r^{\prime}i^{\prime}$ photometry using $gri$ PS1 standards.

All Swope and LCO photometry are listed in Table 4 and shown in Figure 2. We
estimated the time of maximum light in $V$-band by fitting a low-order
polynomial to the overall light curve and derive a time of $V$-band maximum
light at MJD 58853.64 (2020 January 5.64). Detailed modelling of the light
curves and inferred explosion parameters will be presented by Auchettl et al.
(in prep.).

Figure 2: Swope (circle) and LCO (square) $uBgVri$ light curves of SN 2019yvr
as described in Section 2.3. We denote the epoch of each observation in rest-
frame days (correcting for the redshift of NGC 4666 at $z=0.005080$) from
$B$-band maximum light detected at MJD 58854.28 (Table 4).

###  2.4 Spectroscopy and Classification of SN 2019yvr

We triggered spectroscopic observations of SN 2019yvr on the Faulkes-North 2-m
telescope at Haleakalā, Hawaii with the FLOYDS spectrograph (Program
NOAO2020A-008, PI Kilpatrick). The spectrum was observed on 2020 January 2
roughly 5 days after the initial discovery report from ATLAS and 3 days before
SN 2019yvr reached $V$-band maximum. The observation was a 1500-s exposure at
an average airmass of 1.35 and under near-photometric observing conditions. We
reduced the spectrum following standard procedures in IRAF, including
corrections for telluric absorption and correcting the wavelength solution for
atmospheric diffraction using the sky lines. The final reduced spectrum is
shown in Figure 3.

We also observed SN 2019yvr on the Keck-I 10-m telescope on Maunakea, Hawaii
with the Low-Resolution Imaging Spectrograph (LRIS; Program 2019B-U169, PI
Foley) on 27 Jan 2020, approximately 22 days after $V$-band maximum as seen
from our light curve. The observation was a 180-s exposure obtained during
morning twilight at an average airmass of 1.16 and under near-photometric
conditions. We reduced these data using a custom pyraf-based LRIS pipeline
(Siebert et al., 2020)777https://github.com/msiebert1/UCSC_spectral_pipeline,
which accounts for bias-subtraction, flat-fielding, amplifier crosstalk,
background and sky subtraction, telluric corrections using a standard observed
on the same night and at a similar airmass, and order combination. The final
combined spectrum is shown in Figure 3.

Our spectra reveal characteristic SN Ib features with strong, broad absorption
lines of He i $\lambda\lambda$4471, 5876, 6678, and 7065 (Figure 3). These
features and the lack of any apparent Balmer line emission indicate that SN
2019yvr is a typical SN Ib, and our comparisons to other SNe Ib such as
iPTF13bvn (Srivastav et al., 2014) suggest it is well matched to this
spectroscopic class as a whole. From the spectrum obtained at 3 days before
maximum light, we infer a velocity from Ca absorption of 22,000 km s-1. We
also note prominent lines of Na i D absorption at the redshift of NGC 4666
($z=0.005080$). The complete spectroscopic evolution of SN 2019yvr will be
addressed by Auchettl et al. (in prep.).

Figure 3: Our spectra of SN 2019yvr (black) with comparison to other SNe Ib
(red). All dates are indicated with a “d” with respect to $V$-band maximum
light. The comparison spectra have been dereddened for Milky Way extinction
based on values in Schlafly & Finkbeiner (2011) and dereddened for host
extinction based on values in Deng et al. (2000); Benetti et al. (2011) (for
SN 1999dn), Stritzinger et al. (2009) (for SN 2007Y), Valenti et al. (2011)
(for SN 2009jf), and Srivastav et al. (2014) (for iPTF13bvn). We removed the
recessional velocity for $z=0.005080$ from the SN 2019yvr spectra and
dereddened them following the methods given in Section 3.3. The best-fitting
extinction and $R_{V}$ parameter are given next to each SN 2019yvr spectrum.
We highlight lines of He i at $\lambda\lambda$4471, 5876, 6678, and 7065,
which are present in both epochs, demonstrating that SN 2019yvr is a SN Ib.

##  3 Extinction Toward SN 2019yvr and Its Progenitor System

Stripped-envelope SNe Ib are known to occur in regions of high extinction in
their host galaxies (Drout et al., 2011; Galbany et al., 2016a, b; Stritzinger
et al., 2018). However, if there is significant extinction due to dust in the
circumstellar environment of SN 2019yvr, it may be variable between the time
the HST images and imaging and spectra of SN 2019yvr were obtained. Moreover,
we have no a priori constraint on the dust composition or gas-to-dust ratio in
the local interstellar environment of SN 2019yvr, which is a major factor in
understanding the magnitude of extinction at all optical wavelengths.

Based on the relatively low Milky Way reddening of $E(B-V)=0.02$ mag and the
fact that SN 2019yvr exhibited red colours (Figure 2) and strong Na i D
absorption, we infer that SN 2019yvr and its progenitor system are heavily
extinguished by its host’s interstellar and/or its own circumstellar
environment. Moreover, if we do not correct for any additional extinction, the
$V$-band light curve would peak at only $-15.1$ mag. This is extremely faint
compared with other SNe Ib/c and suggests $A_{V}>1$ mag (Drout et al., 2011;
Stritzinger et al., 2018, although this inference may be affected by Malmquist
bias if known samples of SNe Ib are not representative of the overall
luminosity function).

Throughout the remainder of this section, we consider contextual information
about the host galaxy NGC 4666, observations of SN 2019yvr, and the extinction
properties of circumstellar dust around analogous stripped-envelope SN Ib
progenitor systems in order to infer the total extinction to the SN 2019yvr
progenitor system. Our goal is to derive a $V$-band extinction $A_{V}$ and
reddening law parameter $R_{V}$ that can be used to estimate the total
extinction in the HST bandpasses as observed in pre-explosion data.

###  3.1 Extinction Inferred from Na i D

One quantity that is correlated with line-of-sight reddening in both SNe
(Stritzinger et al., 2018) and quasars (Poznanski et al., 2012) is the
equivalent width of Na i D. We detect Na i D in our 2019yvr LRIS spectrum with
equivalent width of 4.2$\pm$0.2 Å, which is significantly larger than the
maximum Na i D equivalent width (2.384 Å) from the quasars used to derive the
reddening relation in Poznanski et al. (2012), implying that we might
overestimate the total extinction by applying their relation. Indeed, our
measured Na i D equivalent width combined with the Poznanski et al. (2012)
relation would indicate SN 2019yvr has a light-of-sight $E(B-V)>1000$ mag,
which is impossible for any extragalactic optical transient. This finding
could be due in part to saturation in the Na i D line for the original sample
of quasars in Poznanski et al. (2012), which prevents an accurate measurement
of the true column of Na i D as a function of the total column optical
extinction. We infer that the Poznanski et al. (2012) relationship is not
accurate in this high extinction and large Na i D equivalent width regime
where we find SN 2019yvr (consistent with findings in Stritzinger et al.,
2018).

If we instead use the relation between $A_{V}$ and Na i D equivalent width in
Stritzinger et al. (2018), which was derived specifically from SN Ib/c colour
curves, we find SN 2019yvr has a line-of-sight extinction $A_{V}=3.4\pm 0.6$
mag. However, we emphasize that the validity of this relationship at such
large equivalent widths has not been tested, and, more broadly, there is
significant scatter in the correlation between Na i D equivalent width and
optical extinction (Phillips et al., 2013). Therefore, we turn to other
extinction indicators to better estimate the line-of-sight extinction.

###  3.2 Extinction Inferred from SN 2019yvr Spectra

Spectra and light curves of SNe Ib similar to SN 2019yvr can be used to
constrain its line-of-sight extinction. As host extinction is a dominant
systematic uncertainty in estimating intrinsic stripped-envelope SN colours,
any differences in broadband colours between SNe at similar epochs can be
attributed to extinction. Here we compare our SN 2019yvr spectra to those of
other SNe Ib applying a Cardelli et al. (1989) extinction law with variable
$E(B-V)$ and $R_{V}$ to deredden our SN 2019yvr until they closely match.

Our template spectra are chosen from those of well-observed SNe Ib with low
host reddening ($E(B-V)<0.2$ mag) measured and reported in the literature.
These include SN 1999dn ($E(B-V)=0.05$ mag; Deng et al., 2000; Benetti et al.,
2011), SN 2007Y ($E(B-V)=0.11$ mag; Stritzinger et al., 2009), SN 2009jf
($E(B-V)\approx 0.0$ mag; Valenti et al., 2011), and iPTF13bvn ($E(B-V)=0.17$
mag; Srivastav et al., 2014). We use spectra obtained from the Open Supernova
Catalog888sne.space (Guillochon et al., 2017). All template spectra were
chosen to correspond to roughly the same epoch relative to $V$-band maximum as
one of our two SN 2019yvr spectra. For the purposes of our fitting procedure,
we assume that these extinction values are exact with no additional
uncertainty. Furthermore, we assume all template spectra experienced Milky
Way-like host reddening with $R_{V}=3.1$ as $R_{V}$ is either unconstrained or
poorly constrained for all of these objects. We acknowledge that this possibly
biases our $A_{V}$ and $R_{V}$ estimates for SN 2019yvr based on the
spectroscopic fitting method, although $E(B-V)$ is small for our templates and
so this may not be a major systematic uncertainty. For both the SN 2019yvr and
template spectra, we estimate the uncertainty in the specific flux
($\sigma_{\lambda}$) by taking

$\sigma_{\lambda}=\tilde{f}_{\lambda}\sqrt{\left|1-\frac{f_{\lambda}}{\tilde{f}_{\lambda}}\right|},$
(1)

where $\tilde{f}_{\lambda}$ is the specific flux $f_{\lambda}$ passed through
a smoothing function with a 50 Å window and rebinned to 1 Å resolution over
the maximum overlap range between the SN 2019yvr and template spectrum. Thus,
the SN 2019yvr and template spectrum flux uncertainties are propagated through
our entire analysis. We then fit our LRIS and FLOYDS spectra of SN 2019yvr to
the templates by calculating a dereddened spectral template
$\tilde{f}_{\lambda,d}$ assuming the appropriate Milky Way extinction and the
interstellar host extinction given above. We also deredden SN 2019yvr for
Milky Way extinction following the same procedure yielding
$f_{\lambda,\mathrm{19yvr}}$. For both SN 2019yvr and the template, we rescale
the uncertainty $\sigma_{\lambda}$ by the same factor as the dereddened
spectrum. Finally, we derive the best-fitting host extinction
$A_{V,\mathrm{19yvr}}$ and reddening law parameter $R_{V,\mathrm{19yvr}}$ by
calculating $A_{\lambda}$ from Cardelli et al. (1989) and minimizing the
reduced $\chi^{2}$ value

$\chi^{2}=\sum^{N}_{\lambda}\frac{(\tilde{f}_{\lambda,d}-Cf_{\lambda,\mathrm{19yvr}}10^{0.4A_{\lambda}})^{2}}{N(\sigma_{\lambda}^{2}+\sigma_{\lambda,\mathrm{19yvr}}^{2})}$
(2)

where $N$ is the total number of 1 Å wavelength bins and $C$ is a scaling
constant between the two spectra. Thus, our spectral fitting method is
primarily sensitive to the overall shape of the two spectra rather than the
ratio between their fluxes. We show our best-fitting dereddened SN 2019yvr
spectra in Figure 3 and we list our best-fitting $A_{V}$ and $R_{V}$
parameters for SN 2019yvr in Table 1.

Epoch | Template (Epoch) | $A_{V,Temp.}$ | $A_{V,\mathrm{19yvr}}$ | $R_{V,\mathrm{19yvr}}$ | $\chi^{2}$
---|---|---|---|---|---
(days) | (days) | (mag) | (mag) |  |
$-$3 |  iPTF13bvn ($-$2.1) | 0.53 |  3.33$\pm$0.24 |  4.91$\pm$0.37 | 1.00
$-$3 |  SN 2007Y ($-$0.9) | 0.35 |  3.14$\pm$0.29 |  3.22$\pm$0.41 | 4.40
$-$3 |  SN 2009jf ($-$3.3) | 0.00 |  2.46$\pm$0.32 |  3.01$\pm$0.40 | 4.08
$-$3 |  SN 1999dn ($-$3.0) | 0.15 |  3.62$\pm$0.24 |  4.90$\pm$0.49 | 5.31
$+$22 |  SN 2009jf ($+$25.6) | 0.00 |  3.94$\pm$0.38 |  4.97$\pm$0.56 | 17.42
Mean |  |  |  3.3$\pm 0.4$ |  4.1$\pm$0.9 |
Table 1: Our best-fitting parameters for $V$-band extinction ($A_{V}$) and
$R_{V}$ inferred for SN 2019yvr based on matching to template spectra as shown
in Figure 3 and described in Section 3.2. As in Figure 3, the epoch of each SN
2019yvr and template spectrum is given in days with respect to $V$-band
maximum light. $\chi^{2}$ is given in units of reduced
$\chi^{2}/\chi^{2}_{\mathrm{min}}$. We give parameters for each template
spectrum used and the inverse $\chi^{2}$-weighted average for $A_{V}$ and
$R_{V}$. However, see caveats in Section 3.2. Figure 4: Colour curves of SN
2019yvr corrected for Milky Way and interstellar host extinction (with
$A_{V}=2.3$ mag and $R_{V}=4.4$) as discussed in Section 3.3. Circles
correspond to our Swope photometry while squares are for LCO photometry. We
overplot templates for extinction-corrected SN Ib colour curves from
Stritzinger et al. (2018) as black lines with the 1$\sigma$ uncertainties in
each template as a gray shaded region. Figure 5: $\chi^{2}$ values as a
function of assumed interstellar $V$-band extinction $A_{V}$ and reddening law
parameter $R_{V}$ and comparing SN 2019yvr colours to the colour templates in
Section 3.3 and Figure 4. The best-fitting extinction parameters are
$A_{V}=2.4$ mag and $R_{V}=4.7$ (yellow star) with an implied a best-fitting
$E(B-V)=0.51$ mag. The yellow dashed lines show the 1-$\sigma$ best-fitting
limits of $E(B-V)$.

