11institutetext: Department of Astronomy, The Oskar Klein Center, Stockholm
University, AlbaNova, 106 91 Stockholm, Sweden 22institutetext: Department of
Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl
St, 76100 Rehovot, Israel 33institutetext: Caltech/IPAC, Mailcode 100-22, 1200
E. California Blvd., Pasadena, CA 91125, USA 44institutetext: The Raymond and
Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel
Aviv, 69978 Israel 55institutetext: Department of Astronomy, University of
California, Berkeley, CA 94720-3411, USA 66institutetext: Cahill Center for
Astrophysics, California Institute of Technology, 1200 E. California Blvd.
Pasadena, CA 91125, USA 77institutetext: Miller Institute for Basic Research
in Science, University of California, Berkeley, CA 94720, USA 88institutetext:
Astrophysics Research Institute, Liverpool John Moores University, Liverpool
Science Park, 146 Brownlow Hill, Liverpool L35RF, UK 99institutetext: W. M.
Keck Observatory, Kamuela, HI 96743, USA 1010institutetext: School of Physics
and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
1111institutetext: University Claude Bernard Lyon 1, CNRS, IP2I Lyon / IN2P3,
IMR 5822, F-69622, Villeurbanne, France 1212institutetext: School of Physics,
University of New South Wales, NSW 2052, Australia 1313institutetext: Research
School of Astronomy and Astrophysics, Australian National University,
Canberra, ACT 2611, Australia

# Three Core-Collapse Supernovae with Nebular Hydrogen Emission.

J. Sollerman 11 S. Yang 11 S. Schulze 11 N. L. Strotjohann 22 A. Jerkstrand 11
S. D. Van Dyk 33 E. C. Kool 11 C. Barbarino 1144 T. G. Brink 55 R. Bruch 22 K.
De 66 A. V. Filippenko 5577 C. Fremling 66 K. C. Patra 55 D. Perley 88 L. Yan
66 Y. Yang 55 I. Andreoni 66 R. Campbell 99 M. Coughlin 1010 M. Kasliwal 66
Y.-L. Kim 1111 M. Rigault 1111 K. Shin 77 A. Tzanidakis 66 M. C. B. Ashley
1212 A. M. Moore 1313 T. Travouillon 1313

We present the discovery and extensive follow-up observations of SN 2020jfo, a
Type IIP supernova (SN) in the nearby (14.5 Mpc) galaxy M61. Optical light
curves (LCs) and spectra from the Zwicky Transient Facility (ZTF),
complemented with data from Swift/UVOT and near-infrared photometry is
presented. These are used to model the 350-day duration bolometric light
curve, which exhibits a relatively short ($\sim 65$ days) plateau. This
implies a moderate ejecta mass ($\sim 5$ $\mathrm{M}_{\odot}$) at the time of
explosion, whereas the deduced amount of ejected radioactive nickel is $\sim
0.025$ $\mathrm{M}_{\odot}$.

An extensive series of spectroscopy is presented, including
spectropolarimetric observations. The nebular spectra are dominated by
H$\alpha$ but also reveal emission lines from oxygen and calcium. Comparisons
to synthetic nebular spectra indicate an initial progenitor mass of $\sim 12$
$\mathrm{M}_{\odot}$. We also note the presence of stable nickel in the
nebular spectrum, and SN 2020jfo joins a small group of SNe that have inferred
super-solar Ni/Fe ratios.

Several years of pre-discovery data are examined, but no signs of pre-cursor
activity is found. Pre-explosion Hubble Space Telescope imaging reveals a
probable progenitor star, detected only in the reddest band ($M_{\rm
F814W}\approx-5.8$) and is fainter than expected for stars in the 10 – 15
$\mathrm{M}_{\odot}$range. There is thus some tension between the LC analysis,
the nebular spectral modeling and the pre-explosion imaging.

To compare and contrast, we present two additional core-collapse SNe monitored
by the ZTF, which also have nebular H$\alpha$-dominated spectra. This
illustrates how the absence or presence of interaction with circumstellar
material (CSM) affect both the LCs and in particular the nebular spectra. Type
II SN 2020amv has a LC powered by CSM interaction, in particular after $\sim
40$ days when the LC is bumpy and slowly evolving. The late-time spectra show
strong H$\alpha$ emission with a structure suggesting emission from a thin,
dense shell. The evolution of the complex three-horn line profile is
reminiscent of that observed for SN 1998S. Finally, SN 2020jfv has a poorly
constrained early-time LC, but is of interest because of the transition from a
hydrogen-poor Type IIb to a Type IIn, where the nebular spectrum after the
light-curve rebrightening is dominated by H$\alpha$, although with an
intermediate line width.

###### Key Words.:

supernovae: general – supernovae: individual: ZTF20aaynrrh, SN 2020jfo,
ZTF20aahbamv, SN 2020amv, ZTF20abgbuly, SN 2020jfv, SN 1993J, SN 1998S.
galaxy: individual: M61

##  1 Introduction

Core-collapse (CC) supernovae (SNe) are explosions of massive stars ($\gtrsim
8\leavevmode\nobreak\ M_{\odot}$) ending their stellar lives. The variety of
CC SNe is determined primarily by the progenitor mass at the time of CC, but
also by the mass-loss history leading up to the explosion. The most common
category is the hydrogen-rich class; Type II SNe. Hydrogen-poor CC SNe also
originate from massive progenitor stars, but stars that have lost most — or
even all — of their H envelopes prior to explosion; Type IIb SNe (some H
left), SNe Ib (no H, some He), and SNe Ic (neither H nor He); see Filippenko
(1997) and Gal-Yam (2017) for reviews. There are few observational constraints
on mass loss for massive stars, and the processes involved are poorly
understood, but it is well established that the interaction between the ejecta
and the circumstellar material (CSM) can make a significant contribution to
the total SN luminosity (e.g., Chevalier & Fransson 2017).

In this paper, the primary aim is to present the discovery and follow-up
observations of a particularly nearby supernova, the relatively normal Type II
SN 2020jfo in the grand-design spiral galaxy M61, only 14.5 Mpc away. We
present the discovery by the Zwicky Transient Facility (ZTF) of this transient
and the optical light curves (LCs), as well as a spectral sequence covering
the first 350 days of its evolution. For comparison, we include two additional
ZTF SNe that (like SN 2020jfo) are dominated by strong H$\alpha$ emission in
their nebular spectra. The Type II SN 2020amv has a LC dominated by CSM
interaction, which also reveals itself in the complex line profiles seen in
the nebular spectrum. SN 2020jfv was originally classified as a Type IIb SN
(i.e. a relatively hydrogen-poor transient). However, the LC at later times
starts rebrightening and the nebular spectrum is also dominated by H$\alpha$,
with a distinct emission-line profile. These three objects from the ZTF survey
are thus used to exemplify the appearance of the nebular Balmer lines, and how
these are connected to different LC shapes and powering mechanisms.

The paper is organised as follows. In Sect. 2 we present the observations,
including optical photometry and spectroscopy as well as near-infrared (NIR)
photometry and near-ultraviolet (UV) space-based data. Section 3 presents the
similarities and differences between the objects, and Sect. 4 summarises our
conclusions and discusses our observations in context with other SNe.

##  2 Observations and Reductions

###  2.1 Detection and classification

####  2.1.1 SN 2020jfo

SN 2020jfo (also known as ZTF20aaynrrh) was discovered on 2020 May 6 (UT dates
are used throughout this paper; first detection on $\mathrm{JD}=2458975.70$),
with the Palomar Schmidt 48-inch (P48) Samuel Oschin telescope as part of the
ZTF survey (Bellm et al. 2019; Graham et al. 2019). It was reported to the
Transient Name Server (TNS111https://wis-tns.weizmann.ac.il) on the same day
(Nordin et al. 2020b), less than 2 hours after first detection. The first
detection is in the $r$ band, with a host-subtracted magnitude of $16.01\pm
0.04$, at the J2000.0 coordinates $\alpha=12^{h}21^{m}50.48^{s}$,
$\delta=+04\degr 28\arcmin 54.1\arcsec$. The first report also mentions that
the last upper limit ($g>19.7$ mag) from ZTF was on May 2, 4 days before
discovery. Following Bruch et al. (2021), we perform a power-law fit to the
early-phase $g$ and $r$ light curves, and obtain an estimated explosion date
of $\mathrm{JD_{explosion}^{SN2020jfo}}=2458975.20$. A conservative
uncertainty is ${}^{+0.5}_{-3.5}$ days as provided by the first detection and
the last nondetection. We use this as the explosion date throughout the paper,
and measure the phases in rest-frame days with respect to it. This transient
was subsequently also reported to the TNS by several other surveys (for
example, by ATLAS and PS2 in May and by Gaia and MASTER in June), but was also
bright enough to be followed by many amateur astronomers around the
globe222https://www.rochesterastronomy.org/sn2020/sn2020jfo.html.

SN 2020jfo is positioned in the spiral galaxy M61 (NGC 4303), which has a
redshift of $z=0.00522$. It lies in the Virgo cluster and has hosted 7 known
SNe before SN 2020jfo, the most recent being the Type Iax SN 2014dt (e.g.,
Kawabata et al. 2018). As is often the case, the distance of this nearby host
is relatively uncertain. We follow Kawabata et al. (2018) and adopt a distance
modulus of $30.81\pm 0.20$ mag, which is 14.5 Mpc. The implications for the
uncertainty in the distance is discussed in Sect. 3.6. The position of SN
2020jfo in M61 is shown in Fig. 1.

Figure 1: SN 2020jfo in M61. The $r$-band image subtraction is shown in the
top panels, with the SN image observed on 2020 May 6 on the left; this is the
ZTF discovery frame. The SN position is marked. The middle top panel is the
reference image (the template), and the right panel shows the SN standing out
in the image subtraction. The bottom panel displays a $gri$-colour composite
image of the host galaxy and its environment. It was composed of ZTF $g$, $r$,
and $i$ images of the field observed on 2020 December 21, $\sim 7$ months
after the first ZTF detection. The SN is still visible and marked by the red
cross.

The ZTF on-duty astronomer (J.S.) who first noticed the SN immediately
triggered the robotic Palomar 60-inch telescope (P60; Cenko et al. 2006)
equipped with the Spectral Energy Distribution Machine (SEDM; Blagorodnova et
al. 2018). Unfortunately, the observations could not be scheduled the same
night, so we instead triggered observations from La Palma. We thus classified
SN 2020jfo (Perley et al. 2020b) as a Type II SN based on a spectrum obtained
on 2020 May 6 with the Liverpool telescope (LT) equipped with the SPectrograph
for the Rapid Acquisition of Transients (SPRAT), as well as on a spectrum from
the Nordic Optical Telescope (NOT) using the Alhambra Faint Object
Spectrograph (ALFOSC). These spectra were obtained 17.27 and 17.55 hr after
the first ZTF detection. The first SEDM spectrum came in a few hours
thereafter, and corroborated the classification (Perley et al. 2020a).

####  2.1.2 SN 2020amv

Our first ZTF photometry of SN 2020amv (ZTF20aahbamv) was obtained on 2020
January 23 ($\mathrm{JD}=2458871.72$) with the P48. It was saved by the on-
duty astronomer (J.S.) to the GROWTH Marshal (Kasliwal et al. 2019). The first
detection was in the $g$ band, with a host-subtracted magnitude of $18.68\pm
0.08$, at $\alpha=08^{h}49^{m}40.68^{s}$, $\delta=+30\degr 11\arcmin
14.5\arcsec$ (J2000.0). The source was reported to TNS on the same day (Nordin
et al. 2020a), with a note saying that the latest nondetection from ZTF was 4
days prior to discovery. This was also a ZTF discovery, but ATLAS reported the
same object just a few hours later. We include the forced photometry LCs from
ATLAS (Tonry et al. 2018; Smith et al. 2020) when available, for completeness.
With power-law fits to the early $g$ and $r$ data, we set the explosion date
as $\mathrm{JD_{explosion}^{SN2020amv}}=2458871.22\pm 0.29$.