As reported in Section 2.4, our SN 2019yvr spectra correspond to approximately
$-3$ days and $+22$ days relative to $V$-band maximum. For the latter
spectrum, only SN 2009jf had a spectrum sufficiently close in $V$-band epoch
to perform a robust comparison between spectral shape. Thus, while the best-
fitting cases all correspond to the early-time spectrum, our second epoch
serves to validate the results of this analysis. In this way, we derive a
line-of-sight extinction to SN 2019yvr of $A_{V}=2.4$–$3.9$ mag, although most
of our best-fitting values are around $A_{V}=3.2$–$3.6$ mag. These values are
consistent with Na i D, but there is significant scatter in $A_{V}$, implying
that there are systematic uncertainties in our method.

###  3.3 Extinction Inferred from SN 2019yvr Colour Curves

We further investigate the interstellar host reddening using colour curve
templates from Stritzinger et al. (2018) and compare to our Swope and LCO
colour curves of SN 2019yvr (Figure 4). Using a Cardelli et al. (1989)
reddening law, we vary the values of $A_{V}$ and $R_{V}$ in order to derive
colour corrections due to interstellar reddening. We apply these corrections
to our $B-g$, $B-V$, $g-r$, and $g-i$ colour curves to find the best fit with
Stritzinger et al. (2018) template colours as shown in Figure 4. The best-
fitting values are quantified with respect to the summed $\chi^{2}$ values in
$B-g$, $B-V$, $g-r$, and $r-i$ and across all epochs. The final reduced
$\chi^{2}$ value for different values of $A_{V}$ and $R_{V}$ is shown in
Figure 5.

We find the best-fitting values using our colour curve matching are
$A_{V}=2.4\begin{subarray}{c}+0.7\\\ -1.1\end{subarray}$ mag and
$R_{V}=4.7\begin{subarray}{c}+1.3\\\ -3.0\end{subarray}$ and implying a best-
fitting $E(B-V)=0.51\begin{subarray}{c}+0.27\\\ -0.16\end{subarray}$ mag. The
value of $R_{V}$ is limited at the high end by our boundary condition that
$R_{V}<6.0$. Based on well-measured values of $R_{V}$ for SN host galaxies
(e.g., the wide variety of SN Ia hosts presented in Amanullah et al., 2015),
which tend to have $1.3<R_{V}<3.6$, we infer that our prior $R_{V}<6.0$ is a
conservative upper bound on realistic values of the reddening law parameter.

###  3.4 Final Extinction Value Adopted for SN 2019yvr

Overall, the level of extinction inferred from our spectral analysis is
consistent with our estimate from the $V$-band light curve as well as the
value inferred from the Stritzinger et al. (2018) Na i D relation. While the
latter relationship diverges significantly at large extinction values (also
similar to Poznanski et al., 2012), we infer from the agreement between these
three estimates that the line-of-sight host extinction inferred for SN 2019yvr
is close to the value inferred from our spectral and colour curve analyses.
However, the colour curve analysis involves more independent measurements of
the SN 2019yvr optical spectrum, and this analysis has been validated for
several SNe Ib by Stritzinger et al. (2018). Thus, although there is agreement
between all of our methods, we infer that $A_{V}=2.4\begin{subarray}{c}+0.7\\\
-1.1\end{subarray}$ mag and $R_{V}=4.7\begin{subarray}{c}+1.3\\\
-3.0\end{subarray}$ is most representative of the line-of-sight extinction to
SN 2019yvr, and we use these values and our $\chi^{2}$ distribution on
extinction in Figure 5 below.

However, the critical question to the analysis below is how much extinction
did the SN 2019yvr progenitor star experience? While it is reasonable to
assume that the interstellar host extinction inferred from SN 2019yvr would be
the same as the extinction that its progenitor star experienced (especially on
the 2.4–15.0 yr timescale of our pre-explosion data), there could be
additional sources of extinction present when the pre-explosion data were
obtained to which our SN 2019yvr observations are not sensitive or vice versa.
In particular, circumstellar dust could have been present in the pre-explosion
environment but vaporized soon after explosion, or else there could be
material ejected by the progenitor star very soon before explosion that was
not present when the HST or Spitzer data were obtained. Below we consider both
scenarios and the effects of circumstellar material and extinction on our
overall data set.

###  3.5 Possibility of More Circumstellar Extinction 2.6 Year Prior to Core
Collapse

All massive stars exhibit winds that pollute their environments with gas and
dust (Smith, 2014), and this material can lead to significant circumstellar
extinction when the wind is dense, clumpy, and relatively cool. Thus it is
possible that the SN 2019yvr progenitor star experienced significant
circumstellar extinction from a shell of dust that was vaporized before it
could be observed in the SN. Auchettl et al. (in prep.) find evidence for a
significant mass of hydrogen-rich CSM from H$\alpha$, X-ray, and radio
emission. Rebrightening in the light curve of SN 2019yvr beginning $>$150 days
after discovery suggests that this material is in a shell likely at $>$1000 AU
from the progenitor star, thus ejected years or decades before core collapse.

The question we address here is whether there could also be material closer to
the progenitor star that contributes to circumstellar extinction but was
vaporized soon after core collapse, implying that the line-of-sight extinction
estimated above underestimates the extinction at the time of the HST
observations. There is no obvious sign of any such material, for example, in
evolution of the Na i D profile or excess emission in early-time light curves
and spectra.

Dust geometries and properties most likely to be associated with circumstellar
extinction due to material close (2–10$\times$ the photospheric radius as in
Kochanek et al., 2012) to the progenitor star but unconstrained by our SN
2019yvr observations can be probed with our mid-infrared Spitzer/IRAC limits.
Assuming this material was present on the timescale of the IRAC observations,
we model an optically thin shell of dust to our limits of 22.8, 23.1, and 21.5
mag in IRAC bands 1, 2, and 4, respectively (see Section 4.1 for a discussion
of the IRAC limits). A warm shell of gas and dust ($T>200$ K) would result in
bright mid-infrared emission even in cases where it is relatively compact
($<$1 AU). Following analysis in Kilpatrick et al. (2018a), Kilpatrick & Foley
(2018), and Jacobson-Galán et al. (2020), we modeled optically thin shells of
silicate dust with grain sizes $>$0.1$~{}\mu$m and a range of temperatures
from $200$–$1500$ K. At hotter temperatures, the dust would likely sublimate
and thus would not exhibit the same extinction properties or attendant mid-
infrared emission. Similarly, a shell at large distances from its progenitor
star might be so cool that it does not emit significant flux at $<10~{}\mu$m
where our IRAC data probe, even if it has a large mass.

The dust mass limits we derive are strongly temperature dependent, with the
coolest temperatures yielding the weakest limits on mass ($M_{d}<9\times
10^{-4}~{}M_{\odot}$ and $L_{d}<4\times 10^{4}~{}L_{\odot}$ at $200$ K)
whereas hotter dust leads to relatively strong limits on dust mass
($M_{d}<2\times 10^{-8}~{}M_{\odot}$ and $L_{d}<9\times 10^{4}~{}L_{\odot}$ at
$1500$ K). We used the $0.1~{}\mu$m silicate dust grain opacities from Fox et
al. (2010, 2011) to calculate these limits. Assuming the same dust grain
composition, we approximate the limits on optical depth in $V$-band as
$\tau_{V}=\rho\kappa_{V}r_{\mathrm{dust}}$, where $r_{\mathrm{dust}}$ is the
implied blackbody radius of the dust shell, $\rho\approx M_{d}/(4/3\pi
r_{\mathrm{dust}}^{3})$, and $\kappa_{V}$ is the opacity in $V$-band. Under
these assumptions, the optical depth must be $\tau_{V}<3$–$187$, with the
strongest limits again coming from the hottest dust temperatures.

Approximating $A_{V}=0.79\tau_{V}$ as in Kochanek et al. (2012) and Kilpatrick
& Foley (2018), these limits are not constraining on the total circumstellar
extinction due to a compact dust shell. Indeed, circumstellar dust absorption
could be the dominant source of extinction in the SN 2019yvr progenitor
system, but we would have no contextual information from the pre-explosion
Spitzer/IRAC photometry to constrain the magnitude of that extinction.

The strongest argument against such a compact, warm shell of gas and dust is
the lack of any hydrogen or helium emission associated with circumstellar
interaction in early-time spectra or any near-infrared excess in the
photometry as shown in Figure 3 and Figure 2. However, these arguments are
biased by the epoch of the first observations. SN 2019yvr had a reported
discovery on 2019 December 27 by ATLAS with the last previous non-detection
occurring on 2019 December 11 at $>18.6$ mag in $o$-band (Smith et al., 2019).
Subsequent non-detection reports by the Zwicky Transient Facility give a more
constraining non-detection in $g$-band at $>$19.5 mag on 2019 December
13999https://www.wis-tns.org/object/2019yvr. However, this still allows for 14
days when SN 2019yvr could have interacted with CSM in its immediate
environment. Although the first spectrum of SN 2019yvr did not exhibit
evidence for flash ionization or narrow emission lines due to CSM interaction,
this would not be surprising if the explosion was already more than several
days old (e.g., flash ionization lasted for $<$6 days for the type IIb SN
2013cu; Gal-Yam et al., 2014). Deeper and higher cadence early time
observations and pre-explosion limits, especially from high-resolution, near-
and mid-infrared imaging, would have been needed to provide meaningful
constraints on the presence and total mass of such material.

###  3.6 Possibility of Less Circumstellar Extinction 2.6 Years Prior to Core
Collapse

There is strong evidence for circumstellar interaction around SN 2019yvr in
optical spectra, radio, and X-ray detections starting around 150 days after
discovery (Auchettl et al. in prep.). The development of narrow Balmer lines
at these late times indicates this material is hydrogen rich. A delayed
interaction points to a shell of material at a large projected separation from
the progenitor ($\approx$1000 AU assuming a SN shock velocity of
$\approx$10,000 km s-1).

A key consideration above is whether this CSM was present at the time of the
HST observations or if it was ejected in the subsequent 2.6 yr before core
collapse. In the latter case, any dust synthesized in the CSM would not be
present in the HST data and thus $A_{V}=2.4$ mag would be an overestimate of
the extinction affecting any emission we detect in pre-explosion data.

Based on the HST observations and follow up data of the SN, we cannot
constrain this scenario. However, one prediction from this scenario would be
an intermediate-luminosity transient associated with an extreme mass loss
episode over this time. We analyzed this location of the sky and found no
luminous counterparts in pre-explosion imaging from the ASAS-SN Sky
Patrol101010https://asas-sn.osu.edu/ (Shappee et al., 2014; Kochanek et al.,
2017) or the Catalina Surveys Data Release 2 (Drake et al., 2009), but these
limits only extend to $<13$ mag given contamination from the bright center of
NGC 4666. Thus we cannot provide a meaningful estimate on any such CSM, but we
consider the possibility that $A_{V}<2.4$ mag in our analysis of pre-explosion
counterparts in Sections 4 and 5 below. In general, we assume that the
extinction inferred from SN 2019yvr is the same between the epoch of HST
observations and the time of explosion. Overall, we consider
$A_{V}=2.4\begin{subarray}{c}+0.7\\\ -1.1\end{subarray}$ mag and
$R_{V}=4.7\begin{subarray}{c}+1.3\\\ -3.0\end{subarray}$ to represent the
total line-of-sight extinction to the SN 2019yvr progenitor system at the time
the pre-explosion HST imaging was obtained.