The transient was classified as a Type II SN by ePESSTO+ (Smartt et al. 2015)
using a spectrum from February 2 (Irani et al. 2020), and the SEDM spectrum
from the Bright Transient Survey (Fremling et al. 2020) was also made public
on TNS (Dahiwale & Fremling 2020a). SN 2020amv was observationally well
covered at early phases since it showed narrow features that could indicate
CSM interaction (“flash features”; see, e.g., Gal-Yam et al. 2014; Bruch et
al. 2021). The host galaxy has $z=0.0452$, and using a flat $\Lambda$CDM
cosmology with $\Omega_{\rm m}=0.3$ and H${}_{0}=70$ km s-1 Mpc-1 this
corresponds to 200 Mpc (distance modulus 36.5 mag).

####  2.1.3 SN 2020jfv

SN 2020jfv (ZTF20abgbuly) was first reported to TNS by ATLAS (Tonry et al.
2020), with a detection on 2020 May 5. Gaia actually detected the transient
earlier (on April 30), and claimed a nondetection the previous night. With ZTF
the first observations were obtained later (June 18), when the object was
already clearly declining. For SN 2020jfv we adopt 2020 April 30
($\mathrm{JD_{explosion}^{SN2020jfv}}=2458969.51$) as both the discovery date
and the explosion date, but note that there is quite some uncertainty in the
actual date of explosion. The object is positioned at
$\alpha=23^{h}06^{m}35.75^{s}$, $\delta=+00\degr 36\arcmin 43.7\arcsec$
(J2000.0), not far from the centre of the face-on spiral galaxy WISEA
J230635.97+003641.9. This galaxy had no previously reported redshift, and our
estimate of $z=0.017$ comes from the measured host-galaxy narrow emission
lines in our late-time nebular spectra. The distance is thus estimated to be
73.8 Mpc adopting the same cosmology as provided above. The SN was classified
as Type IIb based on an SEDM spectrum obtained on June 20 (Dahiwale & Fremling
2020b). It continued to fade in a linear fashion for the next 100 days, but it
thereafter began rebrightening, especially in the $r$ band. Late-time spectra
demonstrate that this is due to CSM interaction driving a conspicuous nebular
H$\alpha$ line. The transformation from a stripped-envelope SN to a CSM
interacting Type IIn is reminiscent of the evolution discussed by e.g.,
Sollerman et al. (2020); Milisavljevic et al. (2015); Mauerhan et al. (2018);
Prentice et al. (2020); Chandra et al. (2020); Tartaglia et al. (2021). Some
of the data for these three SNe and their host galaxies is summarised in Table
1.

###  2.2 Optical photometry

Following the discoveries as outlined above, ZTF obtained regular follow-up
photometry in the $g$, $r$, and sometimes $i$ bands with the ZTF camera
(Dekany et al. 2020) on the P48. Additional images were obtained with the LT
and with the P60. Some late-time photometry was also obtained with ALFOSC on
the NOT. P48 LCs come from the ZTF pipeline (Masci et al. 2019), where we have
also applied forced photometry (see, e.g., Yao et al. 2019). Photometry from
the P60 and LT was produced with the image-subtraction pipeline described by
Fremling et al. (2016), with template images from the Sloan Digital Sky Survey
(SDSS; Ahn et al. 2014). This pipeline produces point-spread-function (PSF)
magnitudes, calibrated against SDSS stars in the field. The NOT photometry was
done using template subtractions performed with
hotpants333http://www.astro.washington.edu/users/becker/v2.0/hotpants.html,
using archival SDSS images as templates. The magnitudes of the transient were
measured using SNOoPY444SNOoPy is a package for SN photometry using PSF
fitting and template subtraction developed by E. Cappellaro. A package
description can be found at http://sngroup.oapd.inaf.it/snoopy.html. and
calibrated against SDSS stars in the field. All magnitudes are reported in the
AB system (Oke & Gunn 1983). For completeness and comparison, we have also
included data from the forced photometry service from ATLAS. Those data points
are included in the figures, but are generally not part of the analysis and
measurements.

In our analysis we have corrected all photometry for Galactic extinction,
using the Milky Way (MW) colour excess $E(B-V)_{\mathrm{MW}}$ toward the
position of the SNe, as provided in Table 1. All reddening corrections are
applied using the Cardelli et al. (1989) extinction law with $R_{V}=3.1$. No
further host-galaxy extinction has been applied. We discuss the effects of
this assumption in Sect. 3.6. All the photometry is given in Tables 2, 3 and
4. The three light curves are shown in Fig. 2.

Figure 2: Light curves of the three supernovae. Left: SN 2020jfo in multiple
bands with symbols and offsets as provided in the legend to the very right. In
this case it includes six passbands from Swift and three from ZTF and LT, as
well as near-infrared $J$-band data. These are observed (AB) magnitudes
plotted versus rest-frame time in days since explosion. For the $J$-band data,
the Vega/AB magnitude conversion follows Blanton & Roweis (2007). ATLAS forced
photometry in the $c$ and $o$ bands is also included, and the ZTF and ATLAS
data are in 3 day bins. The arrows on top indicate the epochs of spectroscopy,
and the lines with error regions are Gaussian Process estimates of the
interpolated LCs (for $g$ band). The middle and right panels show the LCs for
SN 2020amv and SN 2020jfv, respectively.

###  2.3 Near-infrared photometry

SN 2020jfo was observed in the NIR $J$ band as a part of regular survey
operations of the Palomar Gattini-IR (PGIR) survey. PGIR is a wide-field NIR
survey at Palomar Observatory, observing the entire visible sky at a median
cadence of $\sim 2$ days, and to a median 5$\sigma$ depth of $J\approx 15.7$
AB mag (Moore & Kasliwal 2019; De et al. 2020b). The transient was clearly
detected in the PGIR data for $\sim 30$ days after discovery. We derived
$J$-band photometry (calibrated to the 2MASS catalogue in the Vega system) of
SN 2020jfo by performing forced PSF photometry at the location of the
transient on the PGIR difference images, using the method described by De et
al. (2020a). The photometry is given in Table 5 and included in Fig. 2.

###  2.4 Swift observations

####  2.4.1 UVOT photometry

For SN 2020jfo, we also have observations in the ultraviolet (UV) and optical
from the UV Optical Telescope onboard the Neil Gehrels Swift observatory
(UVOT; Gehrels et al. 2004; Roming et al. 2005). As shown in Table 6, 25
epochs were obtained over the first 300 days. Our first Swift/UVOT observation
was performed on 2020 May 7 (just 1.4 days after estimated explosion), and
provided detections in all bands.

The brightness in the UVOT filters was measured with UVOT-specific tools in
HEAsoft555https://heasarc.gsfc.nasa.gov/docs/software/heasoft/ version 6.26.1.
Source counts were extracted from the images using a circular aperture with a
radius of $3^{\prime\prime}$. The background was estimated using a
significantly larger circular region. The count rates were measured from the
images using the Swift tool uvotsource. They were converted to magnitudes
using the UVOT photometric zero points (Breeveld et al. 2011) and the latest
calibration files from 2020 September. To remove the host-galaxy contribution,
we used observations from before the SN explosion. We measured the flux at the
SN site using the same source and background apertures and arithmetically
subtracted the host contribution from the SN photometry. All magnitudes were
transformed into the AB system using Breeveld et al. (2011). These
measurements are included in Fig. 2.

####  2.4.2 X-rays

The field of SN 2020jfo was observed with Swift’s onboard X-ray telescope
(XRT; Burrows et al. 2005) in photon-counting mode several times between 2020
May 7 and 2021 January 30. Swift also observed this field between 2008 and
2016, long before the SN explosion. We analysed all data with the online tools
of the UK Swift team666https://www.swift.ac.uk/user_objects/ that use the
methods described by Evans et al. (2007) and Evans et al. (2009) and the
software package HEAsoft.

We detect emission at the SN position in the 2008–2016 datasets. To recover
any SN flux in the later data, we numerically subtracted that baseline flux,
but this did not result in any significant detections ($<1.4\sigma$). To
convert count-rate limits to flux, we use
WebPIMMS777https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl,
assume a power law with a photon index of 2 and a Galactic equivalent neutral-
hydrogen column density of $1.58\times 10^{20}\leavevmode\nobreak\ {\rm
cm}^{-2}$. We conclude that any X-ray emission from SN 2020jfo must be fainter
than a few $10^{39}\,{\rm erg\leavevmode\nobreak\ s}^{-1}$.

###  2.5 Pre-explosion imaging

####  2.5.1 Progenitor

The explosion in a nearby Messier galaxy could allow for investigation of the
site of the progenitor star. Fortunately, the site of SN 2020jfo was
serendipitously imaged prior to explosion with the Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS)/Wide Field Channel (WFC) in bands
F435W and F814W (1090 s total exposure time in each band) on 2012 May
24888GO-12574; PI D. Leonard and in F814W (2112 s) on 2019 April 7999GO-15645;
PI D. Sand, as well as with the Wide Field Camera 3 (WFC3)/UVIS channel on
2020 March 29101010GO-15654; PI J. Lee in F275W (2190 s), F336W (1110 s),
F438W (1050 s), F555W (670 s), and F814W (803 s). The SN site is also located
in archival HST Wide-Field Planetary Camera 2 (WFPC2) data, but owing to the
comparatively inferior spatial resolution and sensitivity of the WFPC2 data,
we do not consider them further.

To locate the precise position of the SN in the archival HST datasets, we
obtained high-spatial-resolution images in the $K^{\prime}$ band with the
adaptive optics (AO)-assisted OSIRIS Imager (Larkin et al. 2006) on the Keck-I
10 m telescope on 2020 July 7. The SN field required laser-guide-star AO and
short-turnaround clearance for satellite avoidance. There were 10 frames added
together, with each coadd consisting of 8 dithered images of duration 14.75 s,
so the total integration time was 1180 s.

We astrometrically registered the Keck AO image to the 2012 ACS/WFC F814W
image using 32 stars in common between the two image datasets, employing the
task geomap in PyRAF with parameter “calctype” set to “double.” The geometric
distortion for the OSIRIS AO imager is quite small (${<}5$ milliarcsec), so a
distortion correction is not strictly required111111J. Lu, private
communication. We were able to achieve a satisfactory solution to a precision
of 0.15 WFC pixel (7.5 milliarcsec). Figure 3 shows the Keck and ACS images in
a broader view, as well as a zoom-in on the SN site. With regards to a version
of the 2012 F814W image mosaic available from the Hubble Legacy
Archive121212http://hla.stsci.edu/, from the position of the SN in the OSIRIS
image, and referencing our astrometric solution, we expect the precise SN
location at pixel (2110.05, 3062.02). In the image mosaic a faint object can
be seen with a centroid position of (2110.06, 3061.93). The difference between
these two pixel values is within the astrometric uncertainty, and we therefore
consider the object to very likely be the candidate for the SN progenitor.

In order to determine the brightness of this object from the archival HST
data, we extracted photometry from the individual frames for both the ACS and
WFC3 observations with Dolphot (Dolphin 2016). We used recommended parameters
for Dolphot appropriate for a crowded extragalactic environment131313FitSky=3
and RAper=8. The object is only detected in F814W. We measured F814W =
$25.02\pm 0.07$ mag in 2021, and $25.56\pm 0.09$ and $25.08\pm 0.13$ mag in
2019 and 2020, respectively. Upper limits were placed in the other bands;
these are all at the 5$\sigma$ level. The detection limit in 2012 in F435W is
26.8. The limits in 2020 are 25.1, 25.2, 26.5, and 26.8 mag in F275W, F336W,
F438W, and F555W, respectively. The lack of detection of the star in bands
bluer than F814W indicates that this is a cool, red star.