##  4 The Progenitor Candidate to SN 2019yvr

###  4.1 Aligning Adaptive Optics and Pre-Explosion Imaging

We obtained positions for 114 point sources in our GSAOI adaptive optics image
using sextractor (Bertin & Arnouts, 1996) and compared these to the positions
of the same sources in the F555W HST/WFC3 image as obtained in dolphot. From
these common sources, we derived a coordinate transformation solution from
GSAOI$\rightarrow$HST. We also derived the systematic uncertainty in this
transformation by splitting our sample of common astrometric sources in half,
re-deriving the coordinate transformation, and then comparing the offset
between the remaining HST sources and their positions from our GSAOI image and
transformation. Repeating this procedure, we are able to derive an average
systematic offset between our GSAOI and HST sources. We assume the root-mean-
square of these offsets dominates the error in our astrometric solution, which
we find is $\sigma_{\alpha}=0.16$ WFC3/UVIS pixels
($0.008$${}^{\,\prime\prime}$) and $\sigma_{\delta}=0.18$ WFC3/UVIS pixels
($0.009$${}^{\,\prime\prime}$).

The position of SN 2019yvr in our GSAOI image corresponds to a single point
source in the WFC3/UVIS imaging to a precision of $0.1$ WFC3/UVIS pixels
($\approx$0.6$\sigma$ as the uncertainty on the position of this source from
our GSAOI is negligible). We detect this source in the drizzled F555W image at
34$\sigma$ significance, and there are no other sources at the $>$5$\sigma$
level within a separation of 0.27${}^{\,\prime\prime}$ or 30 times the
astrometric uncertainty. Our WFC2/UVIS photometry is listed in Table 2.

We also examined the position of SN 2019yvr in pre-explosion Spitzer/IRAC
imaging. Using the same alignment method as above, we determined the location
of SN 2019yvr in the Spitzer/IRAC stacked images using our GSAOI image of SN
2019yvr. Our alignment uncertainty is typically $\sigma\approx 0.2$ IRAC
pixels ($0.12$${}^{\,\prime\prime}$) from GSAOI$\rightarrow$IRAC in each
channel. We found no evidence of a counterpart in any epoch or the cumulative,
stacked pre-explosion frames. Therefore, we place an upper limit on the
presence of a pre-explosion counterpart in the stacked IRAC frames by
injecting and recovering artificial stars at the location of SN 2019yvr and
using the native IRAC point response function for each channel. Our pre-
explosion limits for IRAC are reported in Table 3.

###  4.2 The Nature of the HST Counterpart to SN 2019yvr

Stripped-envelope SNe are known to occur in the brightest, highest extinction,
and highest metallicity regions of their host galaxies (Galbany et al., 2016a,
b). The iPTF13bvn progenitor system was identified in a relatively uncrowded
region of NGC 5086 (Cao et al., 2013) and subsequently confirmed as the actual
progenitor by its disappearance (Eldridge & Maund, 2016; Folatelli et al.,
2016), but in general SNe Ib/c are found in crowded regions of their host
galaxies (when the surrounding environment can be resolved, as in Eldridge et
al., 2013). For example, the candidate progenitor system of the stripped-
envelope SN Ic 2017ein was in an environment with several other luminous
sources (Kilpatrick et al., 2018b). This fact and the counterpart’s high
optical luminosity suggest it may in fact have an unresolved star cluster or a
chance coincidence.

The candidate progenitor system to SN 2019yvr does not appear extended in any
of the WFC3/UVIS frames, with dolphot average sharpness=$-0.02$,
roundness=$0.36$, and classified as a bright star, which is consistent with a
circular point source at WFC3/UVIS resolution. The source is not blended with
any other nearby sources and has an average crowding=$0.09$. Therefore, we
conclude that the candidate counterpart is consistent with being a single,
isolated point source in all of our images.

One possible scenario is that the candidate source is dominated by emission
from multiple stars in a single system or open cluster (similar to those in
Bastian et al., 2005; Gieles et al., 2006; Gieles & Portegies Zwart, 2011).
Although the candidate source is point-like, the PSF size of HST/WFC3 in F555W
is $\approx$0.067${}^{\,\prime\prime}$, or 4.7 pc at the distance of NGC 4666.
Many open clusters are smaller than this, and might be so compact as to
resemble a point source. The F555W (roughly $V$-band) absolute magnitude we
infer for this source is $-$7.8 mag (assuming $A_{V}=2.4$ mag), which would be
extremely low luminosity for the population of clusters in Gieles et al.
(2006). Thus, while we cannot currently rule out the possibility that the
source is a cluster we find it much more likely that the source is dominated
by emission from a single star or star system associated with SN 2019yvr.

We estimate a single-trial probability of chance coincidence by considering
that there are 3281 sources (of any type) detected at $>$5$\sigma$ in a
10${}^{\,\prime\prime}$ region surrounding the candidate SN 2019yvr
counterpart in any of the HST frames. Thus, at most $6.7$ arcsec2 or 2% of
this region is subtended by area within 3$\sigma$ (astrometric uncertainty) of
any source, which is a conservative upper limit on the probability of chance
coincidence between the counterpart and SN 2019yvr. We find it is unlikely
that SN 2019yvr coincides with this source by chance, although we acknowledge
that this scenario cannot be ruled out definitively before we demonstrate that
the source has disappeared (as in the case of iPTF13bvn; Eldridge & Maund,
2016; Folatelli et al., 2016).

Given that SN 2019yvr coincides with a single, bright source, that source is
point-like and isolated from nearby sources, and the relatively low likelihood
of a chance coincidence, we consider this source to be a credible progenitor
candidate to SN 2019yvr. Below we assume that this object is dominated by
emission from a single stellar system that hosted the SN 2019yvr progenitor
star.

WFC3/UVIS Photometry of 2019yvr Progenitor Candidate  MJD Filter Exposure (s)
Magnitude Uncertainty 57864.06972 F438W 1140 26.2028 0.2207 57864.11750 F625W
1134 24.8352 0.0466 57864.17879 F555W 1200 25.4011 0.0720 57864.24510 F814W
1152 24.1778 0.0498 57890.23024 F555W 1143 25.1599 0.0612 57890.24828 F625W
1140 24.8008 0.0443 57917.36677 F438W 1140 26.3646 0.4810 57917.39478 F625W
1134 24.9420 0.0538 57917.43295 F555W 1200 25.5264 0.0853 57917.46140 F814W
1152 24.3412 0.0588 57944.68250 F555W 1143 25.2253 0.0621 57944.75022 F625W
1140 24.8760 0.0478 57972.22031 F438W 1140 25.9439 0.2410 57972.23837 F625W
1134 25.0531 0.0559 57972.28531 F555W 1200 25.4908 0.0829 57972.30377 F814W
1152 24.2492 0.0575 Average Photometry  MJD Filter Exposure (s) Magnitude
Uncertainty 57917.88560 F438W 3420 26.1382 0.1622 57904.13112 F555W 5886
25.3512 0.0319 57917.74983 F625W 5682 24.8971 0.0221 57918.00342 F814W 3456
24.2533 0.0319

Table 2: HST WFC3/UVIS photometry of the SN 2019yvr progenitor candidate. All
magnitudes are on the AB system.

Spitzer/IRAC Pre-explosion Limits  Average MJD Wavelength Exposure Limit
($\mu$m) (s) (mag) 57917.88560 3.6 1530.0 $>$22.8 57904.13112 4.5 1864.8
$>$23.1 57918.00342 7.9 278.0 $>$21.5

Table 3: IRAC limits on the presence of a pre-explosion counterpart to SN
2019yvr progenitor candidate. All magnitudes are on the AB system.

###  4.3 Photometric Properties of the Pre-Explosion Counterpart

We show the light curve of the SN 2019yvr progenitor candidate at times
relative to explosion in Figure 6. The source is relatively stable with at
most $0.47$ mag peak-to-peak variability (corresponding to 3.4$\sigma$) in
F555W over a baseline of 110 days. Thus we infer that the progenitor candidate
did not exhibit any extreme variability with constant flux at the $<$0.24 mag
level in all bands. This suggests that if the counterpart is a star, it was
not in an eruptive or any other highly variable phase during these
observations, as these events are typically accompanied by large differences
in luminosity or colour (as in pre-SN outbursts associated with SNe IIn, e.g.,
Smith et al., 2009; Mauerhan et al., 2013; Kilpatrick et al., 2018a).

Thus we are confident that the average photometry across all four HST bands in
which we detect the progenitor candidate is representative of its overall
spectral energy distribution (SED). Taking an inverse-variance weighted
average of across all epochs in each band, we derive average photometry
$m_{\rm F438W}=26.138\pm 0.162$ mag, $m_{\rm F555W}=25.351\pm 0.032$ mag,
$m_{\rm F625W}=26.897\pm 0.022$ mag, and $m_{\rm F814W}=24.253\pm 0.032$ mag
as shown in Table 2. Temporarily, ignoring any correction due to host
extinction but accounting for Milky Way extinction, the source has $m_{\rm
F555W}-m_{\rm F814W}$ (roughly $V-I$) of 1.065$\pm$0.045 mag and $M_{\rm
F555W}=-5.5$ mag assuming our preferred distance modulus above (both values
are in AB mag). This is roughly consistent with temperatures of
$T_{\mathrm{eff}}=3360$ K, which is broadly comparable, although slightly
hotter, than most terminal RSGs at solar metallicity (Choi et al., 2016). This
suggests that the source either has a cool photosphere or is heavily
extinguished, in agreement with expectations from our analysis of SN 2019yvr.
However, if the source is extinguished due to CSM, there is no clear evidence
from pre-explosion variability whether any of this material was ejected during
the window of the HST or Spitzer observations.

Figure 6: The pre-explosion light curve of the SN 2019yvr progenitor candidate
in all four HST filters for which we have imaging. The source is not
significantly variable, with at most $0.47$ mag peak-to-peak variations as
discussed in Section 4.3.

###  4.4 Comparison to Blackbodies and Single-star Spectral Energy
Distributions

Assuming that the counterpart is dominated by the SED of a single star, we
estimate the luminosity and temperature of that star by fitting various SED
models to the HST photometry. Broadly, we use blackbody and stellar SEDs
obtained from Pickles & Depagne (2010).

We use a full forward modeling and Monte Carlo Markov Chain (MCMC) approach to
simulate the in-band apparent magnitudes assuming the distance above and
drawing extinction ($A_{V}$) and reddening ($R_{V}$) parameters following the
$\chi^{2}_{\mathrm{ext}}$ probability distribution from our light curve
analysis in Section 3 and as shown in Figure 5. For a blackbody with a given
effective temperature $T_{\mathrm{eff}}$ and luminosity $L$ as well as
extinction values drawn from the $\chi^{2}$ distribution discussed above, we
simulate an intrinsic, absolute magnitude $M_{i}$ in each band $i$ and convert
to an apparent magnitude $m_{i}$ with in-band Milky Way extinction $A_{{\rm
MW},i}$, the implied host extinction $A_{H,i}$, and our preferred distance
modulus $\mu=30.8$ mag. Thus the simulated apparent magnitude depends on both
the intrinsic model parameters (i.e., $T_{\mathrm{eff}}$,$L$) and extinction
parameters ($A_{V}$, $R_{V}$) drawn from $\chi^{2}_{\mathrm{ext}}$. We then
estimate the log likelihood for our MCMC ($\chi^{2}_{\mathrm{eff}}$) using the
observed magnitude $m_{o,i}$ and uncertainty $\sigma_{o,i}$ from Table 2 as

$\chi^{2}_{\mathrm{eff}}=\sum_{i}\left(\frac{m_{i}-m_{o,i}}{\sigma_{i}}\right)^{2}+\chi^{2}_{\mathrm{ext}}(A_{V},R_{V}).$
(3)

In this way we incorporate the differences between the observed and forward-
modeled magnitudes as well as between the values of $A_{V}$ and $R_{V}$ for
each trial and the best-fitting values from our colour curve template fitting.
Assuming a blackbody SED, we estimate the best-fitting parameters
$T_{\mathrm{eff}}=7700\begin{subarray}{c}+900\\\ -1000\end{subarray}$ K and
$\log(L/L_{\odot})=5.3\begin{subarray}{c}+0.2\\\ -0.3\end{subarray}$. Although
we include the distance modulus uncertainty in our luminosity (and radius)
uncertainty estimates, we did not include this value in our fitting method as
it does not affect the overall shape of the SED. The implied photospheric
radius for the best-fitting blackbodies are $R=250\pm 30~{}R_{\odot}$. As we
incorporate $A_{V}$ and $R_{V}$ from the light curve analysis into our models,
we also constrain these parameters with best-fitting values
$A_{V}=2.8\begin{subarray}{c}+0.3\\\ -0.4\end{subarray}$ mag and
$R_{V}=5.2\begin{subarray}{c}+0.8\\\ -0.7\end{subarray}$ assuming the
intrinsic blackbody spectrum.