The detected star, given the assumed distance modulus and accounting only for
MW extinction, has absolute magnitudes of $M_{\rm F814W}\approx-5.8$, $M_{\rm
F275W}>-5.8$, $M_{\rm F336W}>-5.7$, $M_{\rm F438W}>-4.4$, and $M_{\rm
F555W}>-4.1$. For red supergiants (RSGs) near the terminus of their
evolutionary tracks, from the BPASS solar-metallicity single-star models
(Stanway & Eldridge 2018) at 10–15 $\mathrm{M}_{\odot}$, we would expect
$M_{\rm F814W}\approx-7$ to $-8$ mag. This implies that the progenitor may
have been either more compact or further extinguished by $A_{\rm F814W}\gtrsim
1$ mag, potentially from circumstellar dust.

We also obtained image data in F555W and F814W on 2021 July 28 with the HST
WFC3/UVIS141414GO-16179 PI: A. Filippenko. The SN is still prominent in the
images (Fig. 3), and we confirm, with an astrometric 1$\sigma$ uncertainty of
0.15 UVIS pixel (6 milliarcsec), the identification of the progenitor
candidate, based on the AO image.

####  2.5.2 Precursor study

The ZTF survey first started to monitor the position where SN 2020jfo exploded
more than 800 days (2.4 yr) before the explosion date. We obtained in total
300 pre-explosion observations during 158 different nights. No precursors were
detected when searching the unbinned or binned data following the methods
described by Strotjohann et al. (2021). The median limiting magnitude is $-11$
in the $r$ band and brighter precursors can be excluded in 31 weeks or 25% of
the time, while outbursts as bright as magnitude $-12$ can be ruled out 49% of
the time within the last 2.4 yr before the SN explosion. The SN location was
also regularly observed by the Palomar Transient Factory (PTF) and the
intermediate PTF (iPTF) surveys; 988 observations were obtained spanning 11.1
to 4 yr before the SN explosion. We can rule out week-long precursors that are
brighter than magnitude $-11$ in the Mould $R$ band in 102 weeks or 28% of the
time. The upper panel of Fig. 4 shows the limiting magnitudes for week-long
bins.

Figure 3: Top left: Adaptive optics (AO) image of SN 2020jfo obtained with the
OSIRIS imager on the Keck-I 10 m telescope in the $K^{\prime}$ band on 2020
July 7. Top right: Portion of an archival HST ACS/WFC image mosaic obtained in
F814W on 2012 May 24, which contains the SN site, shown at the same scale and
orientation as the top-left panel. In both top panels, the 32 stars in common
between the two images and used in the astrometric registration are circled.
Bottom left: A zoom-in on the SN site in the HST image, with the SN progenitor
candidate indicated by tick marks. Bottom right: Zoom-in on the SN site after
the explosion, in a HST WFC3/UVIS F814W image from 2021 July 28. The SN is
clearly identified, confirming the progenitor candidate identification based
on the AO image. North is up and east is to the left in all panels.

Figure 4: Pre-explosion light curves from both (i)PTF and ZTF of the three SNe
in 7-day bins. No firm detections were obtained in the 11 yr prior to
explosion, and the limits are discussed in the text.

A similar precursor search for SN 2020amv also did not reveal any pre-
explosion activity, as shown in the middle panel of Fig. 4, but the limits are
less constraining owing to the larger SN distance. The median limiting
magnitude is $-15.7$ in the $r$ band and such a bright outburst can be
excluded in 30 weeks or 25% of the time. The detected flash-spectroscopy
features in the early-time spectra of SN 2020amv indicate that the progenitor
star lost material shortly before the explosion, but apparently this mass-loss
event was not associated with any optical outburst that was bright enough to
be detected in this search. This is consistent with Strotjohann et al. (2021),
who observed no pre-explosion outbursts prior to 20 SNe with flash-
spectroscopy features, including SN 2020amv, even though several of them were
located at small redshifts of $z<0.02$. According to Strotjohann et al.
(2021), this indicates that these flash-spectroscopy SNe likely have fewer or
fainter outbursts than Type IIn SNe, but their sample was too small to
constrain the precursor rate further. SN 2020amv reveals, as described below,
both early flash features, but also later evidence for CSM interaction.

For SN 2020jfv, a single bin surpasses the formal $5\sigma$ threshold of the
precursor search. The detection occurs 1.8 yr before the estimated explosion
date in the $r$ band and is seen when combining data in 7-day or 30-day bins.
However, a more detailed inspection shows that this is likely a false
detection. No point source is seen when coadding the $3$ difference images in
the bin, and the $g$-band images in the same nights yield a significantly
deeper limiting magnitude. The median limiting magnitude is close to $-14$ in
the $r$ band and we can exclude such a bright precursor in 29 weeks during the
final 2 yr before the SN explosion (28% of the time), assuming that the
mentioned single detected flux excess is not real. We also analyzed 158
PTF/iPTF observations and can exclude precursors brighter than magnitude $-14$
in 31 weeks.

###  2.6 Optical spectroscopy

Follow-up spectroscopy of SN 2020jfo was primarily conducted with robotic
telescopes, most of them with the SEDM mounted on the P60, but also with SPRAT
on the LT. Further spectra were obtained with the NOT using ALFOSC. This
included the above-mentioned classification spectrum for SN 2020jfo, but also
later nebular-phase spectra. We also obtained spectra with the Lick 3 m Shane
telescope equipped with the Kast spectrograph. The full log of spectra is
provided in Table 7. As can be seen in the table, P60+SEDM was also
instrumental in providing spectra for the other two SNe. For the three SNe, 42
spectra in total where obtained. Additional spectra in this paper come from
Gemini-North equipped with GMOS, the Palomar P200 telescope and DBSP (Oke &
Gunn 1982), as well as deep nebular spectra taken with the Keck-I telescope
using the Low Resolution Imaging Spectrograph (LRIS; Oke et al. 1995).

The LPipe reduction pipeline (Perley 2019) was used to process the LRIS data.
SEDM spectra were reduced using the pipeline described by Rigault et al.
(2019), and the spectra from La Palma were reduced using standard pipelines
and procedures for each telescope and instrument. The ALFOSC spectra were
often reduced using PypeIt (Prochaska et al. 2020). All spectral data and
corresponding information will be made available via
WISeREP151515https://wiserep.weizmann.ac.il (Yaron & Gal-Yam 2012). All
spectra have been calibrated on an absolute scale using contemporaneous (or
interpolated) photometry.

####  2.6.1 Spectropolarimetry

We obtained two epochs of spectropolarimetry of SN 2020jfo on the nights of
2020 May 25 and May 29 (19.7 and 23.7 rest-frame days past explosion) using
the polarimetry mode of the Kast spectrograph on the Lick 3 m Shane telescope.
On each night, low-polarization and high-polarization standard stars were
observed to calibrate the data. Observations and data reduction were carried
out as in Patra et al. (in prep.).

Linear polarization is calculated from the Stokes $Q$ and $U$ parameters as
$P=\sqrt{Q^{2}+U^{2}}$, and the polarization position angle (PA) on the sky is
defined as $\theta=1/2\arctan(U/Q)$. $P$ is a positive-definitive quantity and
therefore overestimated when the signal-to-noise ratio (S/N) is low. We debias
the polarization as

$P_{db}=P-\left(\frac{\sigma^{2}_{P}}{P}\times
h(P-\sigma_{P})\right)\leavevmode\nobreak\ \mbox{and}\leavevmode\nobreak\
\leavevmode\nobreak\ \theta_{db}=\theta,$ (1)

where $\sigma_{P}$ is the $1\sigma$ uncertainty in $P$ and $h$ is the
Heaviside step function.

We measured the average Stokes $Q$ and $U$ of the low-polarization star HD
110897 to $<0.05\%$, demonstrating the low instrumental polarization. We also
find low interstellar polarization (ISP) in the direction of SN 2020jfo.
Serkowski et al. (1975) showed that an upper limit on ISP owing to dichroic
extinction by dust grains can be derived as $9\times E(B-V)$. In the direction
of SN 2020jfo, the estimated $E(B-V)=0.02$ mag implies that ISP $<0.18$%. We
confirm that the Galactic ISP is low by measuring the polarization of an ISP
”probe-star”161616We observed the star Gaia ID 3894181087039808128., an
unpolarized star close to the line of sight to SN 2020jfo. We found the
polarization of the probe star to be $<0.15$%. Furthermore, the emission peak
of the H$\alpha$ feature of a SN is expected to be depolarized due to
contamination by unpolarized flux diluting the underlying polarized continuum
flux. The minimum observed polarization in the H$\alpha$ feature also
constrains the ISP to be $<0.2$%. Taken together, the three lines of evidence
all show that the ISP in the direction of SN 2020jfo is low.

The continuum polarization, which reflects the global ejecta asymmetry, is
0.4-0.7% over the two epochs, typical of SNe II while on the plateau (see Wang
& Wheeler 2008, and references therein).

The polarization position angle hovers around a mean of $\sim 100\degr$ over
the two epochs. The lack of significant change in PA could imply a global axis
of symmetry, although the data do not have sufficiently long temporal coverage
to confirm this. The Ca II NIR feature is significantly polarized, with levels
exceeding 1% at both epochs (see Fig. 5). The high line polarization suggests
that Ca is not uniformly distributed within the ejecta and likely exists in
clumps. The Ca II line polarization also shows velocity-dependent variation,
with the high-velocity feature more polarized than the normal-velocity
feature.

Figure 5:  Spectropolarimetry of SN 2020jfo on 2020 May 25 and May 29. The
panels (from top to bottom) show total flux, Stokes $Q$, Stokes $U$, debiased
polarization, and the position angle. The vertical bands represent the regions
of potential telluric overcorrection. The data, except for total flux, are
binned to 50 Å to improve the S/N.

##  3 Results and Discussion

###  3.1 Light curves

The LCs of our three SNe are displayed in Fig. 2. We first focus on the LC of
SN 2020jfo.

####  3.1.1 SN 2020jfo

As mentioned above, we have fairly good constraints on the explosion epoch
with an uncertainty of $\pm 2$ days simply based on the last nondetection.
Therefore, we can also measure the rise time with some precision. We used a
Gaussian Processing (GP) algorithm171717https://george.readthedocs.io with a
Matern32 kernel. to interpolate the photometric data and measure the rise time
and the peak magnitudes. The results are provided in Table 8. The SN rises to
maximum brightness in slightly less than 5 days in both $g$ and $r$. The peak
magnitude of $m_{r}^{\rm{peak}}=14.4$ made SN 2020jfo one of the brighter CC
SNe during 2020.

After the initial rise follows a plateau phase in the $r$ band of $\sim 60$\+
days, which establish SN 2020jfo as a Type IIP SN. The $i$ band is well
sampled the first 40+ days, when it follows the plateau, whereas the $g$ band
declines faster. The LCs cover the first 60 days well, whereafter the SN
position was too close to the Sun in the sky. It was recovered after solar
conjunction in $g$ and $r$, declining linearly up to $\sim 350$ days. The
forced-photometry ATLAS LCs confirm the Type IIP classification and the
general shape of the LC. In fact, the fall off the plateau is best seen in the
$o$ band, which indicates a plateau length of $64\pm 3$ days181818As done by
Anderson et al. (2014), we fit $o$-band data using a $\chi^{2}$ minimising
procedure with a composite function of a Gaussian, Fermi-Dirac, and a straight
line, following Olivares E. et al. (2010).. In comparison with the large SN II
sample of Anderson et al. (2014), this is actually one of the shortest plateau
lengths. We follow the SN up to a year after explosion.