We also compared our photometry to single-star SEDs from Pickles & Depagne
(2010). We use stars of all spectral classes, fitting only to a scaled version
of the stellar SED as a function of effective temperature. Using the same MCMC
method, our walkers drew a temperature randomly and then chose the stellar SED
with the closest effective temperature. The best-fitting SEDs are consistent
with stars in the F4 to F0 range (intrinsic
$T_{\mathrm{eff}}=6800\begin{subarray}{c}+400\\\ -200\end{subarray}$ K; see
Figure 7 and Figure 8) with an implied luminosity $\log(L/L_{\odot})=5.3\pm
0.2$ and a photospheric radius $R=320\begin{subarray}{c}+30\\\
-50\end{subarray}$ $R_{\odot}$. Thus the best-fitting values are broadly
consistent between blackbody and Pickles & Depagne (2010) model SEDs. There is
some systematic uncertainty in the exact temperature of the latter models
given the sampling of the Pickles & Depagne (2010) spectra, which are
increasingly sparse for hotter stars. However, this effect is small at
temperatures 5000–10,000 K where there are 40 spectra of varying spectral
classes.

Our treatment of $A_{V}$ and $R_{V}$ is identical to the forward modeling
approach for the blackbodies above, and we derive best-fitting values of
$A_{V}=3.1\begin{subarray}{c}+0.3\\\ -0.2\end{subarray}$ mag and
$R_{V}=5.9\begin{subarray}{c}+0.1\\\ -0.4\end{subarray}$. In both the
blackbody and stellar SED models, our luminosity estimates are on the high-
luminosity end for observed core-collapse SN progenitor stars (e.g., in Smartt
et al., 2015) but consistent with most of the SN IIb progenitor stars (see
Figure 9).

Figure 7: The best-fitting SEDs to the average pre-explosion HST photometry of
the SN 2019yvr progenitor candidate. Our best-fitting blackbody has
$T_{\mathrm{eff}}=7700$ K, while the best-fitting single-star Pickles &
Depagne (2010) model is a F2I star with $T_{\mathrm{eff}}=6800$ K. In both
cases, the implied luminosity is consistent with being
$\log(L/L_{\odot})$$\approx$5.3. The SEDs are completely forward-modeled in
observed flux, thus they include the apparent best-fitting interstellar host
and Milky Way extinction. Although we include the distance uncertainty in our
luminosity estimate, the error bars in this figure only include measurement
uncertainty and uncertainty on extinction. We simply scale the integrated flux
density by our preferred distance.

As a further check on the effect of extinction on our derived parameters, we
show the relationship between the host extinction $A_{V}$ and reddening law
parameter $R_{V}$ and the implied temperature and luminosity for the Pickles &
Depagne (2010) models in Figure 8. Luminosity is highly correlated with
variations in $A_{V}$, with $A_{V}<2.4$ mag implying a lower luminosity but
also a significantly cooler photospheric temperature. In general, such a cool
photosphere is associated with a massive hydrogen envelope, which is in
tension with the SN Ib spectroscopic class (see Section 5 for further
discussion). In contrast, a larger extinction value would imply a
significantly higher luminosity and hotter temperature, although no
combination of parameters we considered for the stellar SED fits allowed an
effective temperature $>$10,000 K to within the 3-$\sigma$ level. This is in
stark contrast with the progenitor of iPTF13bvn with $T_{\mathrm{eff}}\approx
45,000$ K (Cao et al., 2013; Bersten et al., 2014; Eldridge & Maund, 2016;
Folatelli et al., 2016) and He stars generally, which tend to have effective
temperatures $>$20,000 K (as discussed for the progenitors of SNe Ib in Yoon,
2015).

Overall, our constraints on $A_{V}$ and $R_{V}$ enable a relatively tight fit
temperature and luminosity as demonstrated in Figure 8. The minimum
$\chi^{2}/\mathrm{degreesoffreedom}=1.5$, which suggests that a single,
extinguished star is well matched to our data and we cannot effectively
constrain scenarios with more free parameters, such as the inclusion of
another star to the overall SED. However, while this analysis might accurately
reflect the SN 2019yvr progenitor star’s evolutionary state at 2.6 yr before
explosion, it does not place any specific constraints on the pathway that led
to this configuration. We further explore the implications of a SN Ib
progenitor star with these properties and the implications for a single-star
origin in Section 5.1.

Figure 8: Corner plot showing the correlation between our fit parameters
$\log(L/L_{\odot})$and $\log(T/\mathrm{K})$ of a single star following a
Pickles & Depagne (2010) as well as host extinction $A_{V}$ and host reddening
$R_{V}$ as described in Section 4.4. The contours show the 1, 2, and 3$\sigma$
best-fitting values derived from all of our samples. Although the luminosity
and $V$-band host extinction are highly correlated, the resulting luminosity
and temperature are tightly constrained.

###  4.5 Comparison to Binary Star Models

We also compare the SED of the SN 2019yvr progenitor candidate to binary
stellar evolution tracks from BPASS (Eldridge et al., 2017), comprising 12,663
binary star models at a single metallicity. BPASS provides physical parameters
from the binary star system throughout its evolutionary sequence as well as
in-band absolute magnitudes for the individual components and binary system as
a whole. Our analysis involved a direct comparison between the F438W, F555W,
F625W, and F814W magnitudes of the SN 2019yvr counterpart and the total binary
emission estimated via BPASS synthetic magnitudes in F435W, F555W, SDSS $r$,
and F814W, respectively. As BPASS magnitudes are provided in Vega mag, we
transformed our AB mag photometry to Vega mag using the relative Vega mag $-$
AB mag zero points for all four WFC3/UVIS filters (0.15, 0.03, $-$0.15, and
$-$0.42 mag, as in Dressel, 2012).

Assuming that the SN 2019yvr progenitor is one component of a binary system,
we determined what BPASS evolutionary tracks have a terminal state consistent
with the SN 2019yvr candidate photometry. Based on the gas-phase metallicity
estimate of NGC 4666 in Pan et al. (2020), which is consistent with Solar
metallicity, we restrict our analysis to BPASS models with fractional
metallicity $Z=0.014$. However, we emphasize that the Pan et al. (2020)
metallicity estimate is derived from spectra obtained toward the center of NGC
4666 rather than at the site of SN 2019yvr, and so the true metallicity of the
SN 2019yvr progenitor system may be significantly different. Otherwise, we
consider evolutionary tracks for all BPASS initial masses
($M_{\mathrm{init}}=0.1$–$300~{}M_{\odot}$), mass ratios ($q=0.1$–$1.0$), and
binary periods ($\log(P/\mathrm{1~{}day})=0$–$4$).

We used the same MCMC method as above with walkers drawing from initial
masses, mass ratios, and periods and comparing the terminal absolute magnitude
and colours of the closest model in parameter space to the SN 2019yvr
counterpart photometry. We also included $A_{V}$ and $R_{V}$ as free
parameters, but with the walkers drawing from the same $\chi^{2}$ distribution
for these parameters as in the blackbody and stellar SED fits above. For our
BPASS fits, the best-fitting models correspond to initial mass
$M_{\mathrm{init}}=19\pm 1~{}M_{\odot}$, initial mass ratio $q=0.15\pm 0.05$
and initial period $\log(P/\mathrm{1~{}day})=0.5\pm 0.2$. We show our a
Hertzpsrung-Russell diagram with the best-fitting model
($M_{\mathrm{init}}=19$, $q=0.1$, and $\log(P/\mathrm{1~{}day})=0.6$) in
Figure 9 and the luminosity and temperature derived from our Pickles & Depagne
(2010) stellar SED fits.

We performed our fits by comparing the observed photometry of the SN 2019yvr
progenitor candidate to the apparent magnitudes inferred for the combined flux
of both stars in the BPASS models111111In our BPASS v2.2.1 fits, we examined
columns 53–73, representing the absolute magnitude and colours from the
combined flux of both the primary and companion star., and so we are sensitive
to scenarios where the flux from either the primary or companion star
dominates the total emission. In all of the best-fitting models and all four
bands we consider, the counterpart is dominated by emission from a
$\approx$19$~{}M_{\odot}$ primary star and the companion contributes very
little to the overall flux. We found no other binary scenarios where the total
flux was consistent with our photometry at the time one of the stars
terminated, including scenarios where the secondary star produced the SN
explosion instead (i.e., in a neutron star or black hole binary).

In the best-fitting model, the terminal state of the SN progenitor is
$\log(L/L_{\odot})=5.3$ and $T_{\mathrm{eff}}=7300$ K with a terminal mass of
$M_{\mathrm{final}}=7.3~{}M_{\odot}$, implying a consistent luminosity but a
slightly warmer temperature than we derive from the Pickles & Depagne (2010)
models. Similar to above, there are no models at the $<$3-$\sigma$ level where
the exploding star has a terminal temperature $>$11,000 K. In the best-fitting
model, the secondary star (with an initial mass of $1.9~{}M_{\odot}$) is
mostly unchanged with only $0.009~{}M_{\odot}$ of material accreted by the
time the primary reaches core collapse, implying that most of the mass
transfer in this model was non-conservative. In addition, the BPASS models
predict that $0.047~{}M_{\odot}$ (0.6% mass fraction) of hydrogen remains in
the primary in its terminal state and no model with $<$$0.038~{}M_{\odot}$ is
consistent with our HST photometry at the 3-$\sigma$ level. The best-fitting
extinction values for this BPASS model was $A_{V}=2.6\pm 0.3$ mag and
$R_{V}=5.1\begin{subarray}{c}+0.9\\\ -2.1\end{subarray}$.

The primary effect of the BPASS evolutionary models compared to single-star
models is the inclusion of Roche-lobe overflow (RLOF). For our specific best-
fitting model, RLOF turns on in the post-main sequence phase (i.e., Case B
mass transfer; shown with a square in Figure 9), and continues through the end
of the primary star’s evolution. In particular, mass-loss due to RLOF follows
the prescription for common-envelope evolution (CEE) as the radius of the
primary star is smaller than the binary separation throughout post-main
sequence evolution (following prescription in Eldridge et al., 2017). The
binary separation is only $8.6~{}R_{\odot}$ starting in the post-main sequence
and at the onset of CEE, and so the primary mass-loss rate increases
significantly to 1–5$\times 10^{-4}~{}M_{\odot}~{}\mathrm{yr}^{-1}$. This
common envelope mass loss phase largely determines the final mass and state of
the primary star as it is larger than wind-driven mass loss by a factor of
$\approx$1000.

In BPASS, the onset of CEE is highly correlated with small binary separations
and low mass ratios for stars with $M_{\mathrm{init}}>5~{}M_{\odot}$ (Eldridge
et al., 2017). Thus stars that terminate near the progenitor candidate in the
Hertzsprung-Russell diagram require a specific mass-loss scenario where CEE
can strip most of the hydrogen envelope but leave a small amount (at least
$0.038~{}M_{\odot}$ according to our models), leading to relatively tight
constraints on binary mass ratio and period for our BPASS fits. However, these
parameters are subject to significant systematic uncertainty in terms of the
CEE and mass loss prescriptions assumed. In Section 5.1 we discuss whether
these best-fit binary models to the pre-explosion photometry of SN 2019yvr are
consistent with its classification as a type Ib SN.

##  5 What Progenitor Systems Could Explain SN 2019yvr and the Pre-explosion
Counterpart?

Given our analysis in Section 4, we assume throughout this discussion that the
SN 2019yvr pre-explosion counterpart is dominated by emission from the SN
progenitor system. From our inferences about this source above as well as our
knowledge of SN 2019yvr, we consider what evolutionary pathways could lead to
the source observed in the HST photometry as well as the resulting SN. These
pathways need to explain several facts referenced throughout the previous
analysis, which we summarize here as:

  1. 1.

SN 2019yvr was a SN Ib with no evidence for hydrogen in its early-time
spectra, starting from 7 days before peak light (Dimitriadis et al., 2019)
until well after peak light. Following models in Dessart et al. (2012), this
suggests that the progenitor star must have had $<$10${}^{-3}~{}M_{\odot}$
(but possibly as much as $0.03~{}M_{\odot}$; Hachinger et al., 2012) of
hydrogen remaining in its envelope at the time of explosion.

  2. 2.

SN 2019yvr began interacting with CSM starting around $150$ days after
explosion and exhibited strong H$\alpha$, radio, and X-ray emission consistent
with a shock formed in hydrogen-rich material (Auchettl et al. in prep.).
Using conservative assumptions about the SN shock velocity (10,000 km s-1) and
velocity of the CSM (100 km s-1), we infer that this material must have been
ejected at least $\approx$44 yr prior to core collapse and implying a shell or
clump of material at $>$1000 AU from the progenitor system (see estimate from
Equation 4 in Section 5.2). Although we find it more likely that this material
came from the SN progenitor star itself given the frequency of SNe Ib with
late-time interactions (i.e., similar to SN 2014C; Milisavljevic et al., 2015;
Margutti et al., 2017), it is possible that this material came from a binary
companion and the actual SN 2019yvr progenitor star was hydrogen free on this
timescale. Beyond this interaction, there is no evidence for dense CSM in
early-time spectra or light curves, all of which appear similar to SNe Ib
without evidence for these interactions.

  3. 3.

SN 2019yvr exhibited a large line-of-sight extinction ($A_{V}\approx 2.4$ mag)
while its Milky Way reddening was only $E(B-V)=0.02$ mag. We infer from the
strong Na i D line at the redshift of the SN 2019yvr host galaxy NGC 4666 that
this extinction implies a significant dust column in the NGC 4666 interstellar
medium toward SN 2019yvr and/or a correspondingly large column of
circumstellar dust in the environment of the progenitor system itself. There
is no clear evidence for any mass ejections or warm circumstellar gas in a
compact shell around the progenitor system, either in pre-explosion data or
from circumstellar interaction once SN 2019yvr exploded.

  4. 4.

There is a single, point-like progenitor candidate to SN 2019yvr detected in
pre-explosion HST imaging. This source exhibits very little photometric
variability over a 110 day period from 2.7 to 2.4 yr prior to core collapse.
Before applying our host extinction estimate but applying Milky Way
extinction, this progenitor candidate has a red colour of
$m_{\mathrm{F555W}}-m_{\mathrm{F814W}}=1.065$ mag. Accounting for the inferred
extinction and distance modulus above, the source is relatively luminous with
$M_{\mathrm{F555W}}=-7.8$ mag (roughly in $V$-band). This value is consistent
with massive stars but low for a stellar cluster (as in Gieles et al., 2006).

  5. 5.

Accounting for all extinction, the progenitor candidate is consistent with a
$\log(L/L_{\odot})$=5.3$\pm$0.2 and $T_{\mathrm{eff}}\approx 6800$ K star,
which implies a photosphere with a radius of $\approx 320~{}R_{\odot}$ at 2.6
yr prior to the SN 2019yvr explosion. Such a star would be closest in
temperature and luminosity to yellow supergiants confirmed as SN IIb
progenitor stars (Figure 9 and green circles for SNe 1993J, 2008ax, 2011dh,
2013df, and 2016gkg). Comparing to stellar SEDs, the spectral type and
luminosity class are best matched to F4–F0 supergiant stars.

  6. 6.

Comparing the pre-explosion photometry to BPASS binary stellar evolution
tracks in Eldridge et al. (2017), the best-fitting model is a 19
$M_{\odot}$+1.9 $M_{\odot}$ system that undergoes common envelope evolution
and strips most of the material from the progenitor star. Immediately prior to
explosion, the primary star retains 0.047 $M_{\odot}$ of hydrogen in its
envelope, inconsistent with the masses for SN Ib systems given above. No other
BPASS models were consistent with both our pre-explosion photometry and a
system that produced a SN explosion.

Figure 9: (Top): A Hertzpsrung-Russell diagram showing the location of the SN
2019yvr progenitor candidate (blue star) with comparison to SN IIb progenitor
stars (green squares), iPTF13bvn (blue diamond; Cao et al., 2013; Bersten et
al., 2014; Folatelli et al., 2016), and SN II progenitor stars (red squares;
Smartt et al., 2015). We overplot MIST single-star evolutionary tracks from
Choi et al. (2016); Choi et al. (2017) for comparison. Bottom: A 19+1.9
$M_{\odot}$ binary star evolution track from BPASS v2.2 (Eldridge et al.,
2017), which is consistent with the pre-explosion photometry of the SN 2019yvr
progenitor candidate. We highlight the location on the track where the primary
star begins Roche-lobe overflow (RLOF; square), reaches its minimum hydrogen
envelope mass ($0.047~{}M_{\odot}$, circle), and terminates as a supernova
(star). We also show binary star models from Yoon (2017) with outcomes
predicted for type Ib (blue) and IIb SNe (green).

###  5.1 The Anomalously Cool Progenitor and the Type Ib Classification of SN
2019yvr

We compare the SN 2019yvr progenitor candidate in a Hertzsprung-Russell
diagram to other known progenitor systems in Figure 9, including SNe II
(Smartt et al., 2015, and references therein), SNe IIb (SNe 1993J, 2008ax,
2011dh, 2013df, and 2016gkg; Aldering et al., 1994; Crockett et al., 2008;
Maund et al., 2011; Van Dyk et al., 2014; Kilpatrick et al., 2017), and the
progenitor of the SN Ib iPTF13bvn (Cao et al., 2013; Bersten et al., 2014;
Folatelli et al., 2016). We also note that there is a single SN Ic progenitor
candidate for SN 2017ein (Kilpatrick et al., 2018b; Van Dyk et al., 2018), but
the source has a luminosity $\log(L/L_{\odot})$$\approx$6.0 and temperature
$T_{\mathrm{eff}}>10^{5}$ K and so is off our plotting range (and may actually
be a very young open cluster). For comparison, we also overplot single-star
evolutionary tracks from the Mesa Isochrones & Stellar Tracks code (Choi et
al., 2016; Choi et al., 2017).

The most notable feature of the SN 2019yvr progenitor candidate in Figure 9
and compared with other stripped-envelope SN progenitor stars is its
relatively cool effective temperature, which in turn implies an extended
photosphere given our constraints on its SED. Assuming a single-star origin
and judging solely by the source’s inferred luminosity and temperature of
$\log(L/L_{\odot})$=5.3 and $T_{\mathrm{eff}}=6800$ K, the counterpart is
consistent with a $30~{}M_{\odot}$ star in the so-called “Hertzsprung gap”
(see, e.g., de Jager & Nieuwenhuijzen, 1997; Stothers & Chin, 2001). This is
in sharp contrast to the predicted progenitors of SNe Ib, which are thought to
have low-mass, compact envelopes, consistent with a star that has almost no
hydrogen in its outer layers (Yoon, 2015).

Given these facts, we consider the implications of different progenitor
scenarios. In particular, we emphasize the apparent contradiction between a SN
from a star without a significant hydrogen envelope mass and a pre-explosion
counterpart consistent with a massive star with a significantly extended
photosphere, which typically requires a non-negligible hydrogen envelope mass.
Combined with the SN 2014C-like circumstellar interaction observed at late
times, SN 2019yvr may offer significant insight into mass loss in late-stage
stellar evolution for stripped-envelope SNe. Here we review “standard” single
and binary star scenarios and assess whether they can explain all of these
properties.

####  5.1.1 Single-star Models

It is still debated if single massive stars can evolve to the point where they
would explode as yellow supergiants, in part because the Hertzsprung gap is
typically a short-lived and transitional phase in the post-main sequence.
Standard single-star evolution would suggest that a star with an effective
temperature of $T_{\mathrm{eff}}=6800$ K and $\log(L/L_{\odot})$=5.3 retains a
massive hydrogen envelope: a $30~{}M_{\odot}$ initial mass star would retain a
$60\%$ surface hydrogen mass fraction on its first passage through the
Hertzsprung gap (following the structure of model stars in Choi et al., 2016).
Thus, in the context of stripped-envelope SNe, any single-star model would
likely enter the yellow supergiant phase after evolving through the RSG branch
and shedding the remainder of its hydrogen envelope. Georgy (2012) demonstrate
that such evolution is possible if RSG mass loss rates are increased by
approximately an order of magnitude towards the end of their nuclear lives.

While the physics of an enhanced late-stage mass loss is unclear (see, e.g.
Yoon & Cantiello 2010), there do exist a number of luminous yellow supergiants
(termed “yellow hypergiants”) which are hypothesized to be such post-RSG
stars. Many yellow hypergiants are extremely variable with high mass-loss
rates, such as $\rho$ Cas (Smith, 2014; Lobel et al., 2015) and V509 Cas
(Percy & Zsoldos, 1992), and are located toward the end of the luminous blue
variable (LBV) bistability track (Smith et al., 2004). This variability
involves a rapid (years to decades) evolution between quiescent, hot phases
and erupting, cool phases with extreme mass-loss episodes (e.g.,
$10^{-4}~{}M_{\odot}~{}\mathrm{yr}^{-1}$ as in the yellow hypergiants $\rho$
Cas and HR 8752; de Jager, 1998; Humphreys et al., 2002).

Yellow hypergiants undergoing extreme mass loss have been proposed as
candidates for type Ib/c progenitors as their LBV-like mass ejections provide
an efficient way to rid the star of its hydrogen and helium envelope in the
years before core collapse (as in SN 2006jc; Foley et al., 2007; Smith &
Conti, 2008). The presence of these stars in the gap and the fact that some of
them explode as type IIb SNe (the bluest SN IIb progenitor stars span this
gap, e.g., Crockett et al., 2008; Kilpatrick et al., 2017) strengthens the
association between extreme mass loss and stripped-envelope SNe (de Jager,
1998; Stothers & Chin, 1999). While it is debated if such enhanced mass-loss
rates are possible for single stars on the RSG branch (e.g., Beasor et al.,
2020), mass-loss episodes and variability of stars near this point on the
Hertzpsrung-Russell diagram suggests it is possible to rapidly shed their
hydrogen envelopes and increase in temperature over timescales of years (as
observed with HR 8752, for example; de Jager & Nieuwenhuijzen, 1997). However,
yellow hypergiants that we observe in this region of the Hertzsprung-Russell
diagram have massive, hydrogen-rich envelopes and their winds are known to be
hydrogen rich (Smith et al., 2004). Assuming such a star exploded as a SN Ib
only 2.6 yr after being observed in this evolutionary phase, it would either
need to shed its remaining envelope in the final 2.6 yr or remain cool after
retaining only a trace hydrogen envelope. The former scenario will be
discussed in Section 5.2, below.

Alternatively, it is worth considering if a single star with virtually no
remaining hydrogen in its envelope could inflate to a radius of 320
$R_{\odot}$ while exhibiting a photospheric temperature of 6800 K—as required
for SN 2019yvr and its progenitor candidate. The coolest known helium stars
are only $\approx$10,000 K and these examples are significantly less luminous
than the observed counterpart. For example, LSS 4300 is
$T_{\mathrm{eff}}=11,000$ K (Drilling et al., 1984; Schoenberner & Drilling,
1984), KS Per is 10,000 K (Woolf, 1973; Drilling & Schonberner, 1982), and
$\nu$ Sgr is 11,800 K (Frame et al., 1995). The counterpart we observe is only
this hot assuming our SED modeling is inconsistent with the true temperature
at $>3\sigma$ or if there is $>$2 mag more extinction than we assumed at 2.6
yr before explosion. Even if we assume either of these scenarios is true, the
progenitor candidate would still have $\log(L/L_{\odot})$$\geq$5.3, which is
much more luminous than the most luminous helium stars that reach
$\log(L/L_{\odot})$=4.6 (see previous references and Dudley, 1992). Thus an
anomalously high-mass and luminous helium star would be needed to match to SN
2019yvr, which in general is not allowed by standard single star stellar
evolution models.

####  5.1.2 Binary Star Models

As shown above, the best-fitting binary systems for our SN 2019yvr pre-
explosion photometry are short-period, low mass ratio systems that undergo
CEE. While such systems are expected to have complex circumstellar
environments, possibly consistent with SN 2019yvr, we emphasize that all of
the BPASS models terminate with final hydrogen-envelope masses of $>$0.038
$M_{\odot}$. This is inconsistent with a classification as a SN Ib via the
models of Dessart et al. (2012) and Hachinger et al. (2012). In addition,
there is debate as to whether the evolution of such high mass ratio binaries
within BPASS is representative of actual stellar systems. Neugent et al.
(2018); Neugent et al. (2020) argue that any star that will eventually explode
as a core-collapse SN evolves off the main sequence fast enough that companion
stars with initial masses $\lesssim$3 $M_{\odot}$ would not have enough time
to complete their contraction phase and thus would still be protostars. This
is not accounted for in current BPASS models, which initialize all stellar
masses on the ZAMS simultaneously.

More broadly, the progenitor systems of SNe Ib are predicted to be low-mass
helium stars that evolve via binary evolution. While they can expand to
moderately large radii due to shell burning, they typically remain $<$100
$R_{\odot}$ (Kleiser et al., 2018; Laplace et al., 2020). Their photospheric
temperatures are thus significantly hotter than the HST photometry of the SN
2019yvr progenitor candidate implies. The latter allows at most 11,000 K as
opposed to the $\approx$45,000 K photosphere of the progenitor star to
iPTF13bvn (Eldridge & Maund, 2016; Folatelli et al., 2016). Yoon (2015)
predict that the best observational counterparts to SN Ib progenitor systems
are binary, stripped WR stars similar to HD 45166 (Groh et al., 2008) and WR7a
(Oliveira et al., 2003), both of which have $T_{\mathrm{eff}}>20$,000 K.
Nearly all of these stars are known to be in binaries with close ($<$few day)
orbits where the primary has been stripped to $3$–$10~{}M_{\odot}$, likely
through Case B mass transfer (Yoon et al., 2012; Yoon, 2015, 2017). The
companion can span a wide range of luminosities and evolutionary states (e.g.,
WR7a exhibits a 0.204 day binary orbital period, but no secondary is observed
implying a very low-mass star or a compact object; Pereira et al., 1998;
Oliveira et al., 2003). The primary stars are left completely stripped of a
hydrogen envelope. This is in contrast to the best-fitting BPASS model, which
undergoes extreme mass loss due to CEE and RLOF during the post-main sequence
but stops before the hydrogen envelope is completely depleted (Delgado &
Thomas, 1981; Ivanova et al., 2013; Eldridge et al., 2017).