The colour evolution in $g-r$ for SN 2020jfo and also for the Type II SN
2020amv initially become redder for the first 60 days, while SN 2020jfo was on
the plateau. At about 200 days past explosion all our three SNe display
$g-r\approx 1.0$ mag. SN 2013ej (Valenti et al. 2014; Yuan et al. 2016), which
is known to experience little host-galaxy extinction, show a similar initial
reddening. Since our three SNe are bluer than SN 2013ej, this at least
conforms with our omission of extra host extinction corrections.

In Fig. 6 we show the LCs in absolute magnitudes ($M_{r}$) together with the
LCs of a few other SNe II. The magnitudes in Fig. 6 are in the AB
system191919The Vega/AB magnitude conversion follows Blanton & Roweis (2007).
and have been corrected for distance modulus, MW extinction, and host
extinction if any, and are plotted versus rest-frame days past estimated
explosion epoch. SN 2020jfo reached a peak magnitude quite similar to that of
the canonical Type IIP SN 1999em202020 We used $E(B-V)=0.035$ mag and a
distance of 7.5 Mpc from Hamuy et al. (2001) for SN 1999em, data from Faran et
al. (2014). , but the plateau phase is significantly shorter. At nebular
phases SN 2020jfo is fainter than SN 1999em, but instead follows the same
decline rate and tail luminosity as the Type II SN 2013ej212121 We used
$E(B-V)=0.060$ mag and a distance of 9.1 Mpc from Valenti et al. (2014); Yuan
et al. (2016) for SN 2013ej..

Figure 6: Light curves in absolute magnitudes ($M_{{r}}$) for our three SNe.
This accounts for distance modulus and MW extinction as discussed in the text,
but no additional corrections for host extinction. The comparison SNe are
introduced in the text. The photometry has been binned to nightly averages.

In order to estimate the total radiative output, we also attempted to
construct bolometric LCs. For SN 2020jfo, we adopted two approaches: we use a
black-body (BB) function fitted to the GP interpolated fluxes, and we also
employ an analytic bolometric correction (BC) that was constructed by Lyman et
al. (2016) from a sample of SNe II. On the LC plateau, where we have coverage
from the NIR to the UV, we can construct spectral energy distributions (SEDs)
and fit to a diluted BB function. Integrating that BB function provides the
bolometric luminosity. Comparing this with the prescription from Lyman et al.
(2016), we find that the bolometric LC on the plateau agrees very well with
our BB estimate (the ratio is $0.97\pm 0.05$ on the late plateau). Therefore,
for the part of the LC where we only have optical data, we follow the Lyman et
al. (2016) method. The only difference between these methods is for the very
early phases, where our UV data imply slightly higher luminosities; this is
illustrated in Fig. 7. The bolometric LC is also provided on WISEREP.

Figure 7: Bolometric luminosities for our SNe. Left: SN 2020jfo with some
comparison objects (see text). The green solid lines with shaded regions are
estimated with the Lyman et al. (2016) method and is a Gaussian Process fit to
that LC. The blue region is a linear interpolation for the region with less
data. The red fit on the plateau is for a diluted BB fit to our UV through NIR
data, and matches the Lyman et al. method very well on the later part of the
plateau. Right: SNe 2020amv and 2020jfv. These were derived using the Lyman et
al. method, which is rather approximate given the unusual nature of these SNe.

Using this, we can estimate a maximum bolometric luminosity for SN 2020jfo of
$L_{\rm{bol}}=1.63\times 10^{42}$ erg s-1 at 4 rest-frame days and a total
radiated energy over the first 350 rest-frame days of $E_{\rm{rad}}=5.74\times
10^{48}$ erg. The radioactive 56Ni mass ejected in the explosion can be
inferred by measuring the luminosity tail, which is powered by the decay of
radioactive 56Co. Using $L=1.45\times 10^{43}$ exp$(-t/{\tau_{\rm
Co}})({M_{\rm Ni}}/{{\rm M}_{\sun}})$ erg s-1 from Nadyozhin (2003) implies
that we would require $0.024\pm 0.002$ $\mathrm{M}_{\odot}$ of 56Ni to account
for the luminosity (70–100 rest-frame days; see further Sect. 3.1.3).

####  3.1.2 Comparisons with SNe 2020amv and 2020jfv

We here discuss the LCs of the other two SNe presented in this paper, although
not at the same level of detail as for SN 2020jfo. SN 2020amv was
photometrically monitored with P48 for more than a year, with a few data
points also provided by LT, P60, and NOT. The LC is displayed in the middle
panel of Fig. 2. The explosion date is well constrained and the SN rose in a
bit more than two weeks ($r$ band; Table 8) to a Gaussian-shaped LC peak,
where both the rise and the fall are somewhat faster in $g$ than in $r$, with
${\Delta}{m_{15}^{r}}=0.45\pm 0.02$ mag and ${\Delta}{m_{15}^{g}}=0.52\pm
0.01$ mag. After this initial phase, the LC is rejuvenated, as the $r$-band
brightness gently rises again some 60 days past peak. Overall, it is a very
long-lived SN which we followed for more than 450 days. The absolute-magnitude
LC (Fig. 6) demonstrates that SN 2020amv was very luminous,
$M^{\rm{peak}}_{g}=-19.2$ mag, the initial peak resembling that of a Type I
SN, but the remaining bumpy, bright, and long-lived LC reveals that the (late-
time) power source must be something in addition to radioactive decay. In
terms of CC SNe, such LCs are expected to be powered by CSM interaction (e.g.,
Nyholm et al. 2017, 2020), but we note that superluminous SNe, even of Type I,
sometimes display long-lived bumpy LCs, which are not always easily explained
in terms of a central engine (e.g., the large SLSN-I sample from ZTF; Chen et
al. 2021, in prep.).

SN 2020jfv was found while declining and we do not have good constraints for
the date of explosion or the early peak. The right-hand panel of Fig. 2
therefore only shows a declining and eventually flattening LC. The unusual
aspect is that the $r$-band LC, after declining a full magnitude over the
first $\sim 100$ days, rises by $\sim 0.3$ mag in the next 100 days, before
the target was lost in the Sun’s glare. After solar conjunction, we recovered
the LC in $gri$ at virtually the same magnitudes as before the gap.

####  3.1.3 Light-curve modeling for SN 2020jfo

In order to estimate progenitor and explosion parameters for SN 2020jfo from
the bolometric LC, we make use of the semi-analytic Monte Carlo code that was
recently presented by Jäger et al. (2020), as used for the low-luminosity Type
IIP SN 2020cxd (Yang et al. 2021). After marginalisation, our fit provides
estimates with confidence intervals (2$\sigma$) for each of the parameters: SN
2020jfo has $M_{\rm ej}=5.16_{-2.00}^{+0.28}$ $\mathrm{M}_{\odot}$, $E_{\rm
kin}=2.04_{-1.02}^{+0.57}\times 10^{51}$ erg, and $v_{\rm
exp}=8.14_{-1.32}^{+0.54}\times 10^{3}$ km s-1 for the ejecta mass, kinetic
energy, and expansion velocity (respectively). The nickel mass was
simultaneously estimated as $0.029\pm 0.014$ $\mathrm{M}_{\odot}$. The mass of
radioactive nickel is thus similar to that estimated for SN 2013ej (0.023
$\mathrm{M}_{\odot}$; Yuan et al. 2016), as expected from the similar absolute
magnitudes (Fig. 6). It also matches the estimate from Sect. 3.1.1. Moreover,
the LC slope is similar for these two SNe, and Yuan et al. (2016) interpreted
this as being due to gamma-ray escape. The same seems to apply here. In fact,
we can fit for a gamma-leakage LC (Sollerman et al. 1998) with the flux
declining as $e^{(-t/111.3)}\times(1-0.965\times e^{-(t_{0}/t)^{2}}),$ where
$t$ is the time in days and $t_{0}$ is the epoch when the optical depth to the
gamma rays is unity. This epoch is also related to the ejecta mass
(Clocchiatti & Wheeler 1997, their Eq. 5), and our best-fit values
$M_{\rm{Ni}}=0.025$ $\mathrm{M}_{\odot}$ and $t_{0}=166$ days correspond to
$M_{\rm ej}\approx 5-6$ $\mathrm{M}_{\odot}$, for (1–1.7) $\times 10^{51}$ erg
of kinetic energy. We note that the estimated ejecta mass is low, in agreement
with the results from the Monte Carlo fit to the short plateau.

In this respect, the short plateau of SN 2020jfo is similar to those discussed
by Hiramatsu et al. (2021). They presented three Type IIP SNe having plateau
lengths of only 50–70 days, arguing that this was due to low ejecta mass. They
further suggested that their SNe originated from massive progenitors with
normal to large amounts of radioactive nickel, and that the required large
mass loss was also likely the cause for the bright early luminosity. For SN
2020jfo we have a normal initial luminosity and a modest amount of radioactive
nickel. There are no signs of CSM interaction from either the LC or the
spectra. In addition, we have evidence that the initial mass was not very
large (Sect. 3.3). SN 2020jfo is therefore somewhat different from the
discussion of Hiramatsu et al. (2021) in the sense that the plateau is not
generally powered by the radioactivity.

###  3.2 Spectroscopy

The spectroscopic sequence for SN 2020jfo, as provided in Table 7, is
displayed in Fig. 8. The photospheric phase is well covered by the robotic
telescopes, and although the resolution of the SEDM spectra is low, the
overall spectral evolution as the photosphere expands and cools is nicely
covered. The early classification spectra show shallow Balmer lines and a He
II feature at 15,000 km s-1. No signatures of CSM interaction are present in
the spectra. A total of 22 spectra are presented for SN 2020jfo, covering
phases from 2 to 350 days past explosion. The photospheric velocities, as
estimated from the P Cygni Balmer lines, is $\sim 9000$ km s-1 at the early
plateau and $\sim 7000$ km s-1 toward the end of the plateau (44 days past
explosion).

Figure 8: Sequence of spectra of SNe 2020jfo, 2020amv, and 2020jfv. The epochs
and scale factors of the spectra are also provided.

####  3.2.1 SN 2020amv

Thirteen spectra were obtained of SN 2020amv (Table 7). The first spectrum was
acquired with the SEDM about 1.7 days from the estimated explosion date.
Despite the low resolution, several narrow emission lines are identified
(e.g., H${\alpha}$, H${\beta}$), including those of highly ionised species (He
II $\lambda$4686) which correspond to flash-ionisation lines (Bruch et al.
2021). These features disappear within 13 days from the explosion epoch. This
is thus one of the more long-lived flash features in the ZTF sample (Bruch et
al, in prep.). Such transient emission lines emerge from the early interaction
of the shock-breakout radiation with a nearby (typically $\lesssim 10^{15}$cm;
e.g., Yaron et al. 2017) CSM. Two higher-resolution spectra were obtained at
6.6 and 9.8 days. The narrow H$\beta$ emission line in the former indicates a
relatively slow velocity of $\lesssim 335$ km s-1 (full width at half-maximum
intensity; FWHM), consistent with a wind velocity of the CSM. The following
spectra show mainly a blue continuum. After the rebrightening of the light
curve was recognised, additional spectra were obtained with SEDM 230 days past
explosion. These showed a strong H$\alpha$ line, with very high velocities.
This triggered us to obtain a higher-resolution spectrum with the P200 (Table
7), revealing a broad, boxy line profile having several emission peaks. This
complex line profile slowly evolves in later spectra, and the nebular lines
are further explored in Sect. 3.5.

SN 2020amv can in some sense also be seen as a transforming SN (as we argue
for SN 2020jfv below). It seems that CSM interaction is the driving force in
powering SN 2020amv, but that the evidence for this is manifested in different
ways throughout the SN evolution. The early flash spectroscopy provides
evidence for dense CSM close to the exploding star, although the early LC is
similar to Gaussian-shaped LCs of other types of stripped-envelope SNe. Other
studies have found that such LCs might still be consistent with radioactive
powering, as in SN 2018ijp (Tartaglia et al. 2021) , or SN 2020eyj (Kool et
al., in prep.), where CSM interaction came in later to power the prolonged
LCs. SN 2020amv peaks at $-19.24$ mag in $g$, similar to the peak luminosity
of SNe Ia. The CSM evidence in terms of LC evolution is instead obvious after
about 50 days, when the second peak anticipates the long-lived nature of the
overall LC. Here we lack spectroscopy, but when the spectroscopic campaign
resumed it revealed nebular box-shaped emission lines (Sect. 3.5) that again
are a telltale signature for CSM interaction — this time the signature unfolds
a cold dense shell likely caused by the reverse shock produced by ejecta
running into the CSM. These are all different manifestations of CSM
interaction that together unveil the mass-loss history of the exploded star.

####  3.2.2 SN 2020jfv

SN 2020jfv was first spectroscopically observed with the SEDM on P60, and
based on this spectrum classified as a Type IIb SN (Dahiwale & Fremling
2020b). Helium lines at a velocity of 7000 km s-1 are detected. Six months
later, after recognising that the target was rebrightening again, we obtained
another SEDM spectrum that seemed to be dominated by bright, narrow Balmer
emission lines, typical for CSM-driven SNe IIn. We also acquired some spectra
with larger telescopes, namely with the NOT and Keck (Fig. 8). These show a
Type II SN spectrum containing lines from elements such as Mg, Ca, and O (as
are typically seen in CC SNe), but which is very much dominated by the
intermediate-width Balmer lines. We discuss the nebular spectra in the next
sections.

###  3.3 Modelling the oxygen mass of SN 2020jfo

In Fig. 9 we zoom in on the four high-quality late-time spectra of SN 2020jfo
taken with the NOT. These were obtained over a period when the SN was 250–350
days old, and the spectroscopic evolution over that time range is very slow.
Overall, it is a textbook example of a normal SN II spectrum (Jerkstrand
2017), dominated by Balmer lines that still show P Cygni absorption
components, but also strong emission lines of calcium and oxygen. The spectra
are indeed similar to those seen in many other nebular CC SNe, like the famous
SN 1987A or the well-studied SN 2012aw.

For SN 2020jfo, where we have no evidence of CSM interaction, we can compare
the nebular emission-line luminosities with modeling to estimate the oxygen
mass and thus the zero-age main sequence (ZAMS) mass of the star that
exploded. We use the models and methodology advanced by Jerkstrand et al.
(2012), Jerkstrand et al. (2015a), and Jerkstrand et al. (2018). The spectra
are calibrated on an absolute scale using the photometry and corrected for
extinction. Figure 9 shows the 12 $M_{\odot}$ model from the work of
Jerkstrand et al. (2015c). In these comparisons, we have rescaled the model
flux with the ratio of 56Ni mass inferred for SN 2020jfo (0.025
$\mathrm{M}_{\odot}$; see Sects. 3.1.1 and 3.1.3) to the model 56Ni mass
(0.062 M⊙). We have in addition multiplied the model by a factor of 0.5 to
account for the significantly earlier gamma-ray escape occurring in SN
2020jfo.

The 12 $\mathrm{M}_{\odot}$model makes the best fit to the key emission lines
that diagnose the progenitor mass (oxygen, sodium, and magnesium), and
suggests that the best ZAMS-mass match for SN 2020jfo is in the range 10–15
$\mathrm{M}_{\odot}$. H$\alpha$ is too strong in the model, which is
consistent with an unusually low hydrogen envelope mass in this SN, likely
also causing the short plateau. Having a relatively low-mass He core
progenitor that has lost a large part of its H envelope would be most
naturally consistent with binary mass loss.

However, it is also noteworthy that the metal emission lines do not appear
broader than in the model. With a low envelope mass, one would expect a low He
core mass and a quite high explosion energy, as inferred in Sect. 3.1.3, so
the metals should have an unusually high expansion velocity. Thus, no fully
self-consistent scenario is established.

Finally, note that the Ca II NIR triplet mismatch is a well-known shortcoming
of the models. The models give scattering of Ca II $\lambda\lambda$8498, 8542
into the Ca II $\lambda$8662 line — but in most SNe II this is not observed to
happen.

###  3.4 Stable nickel in SN 2020jfo

The nebular spectra also show clear evidence for stable nickel in the form of
the [Ni II] $\lambda$7378 line (Fig. 10). This line is not always seen in SNe
II; a clearly observed line typically requires an unusually weak [Ca II]
doublet near 7300 Å combined with an intrinsically strong nickel line. Here,
it is plausible that the small hydrogen zone damps the calcium doublet, as
much of that emission comes from this zone (Li & McCray 1993).

Stable nickel is an important diagnostic of the explosion mechanism. It is
mainly composed of 58Ni, with a higher production most naturally being
explained by burning and ejection of deeper-lying, more neutron-rich layers in
the progenitor star (Jerkstrand et al. 2015c). The iron comes from decayed
56Ni, which with its zero neutron excess has no such dependency.

We measure the relative luminosities for the lines following the approach of
Jerkstrand et al. (2015b, their section 3.1.1), with a simultaneous fit to
several lines predicted from the models. Even if the exact line shape is not
perfectly matched, we are confident about the line identifications. The
measured luminosity ratio at +306 days is $L_{\rm NiII\leavevmode\nobreak\
7378}/L_{\rm FeII\leavevmode\nobreak\ 7155}=1.7$. The link between luminosity
ratio and mass ratio depends on temperature, although quite weakly. The
temperature can be estimated from the intrinsic line luminosity of [Fe II]
$\lambda 7155$, the best-fitting value for $M({\rm Fe})=0.025$
$\mathrm{M}_{\odot}$ being 2700 K. A range of $T=2500$–3000 K gives a mass
ratio $M(\mbox{Ni})/M(\mbox{Fe})=1.7$–2.1, following the analysis method of
Jerkstrand et al. (2015c).

###  3.5 The nebular H$\alpha$ emission-line profiles

If we focus on the strongest nebular emission line, H$\alpha$, we can see that
it is relatively symmetric in SN 2020jfo, with FWHM $\approx 2510$ km s-1. The
P Cygni absorption has a maximum at 4000 km s-1, and also the red side of the
emission line reaches that velocity. The maximum velocity in the red
absorption component is 6000 km s-1, and this is actually consistent with the
red shoulder of the emission line. Since we were able to properly model the
nebular spectrum of SN 2020jfo in Sect. 3.3 without any input from CSM
interaction, we will adopt the H$\alpha$ profile for this SN as a benchmark
with which to compare our other two SNe, to highlight how CSM interaction (as
evident in the LC evolution) can manifest itself in the line profiles.

SN 2020amv is also a SN II, but the nebular spectrum is strikingly different.
We acquired two SEDM spectra at an age of 240 days, motivated by the endurance
of the SN LC. The SEDM spectra revealed little more than the H$\alpha$
emission line, but it caught our attention since it was very broad. The
resolution of SEDM is low, but the width of the emission line stood out; we
estimated a FWZI (full width at zero intensity) of $\sim 17,000$ km s-1. This
prompted four more nebular spectra with larger telescopes. The first of these,
from P200, is spectacular. It shows mainly H$\alpha$, but also H$\beta$ with
the same line profile as well as the Ca II NIR triplet. The H$\alpha$ line
profile displays three distinct peaks, similar to the late-time spectra of SN
1998S (Pozzo et al. 2004, their fig. 5) and SN 1993J (see e.g., Matheson et
al. 2000, their fig. 11). In Fig. 11 we show the four last nebular spectra of
SN 2020amv, compared to those of other SNe. The H$\alpha$ line in the P200
spectrum has FWHM $\approx 9800$ km s-1 (not measured with a Gaussian fit;
FWZI $\approx 14,000$ km s-1). For the similar SN 1993J, the interpretation
for the shape of the line profile was CSM interaction with emission
originating in a dense thin shell, and this obviously also applies to SN
2020amv.

The P200 H$\alpha$ line profile is asymmetric. The structure on top of the
rectangular, boxy, and flat-topped line profile predicted from a thin shell
can be interpreted either as evidence for an overall asymmetric geometrical
configuration such as a ring-like structure (as mentioned by Pozzo et al.
2004), or alternatively seen as small-scale structure as due to clumps, as
suggested for the forbidden oxygen lines in SN 1993J by Spyromilio (1994). In
the P200 spectrum (Fig. 11) the blue-horn emission is shifted by $5000$ km s-1
with respect to the galaxy rest frame. The same structure remains in the later
NOT spectra, but it can be seen that the relative strengths of the features is
changing. The final Keck spectrum spectacularly shows the three horns at
$\sim-4240$, $-730$, and +1400 km s-1. The evolution of the three components
is particularly conspicuous between the P200 spectrum and the final Keck
spectrum. We see that the emission line becomes more asymmetric with time, and
that the blue horn is dominating the line profile at the last epoch, or rather
that the red-most side of the line profile is suppressed. This is similar to
the case of SN 1998S (Fig. 11). The line-profile evolution of SNe 1993J and
1998S were discussed in some detail by Fransson et al. (2005). Several models
for the geometry and dust distributions were tested, but none could
convincingly explain the profiles of SN 1998S. The central component of the
line profile would require an additional emission component — but the
possibility that the central horn of the H$\alpha$ line in SN 1998S was
affected by host contamination was also mentioned (Fransson et al. 2005). In
this regard, we note that the central component in the SN 2020amv Keck
spectrum is real and clearly resolved at 900 km s-1. The host-galaxy H$\alpha$
line from the same epoch has FWHM $\approx 240$ km s-1. The offset between the
two lines is 730 km s-1, with the central SN component blueshifted. Overall,
the remarkable resemblance and the similar line-profile evolution between SN
1998S and SN 2020amv argue for a generic scenario rather than a fine-tuned
geometry and dust distribution.

Looking finally also at SN 2020jfv, the nebular emission-line spectrum is
somewhat of an intermediate case between the two above-mentioned SNe. This SN
also caught our attention given the photometric behaviour (rebrightening).
Late-time SEDM spectra showed an unusual behaviour — a stripped-envelope SN
transforming into a SN II — which made us activate larger telescopes. The NOT
data revealed a Balmer-dominated spectrum, but also clear [O I]
$\lambda\lambda$6300, 6364 emission and what is likely [Ca II] at 7300 Å. The
later Keck spectrum confirms this, and is even more dominated by H$\alpha$ and
H$\beta$; we measure a flux ratio of H$\alpha$ to [O I] $\lambda\lambda$6300,
6364 of $\sim 6.5$. The Ca II NIR triplet is also very weak. The peak of the
H$\alpha$ line profile is unfortunately damaged by a cosmic-ray hit in the
high-S/N Keck spectrum. The final NOT spectrum, obtained after solar
conjunction, shows basically only the Balmer lines.

The line profile of H$\alpha$ in SN 2020jfv has FWHM $\approx 2500$ km s-1,
which is virtually the same as for H$\beta$ (2800 km s-1, corrected for
instrumental resolution; the host-galaxy lines are 750 km s-1). Whereas the
host lines appear at $z=0.017$, both H$\alpha$ and H$\beta$ are blueshifted by
800 km s-1 from this. The extinction-corrected Balmer ratio is $\sim 5$,
indicating shock interaction. In Fig. 12 we compare two late spectra of SN
2020jfv with our best spectrum of SN 2020jfo, the P200 spectrum of SN 2020amv,
and also with SN 2019oys which was a stripped-envelope SN that transformed
into a SN IIn owing to late CSM interaction (Sollerman et al. 2020). We can
see that the spectrum of SN 2020jfv is different from all of these comparison
objects. The right-hand panel of the figure zooms in on H$\alpha$ in velocity
space, and we note that the intermediate-width emission line of SN 2020jfv
actually has structure on the red side of the line profile. This is
significantly more subtle evidence of CSM interaction than for SN 2020amv.