Thus, as with the single star models, there remains significant tension as to
whether any standard binary evolution model can reproduce both the progenitor
candidate and a system that explodes as a SN Ib. We are left needing to invoke
some additional mechanism that can account for both the cool photospheric
temperature 2.6 yr prior to explosion and the negligible hydrogen envelope
mass at the time of core collapse. In the following sections we discuss two
potential resolutions to this paradox (Section 5.2 and Section 5.3) and
highlight some evolutionary scenarios that may be allowed while also
explaining the dense CSM shell observed around SN 2019yvr (Section 5.4).

###  5.2 Ejection of the Final Hydrogen Envelope in the Last 2.6 Years Prior
to Core Collapse

One way to resolve the apparent conflict between the extended progenitor
radius and lack of hydrogen in the ejecta of SN 2019yvr would be if the
progenitor star _did_ possess a envelope with $\sim$0.01–0.03 M⊙ of hydrogen
(similar to the yellow supergiant progenitor stars of SNe IIb) at the time of
the HST observations, but somehow managed to lose this material in the
intervening 2.6 yr. Such a scenario is not unprecedented: episodic mass
ejections have been invoked to explain stripped-envelope SN progenitor systems
that exhibit dense shells or clumps of CSM (see, e.g., Chugai & Chevalier,
2006; Bietenholz et al., 2014; Chandra, 2018; Mauerhan et al., 2018; Pooley et
al., 2019; Sollerman et al., 2020; Tartaglia et al., 2020). This was also
observed directly for SN 2006jc, a SN Ib with a pre-explosion outburst 2 yr
before explosion (Foley et al., 2007; Smith & Conti, 2008; Pastorello et al.,
2008; Maund et al., 2016), which later manifested as circumstellar
interaction. Indeed, SN 2019yvr shows evidence for relatively narrow
H$\alpha$, radio, and X-ray emission $\approx$150 days after core collapse,
providing evidence for episodic mass ejections throughout the progenitor’s
final evolutionary stages.

In this scenario, this hydrogen-rich material should be located at some radius
around the progenitor star. A key question for SN 2019yvr is whether the shell
of H-rich material encountered by the SN ejecta $\sim$150 days post-explosion
could be the remnants of such an ejection that occurred between the HST
observations and explosion. While the location and timing of this ejection
will be discussed in detail in Auchettl et al. (in prep.), we perform an order
of magnitude calculation to investigate this possibility here. For a CSM wind
velocity ($v_{w}$) and a SN shock velocity ($v_{s}$) the detection of CSM
interaction starting $\sim$150 days after core collapse implies a mass-loss
event that occurred:

$t_{\rm mle}\approx
44\left(\frac{v_{s}}{10^{4}{\rm~{}kms^{-1}}}\right)\left(\frac{v_{\mathrm{w}}}{100{\rm~{}kms^{-1}}}\right)^{-1}\;{\rm
yrs}$ (4)

before core collapse. We have scaled our results to an average shock velocity
of 10,000 km s-1 and wind velocity of 100 km s-1. The former is roughly
consistent with the velocity inferred from helium absorption in our spectra of
SN 2019yvr near maximum light, which is a lower limit for the shock velocity.
While the latter is less constrained, an ejection speed of $\sim$100 km s-1 is
approximately the escape speed for a star with a radius of 320 $R_{\odot}$ (as
inferred from our progenitor candidate) and a mass of 5$-$10 M⊙. For these
assumptions, the mass loss event would have occurred significantly earlier
than the time of the HST observations.

Therefore, if the hydrogen we observe in CSM is the same material inferred
from the progenitor star photosphere, the wind velocity must be at least 1800
km s-1 such that the progenitor star could eject it after the HST data were
obtained. A wind at this speed can only be achieved by a compact and massive
WR-like star (similar to those presented in Rochowicz & Niedzielski, 1995; van
der Hucht, 2001), inconsistent with our pre-explosion photometry. Such a high
velocity would therefore require and ejection mechanism capable of
accelerating material to $\gtrsim$10$\times$ the stellar escape speed. This
would be in contrast to current theoretical models for both wave-driven mass
loss in hydrogen-poor stars (which have terminal velocities of a few hundred
km s-1 Fuller & Ro 2018) and common-envelope ejections (which tend to proceed
at roughly the escape velocity).

Unless a stronger ejection mechanism can be identified, while the mass-loss
event that led to the material at $\approx$1000 AU was timed soon before the
explosion of SN 2019yvr, it may not be directly associated with the relatively
cool photosphere we infer from the HST imaging. Multiple eruptive mass-loss
events would be needed to explain both the CSM and the final depletion of the
progenitor star’s hydrogen envelope assuming the pre-explosion counterpart,
the source of the material around SN 2019yvr, and the SN 2019yvr progenitor
star are all the same. The second ejection would place material closer to the
progenitor star, which is not detected in our light curves or spectra (e.g.,
via enhanced emission due to CSM interaction or flash ionisation features
similar to Gal-Yam et al., 2014). It is possible that this material was missed
due to a delay between the explosion of SN 2019yvr and its discovery, and
detailed analysis of these data will be carried out in Auchettl et al. (in
prep.)to assess whether or not this could be the case.

###  5.3 A Quasi-photosphere Seen as an Inflated Star

If the scenario discussed in Section 5.2 is not viable, then the SN 2019yvr
progenitor candidate must have contained virtually no hydrogen 2.6 yr before
explosion ($<10^{-2}~{}M_{\odot}$ as required for a type Ib classification by
Dessart et al., 2012; Hachinger et al., 2012). In this case, we would require
that the envelope was inflated by some process not accounted for in standard
models of stellar evolution. Here, we consider two scenarios in which a star
with only a trace hydrogen envelope could exhibit a photospheric radius of
$\approx$320 $R_{\odot}$ or roughly 1.5 AU: (a) formation of a pseudo-
photosphere in a dense stellar wind, and (b) inflation due to an additional
heat/energy source that produces a radiation pressure supported envelope.

####  5.3.1 A Stellar Wind

For certain mass-loss rates, a stellar wind can become optically thick and
appear at a radius well beyond that of the underlying star (see, e.g.,
Gallagher, 1992). The radius at which this quasi-photosphere forms
($R_{\tau}$) depends on the density ($\rho$) and opacity ($\kappa$) in the
wind, with optical depth ($\tau$) expressed as

$\tau=\int_{R_{\tau}}^{\infty}\rho\kappa dr.$ (5)

From this equation, a quasi-photosphere will form when $\tau\gtrsim 1$.
Assuming that the radial density profile in the wind follows $r^{-2}$ for a
constant mass-loss rate ($\dot{M}$), these conditions are largely dependent on
the properties of the wind and thus the underlying star. From equation (16) in
de Koter et al. (1996), where the wind opacity is modeled as temperature-
dependent bound-free opacity from the Paschen continuum, the quasi-photosphere
constraint above implies

$\dot{M}>1.9\times 10^{-4}~{}M_{\odot}~{}\mathrm{yr}^{-1}\left(\frac{T_{\rm
eff}}{10^{4}~{}\mathrm{K}}\right)^{3/4}\left(\frac{R_{\tau}}{300~{}R_{\odot}}\right)^{3/2}\\\
\left(\frac{v_{w}}{100~{}\mathrm{km~{}s}^{-1}}\right)^{1/2}\left(\frac{c_{s}}{10~{}\mathrm{km~{}s}^{-1}}\right)^{1/2}$
(6)

with $v_{w}$ the wind velocity and $c_{s}$ the local sound speed.

To estimate mass loss rate required to explain the photosphere observed for
the progenitor candidate of SN 2019yvr, we assume that the wind could be as
fast as $100$ km s-1,as described in § 5.2 (a detailed analysis of the CSM
properties will be presented in Auchettl et al. (in prep.)). We also use our
constraint on the effective temperature of the observed photosphere
$T_{\mathrm{eff}}=6800$ K and radius $R=320~{}R_{\odot}$ above, which gives a
local sound speed $c_{s}\approx 7$ km s-1 and implies $\dot{M}>1.3\times
10^{-4}~{}M_{\odot}~{}\mathrm{yr}^{-1}$.

This mass-loss rate is extreme for a star with $\log(L/L_{\odot})=5.3$ (even
yellow hypergiants or low-luminosity LBVs as in Smith et al., 2004), which
tend to have time-averaged $\dot{M}\approx
10^{-5}~{}M_{\odot}~{}\mathrm{yr}^{-1}$ (Humphreys & Davidson, 1994). However,
the total mass of material needed to form a quasi-photosphere is only
$\dot{M}v_{w}^{-1}R_{\tau}\approx 10^{-5}~{}M_{\odot}$. This material can
easily be shed from the star’s remaining hydrogen/helium envelope over a brief
period of strong mass loss, although the timescale for such a shell to form
would be only $R_{\tau}/v_{w}=26$ days.

However, we do not observe any emission lines or “flash spectroscopy” features
in our early spectra of SN 2019yvr as may be expected if dense wind material
is present. This scenario would also require a significant change in the mass
loss from the SN 2019yvr progenitor timed $\lesssim$3 years prior to
explosion, which is closely aligned with predictions for instabilities during
oxygen burning (Meakin & Arnett, 2006; Arnett et al., 2014). However, there is
no other evidence for a dense wind that could form the observed photosphere.

####  5.3.2 A Radiation Dominated Envelope

An alternative is that the progenitor star retained a small amount of hydrogen
($<10^{-2}~{}M_{\odot}$ as required for a type Ib classification by Dessart et
al., 2012; Hachinger et al., 2012) with an envelope that has relaxed to a
steady-state configuration following some injection of additional energy.
Assuming that radiation pressure dominates over gas pressure, the radial
density profile in the wind will follow $r^{-3}$ (Loeb & Ulmer, 1997).
Hydrostatic equilibrium holds up to the radius at which the radiation is no
longer trapped, and so we associate the outer radius of this envelope with the
photospheric condition $\tau=1$ as

$R_{\tau}\approx 300\left(\frac{M_{\rm
env}}{10^{-4}M_{\odot}}\right)^{1/2}R_{\odot},$ (7)

where we assume Thompson opacity and a hydrogen envelope with a Solar mass
fraction. We note that an inflated progenitor could exist provided that the
mass contained in the envelope is $>10^{-4}M_{\sun}$, which is both
reconcilable with a type Ib classification for SN 2019yvr and in general is
less restrictive than the condition given by Equation 6.

However, even in this case several mysteries remain as to the nature of the
progenitor system. In particular, for a complete description of SN 2019yvr,
the progenitor system would still need to eject a shell of hydrogen-rich
material at $\sim$1000 AU. This will be be discussed in Section 5.4.

###  5.4 Progenitor Scenarios for SN 2019yvr

In the sections above, we have argued that reconciling the extended progenitor
radius and lack of hydrogen in the ejecta of SN2019yvr requires a system that
either:

  1. 1.

ejected the remainder of its hydrogen envelope in the final 2.6 yrs pre-
explosion (Section 5.2).

  2. 2.

undergoes a process not accounted for in standard models of evolution and that
leads to additional inflation or has sufficient material around the progenitor
to form a quasi-photosphere in the CSM (Section 5.3).

In addition, in either case, a viable progenitor scenario for SN 2019yvr must
also explain a shell of hydrogen-rich material observed at $\sim$1000 AU that
was likely ejected 50–100 years prior to explosion. Below we consider two
pathways that may be consistent within these constraints. We emphasize that
this is not an exclusive list of possible scenarios. No existing model
currently explains all the observed properties of SN 2019yvr and its
progenitor system.

####  5.4.1 A Luminous Blue Variable Phase

One evolutionary pathway that naturally explains many of the observables for
SN 2019yvr is a series of eruptions from a massive star in a LBV phase. These
eruptions are known to precede many SNe, some of which are thought to be core-
collapse explosions (as was argued for SN 2009ip Mauerhan et al., 2013),
although this interpretation of LBVs as a phase immediately preceding core
collapse remains controversial (Margutti et al., 2014).