###  3.6 Uncertainties

####  3.6.1 Distance and extinction

For the analysis and discussion so far we made use of the provided distances
and assumed no additional extinction from the host galaxies of our SNe. In
this section we discuss what the main uncertainties are provided the errors in
the distance estimates, and discuss how the derived parameters would change if
more extinction in the host galaxies would dim and redden the light from the
SNe. These are the observational main caveats for most SN studies.

In Sect. 2 we declared that the distance to M61 is uncertain and adopted a
distance modulus of $30.81\pm 0.20$ mag. This estimate comes from the
Expanding Photosphere Method for a Type II SN (Bose & Kumar 2014), and is also
consistent with the peculiar motion corrected luminosity distance derived from
standard cosmology and the observed redshift from NED. Such an uncertainty
directly translates to a 20% error in the nickel mass estimates. There are,
however, other estimates of the distance to M61 that are even larger (by
almost 30%, Pejcha & Prieto 2015).

Extinction is sometimes even more difficult to determine. We have assumed no
host galaxy extinction for the three SNe, mainly based on their blue colors.
For SN 2020jfo there is some evidence for narrow Na I D lines in the Lick
spectrum, where we can estimate an equivalent width of $\lesssim 0.7$ Å for
the doublet. For example using Taubenberger et al. (2006) with
A${}_{V}=3.1\times 0.16\leavevmode\nobreak\ \times\rm{EW(NaID)}$ would give
0.3 mag of extinction in the optical. In most regards such a 30% increase in
flux is similar to adopting a larger distance, as discussed above.

With more color information available from the pre-explosion HST imaging, such
a reddening would also be of importance for the progenitor conclusions.
However, as noted in Sect. 2.5 we detect the probable progenitor only in the
reddest band, and it is fainter than expected by more than a magnitude. A host
extinction correction of the magnitude suggested above would not resolve this
issue.

A main effect from the uncertainty in distance and reddening is on the actual
mass of radioactive nickel as mentioned above, the effects of both a larger
distance and some host extinction could potentially increase the estimate from
0.025 $\mathrm{M}_{\odot}$to 0.04 $\mathrm{M}_{\odot}$. Other parts of the
analysis is less sensitive. For example the modeling of the nebular spectra
relies on relative line luminosities for the 58Ni analysis, which is virtually
independent of both distance and extinction. Also the ZAMS mass estimate from
the oxygen mass is derived by scaling the oxygen luminosity with the derived
nickel mass, and both are similarly affected by the uncertainties.

Shortly, also for SNe 2020bmv and 2020jfo the discussion is similar as above,
but there is not much analysis that is severely affected. The distance
estimates are deduced from the redshifts, and at 74 and 200 Mpc the effects of
peculiar velocities is smaller (V${}_{\rm{pec}}=150\leavevmode\nobreak\ $km
s-1 gives at most 6% uncertainty in flux), and is instead dominated by the
everpresent uncertainty in the Hubble constant ($\pm 3$ km s-1 Mpc-1 gives 9%
in flux). For these two peculiar SNe, the fact that the colors are not red is
not much evidence against host galaxy reddening. None of our spectra show
evidence for narrow ISM lines (SN 2020jfv have no constraining observations,
whereas the NTT spectrum of SN 2020amv would reveal a Na ID of the same
strengths as potentially present for SN 2020jfo). However, our discussion for
these two SNe mostly concerned the line profiles and their evolution, and is
not affected by these uncertainties.

####  3.6.2 Methodology

We can also briefly discuss the different methodologies used to infer the
properties of SN 2020jfo and its progenitor. We have been able to use
relatively well-established procedures to infer properties of the progenitor
star from complementary routes such as pre-explosion progenitor imaging,
bolometric light-curve modeling and nebular NLTE emission-line analysis.
Whereas the derived oxygen mass implies a progenitor of ZAMS mass around 12
$\mathrm{M}_{\odot}$, the short LC plateau indicates a lower ejecta mass and
the progenitor detection is fainter still. This could imply interesting
properties of the supernova, such as circumstellar dust around the progenitor
destroyed in the explosion and extensive mass-loss affecting the hydrogen
envelope, as mentioned in the next section. The tension could also indicate
that some of the many assumptions inherent in these methods may not hold. More
well-studied SNe where several lines of investigation can be applied are
needed.

##  4 Summary, interpretation and conclusions

In this paper we have presented the discovery, classification, and follow-up
observations of the Type II SN 2020jfo, which exploded in the nearby spiral
galaxy M61 in May 2020. We presented optical, NIR, and NUV photometry, as well
as spectroscopy, for the first year of this transient, which was also followed
by many other astronomers (both professional and amateur) around the globe.
Even though the site was covered by PTF and ZTF for 11 yr prior to explosion,
we did not detect any pre-SN outbursts down to a magnitude of $-11$.

The supernova has a well-constrained explosion epoch and rose to a maximum of
$M^{\rm{peak}}_{r}=-16.5$ mag in $\sim 5$ days. It was not detected at any
significance in X-rays using Swift, but the UV LC was well covered. The
plateau length of 65 days is on the short side of the SN IIP distribution.
Using simple modeling we estimate an ejecta mass of $M_{\rm ej}\approx 5$
$\mathrm{M}_{\odot}$, while the mass of radioactive nickel estimated from the
late-time tail of the LC is $0.025$ $\mathrm{M}_{\odot}$, although
uncertainties in distance and host extinction could increase this by
$\sim$50%. The spectroscopic sequence of SN 2020jfo was largely obtained with
robotic low-resolution spectrographs, but reveals normal SN II evolution
dominated by Balmer lines with P Cygni profiles. This sort of sequence can
routinely be achieved with SEDM-like spectrographs.

We also secured two epochs of spectropolarimetry. The S/N is rather low, but
there is evidence of both continuum and line polarization. The interpretation
is uncertain, but the modest continuum polarization may be due to an
asymmetric distribution of radioactive elements, or of an asymmetric electron
density structure. The higher line polarization suggests that Ca is not
uniformly distributed within the ejecta and likely exists in clumps.

For the later nebular phases, we rely on ToO triggers on larger telescopes.
The sequence of spectra of SN 2020jfo from the NOT displays slow evolution in
the nebular phase. Line-flux measurements of the calibrated spectra indicate,
when compared to detailed NLTE models, that the exploding progenitor had a
ZAMS mass of $\sim 12$ $\mathrm{M}_{\odot}$. Nebular emission line analysis
also revealed a high abundance of stable nickel (58Ni), with a mass ratio
$M(\mbox{Ni})/M(\mbox{Fe})\approx 2$. Only a handful of CC SNe have previous
estimates of this mass ratio (Maeda et al. 2007; Jerkstrand et al. 2015b;
Terreran et al. 2016; Tomasella et al. 2018; Müller-Bravo et al. 2020). The
picture so far points to a value of around solar being most common — this
corresponds to burning and ejection of oxygen-rich layers. SN 2020jfo joins a
small group of SNe having an enhanced ratio of 2–4 times solar; others in this
group are the normal Type IIP SN 2012ec (Jerkstrand et al. 2015b) and the
broad-lined Type Ic SN 2006aj (Maeda et al. 2007). The result for SN 2020jfo
adds another important piece to the puzzle. With its moderate progenitor mass,
there appears currently to be no simple dependency on progenitors mass, but
rather that both low-mass and high-mass stars can achieve ejection of neutron-
rich material.

Pre-explosion HST imaging reveal a putative source at the site of SN 2020jfo.
The exact location was obtained with ground-based adaptive optics imaging, and
confirmed with post-explosion HST imaging. The star is only detected in the
reddest band with an absolute magnitude of $M_{\rm F814W}\approx-5.8$, and
nondetections in the bluer bands. This is fainter than expected and might
imply circumstellar dust that was later destroyed by the SN explosion. The
notably short plateau of SN 2020jfo implies that partial stripping of the
envelope occurred at some episode before explosion, either through a stellar
wind or mass exchange with a binary companion. Such mass loss would result in
CSM, which could be consistent with the possible presence of circumstellar
dust. If the CSM were set up by a vigorous pre-SN wind, it was not accompanied
by any luminous outburst (Sect. 2.5).

We have examined BPASS binary models (Eldridge et al. 2019) for the mass range
of 10 – 15 $\mathrm{M}_{\odot}$, as indicated for the ZAMS mass by the nebular
line analysis. Although a number of models terminate near the observed $M_{\rm
F814W}$ (without the presence of further circumstellar extinction), the very
low amount of H mass in these models at the time of explosion is more
consistent with what we would expect for a SN IIb than a SN IIP progenitor.
The reason for the faint, red possible detection of the SN 2020jfo progenitor,
given the properties of the SN itself, therefore remains unknown and requires
further study. Ultimately, the identification of the progenitor candidate must
be confirmed by revisiting the site when the SN has sufficiently faded.

We have compared SN 2020jfo with two other ZTF SNe, all three being dominated
by Balmer lines in the nebular spectra. SN 2020amv is also a SN II, but with a
different LC and spectral evolution. The long-lived bumpy LC is a telltale
sign of interaction with CSM. This is confirmed both by strong flash-
spectroscopy features at early times, and a broad, square-shaped nebular
emission-line profile in H$\alpha$, which furthermore consists of several
components. The interpretation for such a line profile is emission from a cold
dense shell developed by the reverse shock from interaction with dense CSM
that the ejecta run into at later phases. CSM interaction is likely also the
reason why SN 2020jfv rebrightens hundreds of days past explosion. This SN IIb
initially did not have strong signatures of hydrogen, whereas the nebular
spectrum is completely dominated by Balmer lines. The line shapes are somewhat
intermediate between those discussed for the other two SNe above. Instead of
evidence for CSM interaction in terms of a broad, flat-topped line profile, we
see an intermediate-width asymmetric line, where optical-depth effects likely
play a role in shaping the line. Interest in both of the latter targets was
raised only after we realised their slowly evolving and rebrightening light
curves, and in both cases initial SEDM data convinced us to also obtain
spectra at larger telescopes. SEDM can thus operate both as a classification
machine and as a science data provider (as for SN 2020jfo), but also as a way
to investigate whether additional observations should be undertaken for any
given transient. In SN 2020jfo itself, we see no evidence for CSM interaction,
neither in the LC nor in the spectral evolution. However, based on the LC fit
to the short plateau, and to the fast-declining late-time bolometric LC, we
find evidence for a relatively low mass of hydrogen ejecta. Given the ZAMS
mass derived from the nebular-line analysis, this points to a significant
amount of mass loss. Since this CSM is not affecting the observed properties
of the SN, we suspect that this period of mass loss must have occurred at a
substantially earlier phase of the progenitor’s evolution.

Large surveys such as ZTF are now routinely discovering thousands of SNe. Part
of the observations that were previously cumbersome and expensive to obtain,
such as continuous LCs in several bands from early to late times and decent
numbers of low-resolution spectra, are now virtually automatically available
to the astronomical community — in particular, for nearby events such as SN
2020jfo. The challenge has moved to being able to digest and publish the
available data, and to detect, select, and monitor events of particular
interest. SNe 2020amv and 2020jfv were both unusually long lived with LCs that
rebrightened. Dedicated spectroscopic follow-up observations were required to
confirm CSM interaction as the powering mechanism for these objects.

###### Acknowledgements.

We thank the staffs of the various observatories where data were obtained for
their excellent assistance. Based on observations obtained with the Samuel
Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory
as part of the Zwicky Transient Facility project. ZTF is supported by the U.S.
National Science Foundation (NSF) under grant AST-1440341 and a collaboration
including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein
Center at Stockholm University, the University of Maryland, the University of
Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los
Alamos National Laboratories, the TANGO Consortium of Taiwan, the University
of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. ZTF-II
is supported by the National Science Foundation under Grant No. AST-2034437
and a collaboration including Caltech, IPAC, the Weizmann Institute for
Science, the Oskar Klein Center at Stockholm University, the University of
Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO
Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity
College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France.
Operations are conducted by COO, IPAC, and UW. The SED Machine is based upon
work supported by the NSF under grant AST-1106171. The ZTF forced-photometry
service was funded under the Heising-Simons Foundation grant #12540303 (PI M.
Graham). This work was supported by the GROWTH project funded by the NSF under
PIRE grant 1545949. The Oskar Klein Centre was funded by the Swedish Research
Council. Gravitational Radiation and Electromagnetic Astrophysical Transients
(GREAT) is funded by the Swedish Research council (VR) under Dnr 2016-06012.
ECK is further supported by the Wenner-Gren Foundations. Much support to OKC
involvement in ZTF was provided by the Knut and Alice Wallenberg foundation.
Partially based on observations made with the Nordic Optical Telescope,
operated by the Nordic Optical Telescope Scientific Association at the
Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de
Astrofisica de Canarias. Some of the data presented here were obtained with
ALFOSC. Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained from the data archive at the Space Telescope Science Institute
(STScI). STScI is operated by the Association of Universities for Research in
Astronomy, Inc. under NASA contract NAS 5-26555. Support for program GO-16179
was provided by NASA through a grant from STScI. We thank T. de Jaeger, O. D.
Fox, P. Kelly, N. Smith, S. Vasylyev, and W. Zheng for their assistance with
this program. 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 NASA;
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. A major upgrade of
the Kast spectrograph on the Shane 3 m telescope at Lick Observatory was made
possible through generous gifts from the Heising-Simons Foundation as well as
William and Marina Kast. Research at Lick Observatory is partially supported
by a generous gift from Google. Palomar Gattini-IR (PGIR) is generously funded
by Caltech, the Australian National University, the Mt. Cuba Foundation, the
Heising-Simons Foundation, and the Binational Science Foundation. PGIR is a
collaborative project among Caltech, the Australian National University,
University of New South Wales, Columbia University, and the Weizmann Institute
of Science. Y.-L.K. has received funding from the European Research Council
(ERC) under the European Unions Horizon 2020 research and innovation program
(grant 759194 – USNAC). M.C. acknowledges support from the NSF through grant
PHY-2010970. Funding for A.V.F.’s research group at U.C. Berkeley was provided
by the Christopher R. Redlich Fund and the Miller Institute for Basic Research
in Science (where A.V.F. is a Senior Miller Fellow). This work was partially
funded by Kepler/K2 grant J1944/80NSSC19K0112, HST grant GO-15889 from STScI,
and STFC grants ST/T000198/1 and ST/S006109/1. A.J. acknowledges funding from
the European Research Council (ERC) through Starting Grant 803189. This work
has made use of data from the Asteroid Terrestrial-impact Last Alert System
(ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS)
project is primarily funded to search for near-Earth asteroids through NASA
grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the asteroid
search include images and catalogues from the survey area. The ATLAS science
products have been made possible through the contributions of the University
of Hawaii Institute for Astronomy, the Queen’s University Belfast, STScI, the
South African Astronomical Observatory, and The Millennium Institute of
Astrophysics (MAS), Chile.

Figure 9:  The four nebular spectra from NOT for SN 2020jfo are compared with
the nebular model for a 12 $\mathrm{M}_{\odot}$ explosion. The model is scaled
by the ratio of 56Ni masses in SN 2020jfo (0.025 $\mathrm{M}_{\odot}$) and the
model (0.062 $\mathrm{M}_{\odot}$), as well as a factor of 0.5 to account for
a significantly lower degree of gamma-ray trapping in SN 2020jfo. The most
conspicuous emission lines are marked: Mg I] $\lambda 4571$, [O I] $\lambda
5577$, [O I] $\lambda\lambda 6300$, 6364, Na I D, H$\alpha$, [Fe II] $\lambda
7155$, [Ni II] $\lambda 7378$, [Ca II] $\lambda\lambda$7291, 7323, and the Ca
II NIR triplet.  Figure 10:  The 7000–7600 Å spectral range for SN 2020jfo at
+306 days (blue lines). It has been demonstrated that this region is dominated
by emission from [Fe II], [Ni II], and [Ca II] (Jerkstrand et al. 2015c).
Shown in orange is the best-fit multiple-Gaussian model to the emission lines
following the method of Jerkstrand et al. (2015c). This fit gives $L_{\rm
Ni\leavevmode\nobreak\ II\leavevmode\nobreak\ 7378}/L_{\rm
Fe\leavevmode\nobreak\ II\leavevmode\nobreak\ 7155}=1.7$, which maps to a mass
ratio $M(\mbox{Ni})/M(\mbox{Fe})\approx 2$ times solar. The green dashed line
shows the fit without contributions from stable nickel. Figure 11:  Nebular
spectra of SN 2020amv. Left: The full range of the optical spectra, compared
with SN 1993J (top, at 976 days, from Matheson et al. 2000) which also showed
a boxy line profile, and with a sequence of spectra of SN 1998S (305 to 418
days, from Pozzo et al. 2004). Right: Zoom-in on the H$\alpha$ profile in
velocity space. The profile is overall flat-topped and box-shaped in the first
epoch, indicating emission from a thin dense shell formed when ejecta interact
with the CSM. The later spectra show the development of an increasing overall
asymmetry and structure, with a clear blueshifted horn at $-5000$ km s-1. The
evolution, with the suppression of the red side, is reminiscent of the
evolution seen in SN 1998S. Figure 12:  Nebular spectra of SN 2020jfv compared
with the normal SN 2020jfo, the CSM interacting SN 2020amv, and the
transitional SN 2019oys (Sollerman et al. 2020). The right panel shows a zoom-
in on the H$\alpha$ line profile in velocity space. H$\alpha$ dominates all
these spectra, but the spectral evidence for CSM interaction is more subtle in
SN 2020jfv, with a small structure visible at the red end of the line in the
high-S/N Keck spectrum. The peak of this line is affected by a cosmic-ray hit
and is therefore not shown.

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Table 1: Supernova and host-galaxy properties

IAU name | ZTF name | Explosion date | SN Type | Redshift | Host | Distance | $E(B-V)_{\rm MW}$
---|---|---|---|---|---|---|---
|  | (JD) |  |  |  | (Mpc) | (mag)
SN 2020amv | ZTF20aahbamv | 2458871.22 | IIn | 0.0452 | WISEA J084940.38+301115.5 | 200.3 | 0.032
SN 2020jfo | ZTF20aaynrrh | 2458975.20 | II | 0.00522 | M61 | 14.5 | 0.020
SN 2020jfv | ZTF20abgbuly | 2458969.51 | IIb$\rightarrow$IIn | 0.017 | WISEA J230635.97+003641.9 | 73.8 | 0.045

Table 2: Summary of ZTF $gri$-band and ATLAS $co$-band observations (forced
photometry) of SN 2020jfo. ATLAS and ZTF data are binned in 3 and 0.5 days
(respectively). The full table will be made available in machine-readable
format.

Observation | filter | mag
---|---|---
Date (JD) |  |
2458975.70 | r | $16.00\pm 0.01$
2458975.75 | g | $15.40\pm 0.01$
2458976.37 | i | $15.49\pm 0.03$
2458976.94 | o | $15.06\pm 0.00$
2458977.41 | i | $15.06\pm 0.00$
2458978.42 | i | $14.84\pm 0.00$
2458978.69 | g | $14.47\pm 0.00$
2458978.75 | r | $14.52\pm 0.00$
2458978.96 | o | $14.64\pm 0.00$
2458980.41 | i | $14.66\pm 0.00$
2458980.86 | o | $14.52\pm 0.00$
2458981.70 | g | $14.45\pm 0.00$
2458981.75 | r | $14.46\pm 0.00$
2458982.87 | c | $14.51\pm 0.00$
2458983.39 | i | $14.69\pm 0.00$
2458984.90 | o | $14.58\pm 0.00$
2458986.69 | g | $14.56\pm 0.00$
2458986.76 | r | $14.46\pm 0.00$
2458986.87 | c | $14.63\pm 0.00$
2458988.85 | o | $14.59\pm 0.00$
2458989.38 | i | $14.75\pm 0.00$
2458990.83 | c | $14.66\pm 0.00$
2458992.69 | g | $14.70\pm 0.00$
2458992.73 | r | $14.46\pm 0.00$
2458992.85 | o | $14.58\pm 0.00$
2458994.85 | c | $14.76\pm 0.00$
2458995.70 | g | $14.79\pm 0.00$
2458995.73 | r | $14.51\pm 0.00$
2458996.85 | o | $14.61\pm 0.00$
2458998.69 | g | $14.88\pm 0.00$
2458998.73 | r | $14.47\pm 0.00$
2458998.81 | o | $14.60\pm 0.00$
2459005.69 | g | $15.07\pm 0.00$
2459005.83 | o | $14.65\pm 0.00$
2459008.84 | o | $14.66\pm 0.00$
2459009.73 | g | $15.25\pm 0.00$
2459010.82 | o | $14.65\pm 0.00$
2459012.73 | i | $14.66\pm 0.03$
2459014.70 | g | $15.25\pm 0.00$
2459016.80 | o | $14.70\pm 0.00$
2459017.74 | r | $14.65\pm 0.00$
2459017.81 | i | $14.73\pm 0.04$
2459020.70 | g | $15.33\pm 0.00$
2459020.74 | r | $14.68\pm 0.00$
2459020.79 | o | $14.75\pm 0.00$
2459022.78 | c | $15.17\pm 0.01$
2459023.71 | g | $15.41\pm 0.00$
2459024.77 | o | $14.81\pm 0.00$
2459026.77 | o | $14.80\pm 0.00$
2459030.76 | o | $14.87\pm 0.00$
2459032.80 | o | $15.01\pm 0.00$
2459033.68 | r | $15.01\pm 0.00$
2459033.70 | g | $15.90\pm 0.01$
2459035.64 | o | $15.12\pm 0.01$
2459037.71 | g | $16.40\pm 0.02$
2459040.79 | o | $15.72\pm 0.01$
2459042.79 | c | $16.75\pm 0.03$
2459044.76 | o | $16.38\pm 0.02$
2459064.75 | o | $16.67\pm 0.02$
2459158.03 | r | $17.74\pm 0.04$
2459164.03 | r | $17.69\pm 0.02$
2459166.03 | r | $17.79\pm 0.02$