Many types of progenitor systems exhibit pre-SN eruptions in the final years
to weeks before explosion, notably for the progenitors of SNe IIn (SNe with
narrow Balmer lines in their optical spectra, indicative of interaction
between ejecta and a dense mass of CSM; Smith, 2017). These systems must eject
from 0.1–10 $M_{\odot}$ over short timescales (Smith & McCray, 2007; Fox et
al., 2010, 2011) requiring an extreme mass-loss rate and variability on the
timescale of the eruptions. Their progenitor systems have also been observed
in the literature, notably for SN 2009ip whose progenitor star had an initial
mass $>$60 $M_{\odot}$ (Smith et al., 2009; Foley et al., 2011). Although
there is some ambiguity whether these events are actually terminal explosions
of massive stars (Mauerhan et al., 2013; Margutti et al., 2014), the eruptive
mechanism that fills their environments with dense shells of CSM may be more
common among stars at a wide range of initial masses. Indeed, so-called SN
impostors (lower luminosity, likely non-terminal explosions of massive stars
that resemble SNe IIn; Van Dyk et al., 2000; Maund et al., 2006; Pastorello et
al., 2010; Ofek et al., 2016) come from systems with initial masses possibly
as low as 20 $M_{\odot}$ (Kilpatrick et al., 2018a). Such an eruption for SN
2019yvr and SN 2014C-like events (hypothesized by Milisavljevic et al., 2015)
could explain the source of the CSM, although most SN IIn and SN impostor
progenitor stars retain some hydrogen leading to broad Balmer lines in their
spectra.

For SN 2019yvr, we would require a scenario analogous to SNe IIn and Ibn with
strong circumstellar interaction soon after explosion (Smith, 2017;
Hosseinzadeh et al., 2017) but involving multiple mass-loss episodes timed
years or even decades ahead of core collapse instead of months to years. The
main caveat in invoking this progenitor scenario is whether episodic mass
ejections can fully strip the progenitor star’s hydrogen envelope on the
timescale required by our HST observations. While LBV eruptions provide a
compelling mass-loss scenario as progenitor systems extend nearly to the
parameter space of the Hertzsprung-Russell diagram where we find the SN
2019yvr counterpart (see SN 2009ip, 2015bh, Gaia16cfr; Smith et al., 2009;
Mauerhan et al., 2013; Elias-Rosa et al., 2016; Thöne et al., 2017; Kilpatrick
et al., 2018a), these events tend to result in hydrogen-rich explosions with
long-lived Balmer emission. The final eruptions of SN 2019yvr would need to
strip a sufficiently small amount of hydrogen (potentially hundredths of a
Solar mass as in Section 4.5) that interaction with this material would not be
observed in early time spectra. Detailed analysis of the initial observations
of SN 2019yvr could place stronger constraints on whether this scenario
occurred.

####  5.4.2 A Common Envelope Mass Loss Scenario

An alternative evolutionary pathway in the final decades of evolution for the
SN 2019yvr, and put forward for SN 2014C in Margutti et al. (2017), is CEE
leading to ejection of the primary star’s hydrogen envelope $<$100 yr before
explosion. In general, this is not expected as RLOF and CEE are commonly
associated with mass transfer much earlier in the primary star’s life cycle,
for example, immediately after helium core contraction as discussed in Section
1. The progenitor would need to be inflated in a later evolutionary stage
after helium burning to restart mass transfer (i.e., through Case C mass
transfer, see Schneider et al., 2015). This is predicted to occur for only
$\sim$5$-$6% of binary system with primary masses between 8$-$20 M$\odot$
(Podsiadlowski et al., 1992).

However, if the primary star’s envelope can inflate during this phase, the
companion will spiral inward and start CEE, resulting in the ejection of a
significant fraction of the envelope over $<1$ yr (Ivanova et al., 2013). The
velocity of that material would be comparable to the escape velocity of the
primary, and so would likely be $<100$ km s-1 depending on the envelope
structure of the primary star at onset of CEE. In this way, the material could
survive long enough that the SN ejecta can encounter it within the first year
after core collapse, as observed at $\sim$150 days in SN 2019yvr.

The key question for SN 2019yvr is how the post-CEE system evolves in the
final years before core collapse such that it resembles the pre-explosion
counterpart and also explodes as a SN Ib. Case-C CEE is an appealing solution
to this problem as the remaining envelope is predicted to become unstable,
leading to dynamical pulsations and steady mass loss in the remaining years
before core collapse (especially for extremely luminous stars; Heger et al.,
1997). Thus a post-CEE star could still form a quasi-photosphere assuming the
mass-loss rate exceeded 10${}^{-4}~{}M_{\odot}~{}\mathrm{yr}^{-1}$ as in
Section 5.3. Alternatively, the inspiral during CE phase itself supplies a
source of heat in the stellar envelope. Thus, after the ejection of most of
its hydrogen envelope, the resulting post-CEE hydrogen-deficient envelope
could remain partly inflated. If the resulting star was dynamically unstable
and losing mass significantly faster than the $\approx
10^{-5}~{}M_{\odot}~{}\text{yr}^{-1}$ predicted for yellow hypergiants
(Humphreys & Davidson, 1994), it could rapidly shed even the
$10^{-2}$–$10^{-1}~{}M_{\odot}$ that is predicted to remain in the envelope of
comparable stars from BPASS. This would simultaneously explain the
circumstellar material as the result of CEE, the apparently cool photosphere,
and the lack of hydrogen in the primary star’s envelope at core collapse.

###  5.5 Comparisons to Progenitor Star Constraints for SN 2014C and SNe Ib
with Late-time Circumstellar Interaction

SNe Ib with late-time circumstellar interaction similar to SN 2019yvr are not
unprecedented and may in fact represent a large fraction of stripped-envelope
SNe overall. As discussed in Section 1, SN 2014C began interacting with its
circumstellar environment a few weeks after explosion (Margutti et al., 2017),
and both SN 2019yvr and SN 2014C have relatively deep progenitor constraints
considering both have pre-explosion HST imaging and occurred at $\approx$15
Mpc (Milisavljevic et al., 2015). More broadly, there are numerous examples of
stripped-envelope SNe with circumstellar interaction soon after explosion
(Chugai & Chevalier, 2006; Bietenholz et al., 2014; Chandra, 2018; Mauerhan et
al., 2018; Pooley et al., 2019; Sollerman et al., 2020; Tartaglia et al.,
2020), and so any mechanism that we invoke above may need to explain the
presence of CSM for a significant fraction of all core-collapse SNe (Margutti
et al., 2017).

A compact star cluster toward the position of SN 2014C in pre-explosion
imaging (Milisavljevic et al., 2015) had a best-fitting age in the range
$30$–$300$ Myr, although it could be as young as $10$ Myr. This age implies a
main-sequence turnoff mass of $3.5$–$9.5~{}M_{\odot}$ depending on the
metallicity of the cluster. Assuming this cluster hosted the SN 2014C
progenitor star and considering the fact that stars in this mass range either
do not explode as core-collapse SNe or are thought to lead to SNe II,
Milisavljevic et al. (2015) inferred that a more likely explosion scenario for
SN 2014C was through binary star channels for a star with a LBV-like phase and
$M_{\mathrm{ZAMS}}>$20 $M_{\odot}$ (implying that the cluster is younger than
its colours suggest). Given the photometric and spectroscopic similarity
between SNe 2014C and 2019yvr and the presence of a strong density gradient in
the circumstellar environments of both, it is possible that both systems could
be explained through eruptive, LBV-like mass loss or CEE as discussed above.

However, these evolutionary phases would require that the SN 2019yvr
progenitor star had extreme photometric variability, and the lack of any such
variability in the pre-explosion photometry indicates that these episodes
would have occurred outside the short window in which we constrain the
progenitor star’s evolution. Assuming that the mass-loss episode was driven by
an instability in late-stage nuclear burning (e.g., Arnett et al., 2014), the
star could also be inflated temporarily. Smith & Arnett (2014) suggest this
inflation would occur on a timescale comparable to the orbital period in
binary systems, but we see no signature of this variability on $30$–$100$ day
timescales. Deep, high-cadence limits, such as those from the Young Supernova
Experiment (Jones et al., 2019, 2020) or the upcoming Vera C. Rubin
Observatory Legacy Survey of Space and Time (down to r$\approx$24.5 mag on a
3–4 day cadence; Ivezić et al., 2019), will be able to detect or rule out
these mass-loss episodes for future events, potentially constraining the
progenitor scenarios for stripped-envelope SNe in the volume where such
variability is detectable.

##  6 Conclusions

We present pre-explosion imaging, photometry, and spectroscopy of the SN Ib
2019yvr. We find:

  1. 1.

SN 2019yvr was a SN Ib with a large line-of-sight extinction spectroscopically
similar to iPTF13bvn. SN 2019yvr exhibited signatures of interaction $150$
days after discovery consistent with a shock between SN ejecta and dense,
hydrogen-rich CSM (Auchettl et al. in prep.) and similar to SN 2014C
(Milisavljevic et al., 2015; Margutti et al., 2017). This interaction suggests
that the SN 2019yvr progenitor star underwent an eruptive mass-loss episode at
least 44 years before explosion.

  2. 2.

There is a single source in Hubble Space Telescope imaging obtained
$\approx$2.6 year before discovery and consistent with being the progenitor
system of SN Ib 2019yvr. Comparing to blackbodies, single-star SEDs, and
binary star models, we find that the SN 2019yvr progenitor star is consistent
with a star with $\log(L/L_{\odot})=5.3\pm 0.2$,
$T_{\mathrm{eff}}=6800\begin{subarray}{c}+400\\\ -200\end{subarray}$ K, and
thus close ($<$5 day period) binary-star models with initial masses around
$19~{}M_{\odot}$. However, the cool effective temperature and high luminosity
implies a remaining hydrogen envelope mass of at least $0.047~{}M_{\odot}$ in
the binary star model, which is inconsistent with the lack of hydrogen in
spectra of SN 2019yvr.

  3. 3.

Comparison to SN Ib progenitor candidates indicates that SN 2019yvr is much
cooler than what is predicted for a hydrogen-stripped star, much more similar
to the identified progenitors of type IIb SNe. Overall, the progenitor
candidate appears cool and inflated relative to the progenitor of iPTF13bvn
and helium stars.

  4. 4.

We infer that an extreme and episodic mass-loss scenario is required to
produce both a stripped-envelope SN progenitor system and the luminous, cool
progenitor candidate. The binary evolution scenarios discussed above do not
incorporate physical scenarios that can lead to extreme or eruptive mass loss
soon before explosion, and barring such a mass-loss scenario they do not
produce a star whose hydrogen envelope is consistent with the SN Ib
classification. We propose that LBV-like mass ejections or CEE provide natural
explanations for the stellar classification, the lack of a massive hydrogen
envelope, and the presence of dense CSM. We hypothesize that if this mass-loss
mechanism occurs, the star could have formed a quasi-photosphere from CSM in
its environment, requiring either a mass loss at a rate
$>1.3~{}M_{\odot}~{}\mathrm{yr}^{-1}$ or a radiation supported hydrogen
envelope with a mass $>10^{-4}M_{\odot}$ at 2.6 yr before core collapse.

ACKNOWLEDGMENTS

We thank J.J. Eldridge and H. Stevance for helpful comments about our BPASS
analysis, J. A. Vilchez, A. Campillay, Y. K. Riveros and N. Ulloa for help
with the Swope observations, as well as R. Carrasco for support of our
Gemini-S/GSAOI programme. C.D.K. acknowledges support through NASA grants in
support of Hubble Space Telescope programmes GO-15691 and AR-16136. M.R.D.
acknowledges support from the NSERC through grant RGPIN-2019-06186, the Canada
Research Chairs Program, the Canadian Institute for Advanced Research (CIFAR),
and the Dunlap Institute at the University of Toronto. K.A. is supported by
the Danish National Research Foundation (DNRF132) and VILLUM FONDEN
Investigator grant (project number 16599). Parts of this research were
supported by the Australian Research Council Centre of Excellence for All Sky
Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.
D.O.J. is supported by a Gordon and Betty Moore Foundation postdoctoral
fellowship at the University of California, Santa Cruz. Support for this work
was provided by NASA through the NASA Hubble Fellowship grant HF2-51462.001
awarded by the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., for NASA, under
contract NAS5-26555. The UCSC team is supported in part by NASA grant
NNG17PX03C, NASA grants in support of Hubble Space Telescope programmes
AR-14296 and GO-16239 through STScI, NSF grant AST-1815935, the Gordon & Betty
Moore Foundation, the Heising-Simons Foundation, and by a fellowship from the
David and Lucile Packard Foundation to R.J.F. J.H. was supported by a VILLUM
FONDEN Investigator grant (project number 16599). W.J-G is supported by the
National Science Foundation Graduate Research Fellowship Program under Grant
No. DGE-1842165. R.M. acknowledges support by the National Science Foundation
under Award No. AST-1909796. R.M. acknowledges support by the National Science
Foundation under Awards No. AST-1909796 and AST-1944985. R.M. is a CIFAR
Azrieli Global Scholar in the Gravity & the Extreme Universe Program, 2019 and
a Alfred P. Sloan Fellow in Physics, 2019. The Margutti team at Northwestern
is partially funded by the Heising-Simons Foundation under grant #2018-0911
(PI: Margutti). E.R.-R. is supported by the Heising-Simons Foundation, the
Danish National Research Foundation (DNRF132) and NSF. Based on observations
obtained at the Gemini Observatory, which is operated by the Association of
Universities for Research in Astronomy, Inc., under a cooperative agreement
with the NSF on behalf of the Gemini partnership: the National Science
Foundation (United States), National Research Council (Canada), CONICYT
(Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva
(Argentina), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Korea
Astronomy and Space Science Institute (Republic of Korea). The Gemini-S/GSAOI
observations in this paper were obtained under program GS-2020A-Q-221 (PI
Kilpatrick). Some of the data presented herein were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership among the
California Institute of Technology, the University of California and the
National Aeronautics and Space Administration. The Observatory was made
possible by the generous financial support of the W. M. Keck Foundation. The
authors wish to recognize and acknowledge the very significant cultural role
and reverence that the summit of Maunakea has always had within the indigenous
Hawaiian community. We are most fortunate to have the opportunity to conduct
observations from this mountain. This work makes use of observations from the
LCO network through program NOAO2019B-004 (PI Kilpatrick). Based on
observations made with the NASA/ESA Hubble Space Telescope, obtained from the
data archive at the Space Telescope Science Institute. STScI is operated by
the Association of Universities for Research in Astronomy, Inc. under NASA
contract NAS 5-26555. This work is based in part on observations made with the
Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory,
California Institute of Technology under a contract with NASA. (AST-1911206
and AST-1852393).

Facilities: Gemini (GSAOI), HST (WFC3), Keck (LRIS), LCO (FLOYDS, Sinistro),
Spitzer (IRAC), Swope (Direct/4K$\times$4K)

## Data Availability

The photometry in this article are presented in the article. Imaging and
spectroscopy data presented in this article are available upon request. The
Hubble Space Telescope and Spitzer Space Telescope data are publicly available
and can be accessed from the Mikulski Archive for Space Telescopes
(https://archive.stsci.edu/hst/) and Spitzer Heritage Archive
(http://sha.ipac.caltech.edu/applications/Spitzer/SHA/), respectively.

Table 4: SN 2019yvr Photometry MJD (Epoch) | Filter |  Magnitude (Uncertainty)11All magnitudes were calibrated using Pan-STARRS DR2 photometry standards following procedures described in Kilpatrick et al. (2018a) and Section 2.3. All $uBVgri$ magnitudes are on the AB system. All epochs are reported relative to $V$-band maximum. | Source
---|---|---|---
58849.333 (-4.947) | $B$ | 16.781 (0.012) | Swope
58850.355 (-3.925) | $B$ | 16.691 (0.011) | Swope
58851.322 (-2.958) | $B$ | 16.577 (0.013) | Swope
58852.308 (-1.972) | $B$ | 16.606 (0.014) | Swope
58853.365 (-0.915) | $B$ | 16.570 (0.014) | Swope
58869.334 (15.054) | $B$ | 17.772 (0.022) | Swope
58871.311 (17.031) | $B$ | 17.970 (0.020) | Swope
58872.330 (18.050) | $B$ | 18.053 (0.023) | Swope
58874.339 (20.059) | $B$ | 18.267 (0.047) | Swope
58876.301 (22.021) | $B$ | 18.234 (0.018) | Swope
58849.331 (-4.949) | $V$ | 15.822 (0.009) | Swope
58850.354 (-3.926) | $V$ | 15.701 (0.008) | Swope
58851.321 (-2.959) | $V$ | 15.647 (0.009) | Swope
58852.307 (-1.973) | $V$ | 15.648 (0.009) | Swope
58853.364 (-0.916) | $V$ | 15.565 (0.009) | Swope
58869.332 (15.052) | $V$ | 16.302 (0.011) | Swope
58871.309 (17.029) | $V$ | 16.440 (0.010) | Swope
58872.329 (18.049) | $V$ | 16.476 (0.011) | Swope
58874.338 (20.058) | $V$ | 16.679 (0.024) | Swope
58876.298 (22.018) | $V$ | 16.689 (0.009) | Swope
58846.307 (-7.973) | $g$ | 16.590 (0.014) | LCO
58847.304 (-6.976) | $g$ | 16.440 (0.013) | LCO
58848.301 (-5.979) | $g$ | 16.317 (0.012) | LCO
58849.326 (-4.954) | $g$ | 16.293 (0.009) | Swope
58850.296 (-3.984) | $g$ | 16.147 (0.021) | LCO
58850.351 (-3.929) | $g$ | 16.170 (0.008) | Swope
58851.317 (-2.963) | $g$ | 16.123 (0.009) | Swope
58852.302 (-1.978) | $g$ | 16.050 (0.007) | Swope
58852.338 (-1.942) | $g$ | 16.022 (0.011) | LCO
58853.359 (-0.921) | $g$ | 16.036 (0.008) | Swope
58853.420 (-0.860) | $g$ | 15.946 (0.012) | LCO
58854.506 (0.226) | $g$ | 16.011 (0.016) | LCO
58855.506 (1.226) | $g$ | 15.982 (0.010) | LCO
58856.668 (2.388) | $g$ | 15.902 (0.023) | LCO
58857.672 (3.392) | $g$ | 16.024 (0.032) | LCO
58859.020 (4.740) | $g$ | 16.130 (0.014) | LCO
58861.015 (6.735) | $g$ | 16.170 (0.026) | LCO
58862.012 (7.732) | $g$ | 16.226 (0.021) | LCO
58866.340 (12.060) | $g$ | 16.736 (0.014) | LCO
58867.422 (13.142) | $g$ | 16.817 (0.012) | LCO
58869.328 (15.048) | $g$ | 17.019 (0.011) | Swope
58869.636 (15.356) | $g$ | 16.993 (0.014) | LCO
58871.304 (17.024) | $g$ | 17.130 (0.013) | Swope
58872.325 (18.045) | $g$ | 17.204 (0.011) | Swope
58872.329 (18.049) | $g$ | 17.098 (0.015) | LCO
58874.327 (20.047) | $g$ | 17.371 (0.009) | Swope
58875.307 (21.027) | $g$ | 17.230 (0.015) | LCO
58876.287 (22.007) | $g$ | 17.447 (0.008) | Swope
58876.338 (22.058) | $g$ | 17.319 (0.015) | LCO
58877.425 (23.145) | $g$ | 17.340 (0.016) | LCO
58846.309 (-7.971) | $i$ | 15.551 (0.010) | LCO
58847.306 (-6.974) | $i$ | 15.427 (0.009) | LCO
58848.303 (-5.977) | $i$ | 15.321 (0.009) | LCO
58849.300 (-4.980) | $i$ | 15.192 (0.008) | LCO
58849.325 (-4.955) | $i$ | 15.181 (0.007) | Swope
58850.298 (-3.982) | $i$ | 15.133 (0.008) | LCO
58850.350 (-3.930) | $i$ | 15.028 (0.007) | Swope
58851.316 (-2.964) | $i$ | 14.977 (0.007) | Swope
58851.340 (-2.940) | $i$ | 15.063 (0.008) | LCO
58852.301 (-1.979) | $i$ | 14.942 (0.007) | Swope
58852.340 (-1.940) | $i$ | 15.009 (0.008) | LCO
58853.358 (-0.922) | $i$ | 14.878 (0.006) | Swope
58853.422 (-0.858) | $i$ | 14.858 (0.009) | LCO
58854.508 (0.228) | $i$ | 14.880 (0.013) | LCO
58855.508 (1.228) | $i$ | 14.839 (0.007) | LCO
58856.670 (2.390) | $i$ | 14.726 (0.013) | LCO
58857.674 (3.394) | $i$ | 14.789 (0.014) | LCO
58859.023 (4.743) | $i$ | 14.763 (0.008) | LCO
58859.681 (5.401) | $i$ | 14.726 (0.012) | LCO
58861.017 (6.737) | $i$ | 14.740 (0.013) | LCO
58862.015 (7.735) | $i$ | 14.783 (0.012) | LCO
58863.012 (8.732) | $i$ | 14.799 (0.012) | LCO
58866.342 (12.062) | $i$ | 15.077 (0.008) | LCO
58867.425 (13.145) | $i$ | 15.089 (0.008) | LCO
58869.327 (15.047) | $i$ | 15.086 (0.007) | Swope
58869.639 (15.359) | $i$ | 15.216 (0.008) | LCO
58871.258 (16.978) | $i$ | 15.265 (0.009) | LCO
58871.303 (17.023) | $i$ | 15.145 (0.007) | Swope
58872.324 (18.044) | $i$ | 15.166 (0.006) | Swope
58872.332 (18.052) | $i$ | 15.304 (0.008) | LCO
58873.331 (19.051) | $i$ | 15.353 (0.084) | LCO
58874.323 (20.043) | $i$ | 15.327 (0.054) | Swope
58874.328 (20.048) | $i$ | 15.342 (0.018) | LCO
58875.309 (21.029) | $i$ | 15.393 (0.008) | LCO
58876.341 (22.061) | $i$ | 15.473 (0.008) | LCO
58877.427 (23.148) | $i$ | 15.434 (0.008) | LCO
58878.613 (24.333) | $i$ | 15.518 (0.010) | LCO
58882.099 (27.819) | $i$ | 15.562 (0.008) | LCO
58846.308 (-7.972) | $r$ | 15.798 (0.010) | LCO
58847.305 (-6.975) | $r$ | 15.638 (0.009) | LCO
58848.302 (-5.978) | $r$ | 15.562 (0.008) | LCO
58849.299 (-4.981) | $r$ | 15.501 (0.010) | LCO
58849.323 (-4.957) | $r$ | 15.419 (0.007) | Swope
58850.297 (-3.983) | $r$ | 15.370 (0.009) | LCO
58850.349 (-3.931) | $r$ | 15.302 (0.006) | Swope
58851.314 (-2.966) | $r$ | 15.256 (0.007) | Swope
58851.339 (-2.941) | $r$ | 15.293 (0.009) | LCO
58852.299 (-1.981) | $r$ | 15.227 (0.007) | Swope
58852.339 (-1.941) | $r$ | 15.250 (0.009) | LCO
58853.357 (-0.923) | $r$ | 15.141 (0.007) | Swope
58853.421 (-0.859) | $r$ | 15.153 (0.009) | LCO
58854.507 (0.227) | $r$ | 15.174 (0.014) | LCO
58855.507 (1.227) | $r$ | 15.149 (0.008) | LCO
58856.669 (2.389) | $r$ | 15.106 (0.013) | LCO
58857.673 (3.393) | $r$ | 15.078 (0.017) | LCO
58859.021 (4.741) | $r$ | 15.119 (0.012) | LCO
58859.674 (5.394) | $r$ | 15.102 (0.013) | LCO
58861.016 (6.736) | $r$ | 15.197 (0.019) | LCO
58862.014 (7.734) | $r$ | 15.200 (0.013) | LCO
58866.341 (12.061) | $r$ | 15.567 (0.009) | LCO
58867.423 (13.143) | $r$ | 15.601 (0.008) | LCO
58869.326 (15.046) | $r$ | 15.664 (0.007) | Swope
58869.638 (15.358) | $r$ | 15.746 (0.009) | LCO
58871.301 (17.021) | $r$ | 15.748 (0.007) | Swope
58872.323 (18.043) | $r$ | 15.781 (0.007) | Swope
58872.331 (18.051) | $r$ | 15.840 (0.009) | LCO
58873.329 (19.049) | $r$ | 15.958 (0.087) | LCO
58874.321 (20.041) | $r$ | 15.893 (0.009) | Swope
58875.308 (21.028) | $r$ | 15.941 (0.008) | LCO
58876.340 (22.060) | $r$ | 16.056 (0.009) | LCO
58877.426 (23.146) | $r$ | 16.036 (0.008) | LCO
58878.612 (24.332) | $r$ | 16.078 (0.010) | LCO
58849.327 (-4.953) | $u$ | 18.413 (0.031) | Swope
58850.352 (-3.928) | $u$ | 18.454 (0.032) | Swope
58850.352 (-3.928) | $u$ | 18.455 (0.032) | Swope
58851.319 (-2.961) | $u$ | 18.569 (0.032) | Swope
58852.303 (-1.977) | $u$ | 18.622 (0.027) | Swope
58853.360 (-0.920) | $u$ | 18.743 (0.032) | Swope
58869.329 (15.049) | $u$ | 19.946 (0.186) | Swope
58874.329 (20.049) | $u$ | 20.772 (0.268) | Swope
58876.290 (22.010) | $u$ | 20.672 (0.252) | Swope

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