Observation | filter | mag
---|---|---
Date (JD) |  |
2459168.03 | r | $17.91\pm 0.03$
2459176.00 | r | $18.09\pm 0.26$
2459177.02 | g | $18.97\pm 0.06$
2459177.06 | r | $17.99\pm 0.05$
2459181.00 | g | $19.36\pm 0.17$
2459181.06 | r | $18.07\pm 0.05$
2459182.50 | o | $18.23\pm 0.11$
2459185.01 | r | $18.13\pm 0.04$
2459186.15 | o | $18.21\pm 0.10$
2459187.04 | g | $19.03\pm 0.17$
2459189.01 | r | $18.17\pm 0.04$
2459189.04 | g | $18.92\pm 0.07$
2459190.13 | o | $18.24\pm 0.11$
2459191.11 | o | $18.21\pm 0.11$
2459195.02 | r | $18.29\pm 0.03$
2459195.05 | g | $19.15\pm 0.06$
2459197.99 | c | $18.90\pm 0.20$
2459198.98 | r | $18.40\pm 0.05$
2459199.03 | g | $19.13\pm 0.06$
2459200.95 | g | $18.97\pm 0.16$
2459201.03 | r | $18.34\pm 0.06$
2459204.00 | r | $18.60\pm 0.05$
2459204.04 | g | $19.21\pm 0.07$
2459204.97 | g | $19.10\pm 0.05$
2459205.01 | r | $18.42\pm 0.03$
2459205.13 | c | $19.04\pm 0.23$
2459207.10 | c | $19.01\pm 0.23$
2459209.13 | c | $19.22\pm 0.28$
2459217.02 | r | $18.66\pm 0.09$
2459217.04 | g | $19.41\pm 0.17$
2459217.08 | o | $18.62\pm 0.16$
2459221.94 | r | $18.67\pm 0.06$
2459221.97 | g | $19.20\pm 0.11$
2459222.27 | o | $18.78\pm 0.18$
2459223.90 | g | $19.33\pm 0.09$
2459223.97 | r | $18.85\pm 0.06$
2459225.09 | c | $19.23\pm 0.28$
2459225.97 | g | $19.41\pm 0.08$
2459226.00 | r | $19.01\pm 0.28$
2459227.88 | r | $18.79\pm 0.05$
2459227.96 | g | $19.40\pm 0.07$
2459228.07 | c | $19.24\pm 0.29$
2459229.93 | r | $18.93\pm 0.08$
2459231.00 | c | $19.74\pm 0.47$
2459231.96 | g | $19.49\pm 0.06$
2459231.98 | r | $18.89\pm 0.04$
2459233.80 | c | $19.44\pm 0.35$
2459249.11 | o | $19.17\pm 0.27$
2459249.91 | r | $19.35\pm 0.11$
2459249.94 | g | $19.85\pm 0.21$
2459251.79 | i | $19.37\pm 0.19$
2459251.96 | r | $19.30\pm 0.06$
2459253.09 | o | $19.32\pm 0.31$
2459253.88 | g | $19.94\pm 0.12$
2459253.99 | r | $19.32\pm 0.06$
2459255.92 | r | $19.43\pm 0.06$
2459257.88 | g | $19.76\pm 0.15$
2459257.99 | c | $19.63\pm 0.42$
2459260.94 | c | $19.61\pm 0.41$
2459262.86 | r | $19.58\pm 0.09$
2459262.92 | g | $19.79\pm 0.19$
2459264.87 | r | $19.45\pm 0.07$

Observation | filter | mag
---|---|---
Date (JD) |  |
2459266.83 | i | $19.25\pm 0.21$
2459266.86 | r | $19.82\pm 0.19$
2459268.85 | r | $19.67\pm 0.12$
2459268.94 | g | $19.97\pm 0.14$
2459269.64 | o | $19.50\pm 0.37$
2459270.85 | r | $19.85\pm 0.23$
2459270.90 | g | $20.33\pm 0.32$
2459272.91 | g | $19.70\pm 0.31$
2459272.94 | r | $19.49\pm 0.24$
2459275.88 | r | $19.59\pm 0.17$
2459275.94 | g | $19.63\pm 0.21$
2459277.02 | o | $19.71\pm 0.45$
2459278.82 | g | $20.17\pm 0.13$
2459278.92 | r | $19.71\pm 0.11$
2459280.79 | r | $19.91\pm 0.10$
2459280.90 | g | $19.98\pm 0.12$
2459281.00 | o | $19.39\pm 0.33$
2459281.79 | r | $19.95\pm 0.12$
2459285.07 | o | $19.83\pm 0.51$
2459288.94 | o | $19.81\pm 0.50$
2459290.80 | g | $20.32\pm 0.12$
2459290.84 | r | $19.91\pm 0.10$
2459291.79 | g | $20.54\pm 0.16$
2459291.85 | r | $20.12\pm 0.11$
2459292.85 | r | $20.08\pm 0.11$
2459292.99 | o | $19.81\pm 0.50$
2459293.73 | r | $19.84\pm 0.13$
2459293.79 | g | $20.46\pm 0.18$
2459295.74 | r | $20.06\pm 0.20$
2459295.77 | g | $19.97\pm 0.19$
2459296.85 | r | $20.35\pm 0.27$
2459297.50 | o | $19.80\pm 0.49$
2459298.76 | g | $20.44\pm 0.37$
2459298.88 | r | $20.16\pm 0.30$
2459304.81 | r | $20.22\pm 0.31$
2459305.78 | g | $20.35\pm 0.24$
2459306.04 | o | $20.00\pm 0.60$
2459306.77 | r | $20.48\pm 0.16$
2459306.85 | g | $20.50\pm 0.21$
2459307.73 | r | $20.85\pm 0.24$
2459307.80 | g | $21.05\pm 0.26$
2459308.77 | g | $20.60\pm 0.15$
2459308.77 | r | $20.53\pm 0.16$
2459308.94 | c | $20.74\pm 1.23$
2459309.71 | g | $20.69\pm 0.21$
2459309.74 | r | $20.57\pm 0.18$
2459311.73 | g | $20.66\pm 0.24$
2459311.81 | r | $20.61\pm 0.19$
2459312.80 | r | $20.63\pm 0.19$
2459312.83 | g | $20.76\pm 0.22$
2459313.76 | g | $20.81\pm 0.21$
2459313.80 | r | $20.53\pm 0.29$
2459315.73 | r | $20.45\pm 0.15$
2459316.81 | r | $20.77\pm 0.22$
2459316.82 | g | $20.77\pm 0.17$
2459317.71 | r | $20.53\pm 0.15$
2459319.73 | r | $20.43\pm 0.20$
2459319.76 | g | $20.81\pm 0.26$
2459321.75 | g | $21.10\pm 0.25$
2459321.78 | r | $20.75\pm 0.19$
2459322.95 | o | $19.96\pm 0.58$

Table 3: Summary of ZTF $gri$-band and ATLAS $co$-band observations of SN
2020amv. ATLAS and ZTF data are binned in 3 days. The full table will be made
available in machine-readable format.

Observation | filter | mag
---|---|---
Date (JD) |  |
2458871.72 | g | $18.71\pm 0.04$
2458871.77 | r | $19.02\pm 0.04$
2458871.89 | c | $18.76\pm 0.18$
2458873.98 | o | $18.25\pm 0.11$
2458874.77 | r | $17.99\pm 0.02$
2458874.85 | g | $17.63\pm 0.01$
2458875.99 | c | $17.64\pm 0.06$
2458877.78 | r | $17.72\pm 0.02$
2458877.94 | o | $17.74\pm 0.07$
2458878.76 | g | $17.47\pm 0.02$
2458880.82 | r | $17.59\pm 0.01$
2458882.96 | o | $17.65\pm 0.06$
2458883.90 | g | $17.46\pm 0.02$
2458884.70 | r | $17.60\pm 0.03$
2458890.70 | r | $17.83\pm 0.05$
2458890.76 | g | $17.80\pm 0.04$
2458893.76 | r | $17.77\pm 0.02$
2458894.70 | g | $17.82\pm 0.02$
2458898.74 | g | $18.04\pm 0.02$
2458899.76 | r | $17.98\pm 0.02$
2458901.89 | o | $18.07\pm 0.09$
2458903.68 | r | $18.14\pm 0.02$
2458903.76 | g | $18.27\pm 0.02$
2458903.89 | c | $18.26\pm 0.11$
2458906.09 | o | $18.20\pm 0.10$
2458906.72 | r | $18.16\pm 0.02$
2458906.78 | g | $18.46\pm 0.03$
2458909.81 | r | $18.31\pm 0.03$
2458912.64 | o | $18.31\pm 0.12$
2458912.66 | g | $18.56\pm 0.04$
2458917.66 | r | $18.34\pm 0.34$
2458919.03 | o | $18.34\pm 0.12$
2458931.91 | c | $18.76\pm 0.18$
2458933.81 | o | $18.51\pm 0.14$
2458937.88 | o | $18.63\pm 0.16$
2458940.69 | g | $18.97\pm 0.06$
2458940.70 | r | $18.46\pm 0.13$
2458944.69 | r | $18.36\pm 0.05$
2458944.69 | g | $19.16\pm 0.12$
2458948.37 | o | $18.36\pm 0.12$
2458951.81 | c | $18.84\pm 0.19$
2458953.88 | o | $18.49\pm 0.14$
2458954.70 | r | $18.42\pm 0.03$
2458957.80 | o | $18.50\pm 0.14$
2458959.78 | c | $18.96\pm 0.22$
2458961.84 | o | $18.51\pm 0.14$
2458962.68 | r | $18.35\pm 0.03$
2458962.71 | g | $19.13\pm 0.04$
2458966.44 | o | $18.46\pm 0.13$
2458967.64 | r | $18.33\pm 0.03$
2458971.66 | r | $18.24\pm 0.03$
2458971.74 | g | $19.25\pm 0.11$
2458972.66 | o | $18.41\pm 0.13$
2458974.72 | r | $18.24\pm 0.04$
2458975.78 | o | $18.32\pm 0.12$
2458977.65 | g | $19.05\pm 0.09$
2458981.67 | g | $19.25\pm 0.05$

Observation | filter | mag
---|---|---
Date (JD) |  |
2458981.72 | r | $18.42\pm 0.03$
2458983.77 | c | $19.10\pm 0.25$
2458985.78 | o | $18.49\pm 0.14$
2458987.67 | r | $18.40\pm 0.03$
2458987.79 | c | $18.94\pm 0.21$
2458989.76 | o | $18.57\pm 0.15$
2458993.79 | o | $18.56\pm 0.15$
2458999.66 | r | $18.63\pm 0.09$
2459002.78 | o | $18.60\pm 0.15$
2459011.67 | r | $18.65\pm 0.07$
2459014.67 | r | $18.84\pm 0.09$
2459018.68 | r | $18.73\pm 0.06$
2459108.01 | r | $19.12\pm 0.08$
2459123.03 | r | $19.41\pm 0.10$
2459125.01 | g | $20.24\pm 0.30$
2459126.10 | o | $19.47\pm 0.36$
2459127.01 | r | $19.44\pm 0.09$
2459128.95 | g | $19.96\pm 0.27$
2459131.03 | r | $19.41\pm 0.11$
2459135.97 | g | $20.71\pm 0.20$
2459136.01 | r | $19.27\pm 0.06$
2459139.95 | g | $20.58\pm 0.22$
2459140.01 | r | $19.32\pm 0.06$
2459142.11 | o | $19.69\pm 0.44$
2459144.00 | r | $19.41\pm 0.05$
2459144.01 | g | $20.65\pm 0.13$
2459149.89 | r | $19.52\pm 0.18$
2459150.01 | g | $20.92\pm 0.22$
2459153.98 | r | $19.39\pm 0.09$
2459154.02 | g | $20.48\pm 0.31$
2459154.10 | o | $19.67\pm 0.44$
2459157.01 | r | $19.35\pm 0.09$
2459158.04 | o | $19.77\pm 0.48$
2459158.99 | g | $20.76\pm 0.37$
2459165.95 | g | $20.91\pm 0.16$
2459165.96 | r | $19.44\pm 0.05$
2459168.99 | g | $21.07\pm 0.18$
2459171.02 | r | $19.51\pm 0.06$
2459173.95 | g | $20.87\pm 0.16$
2459176.02 | r | $19.67\pm 0.33$
2459176.09 | c | $20.26\pm 0.77$
2459179.97 | g | $21.07\pm 0.22$
2459180.08 | o | $19.77\pm 0.48$
2459181.98 | r | $19.71\pm 0.11$
2459183.14 | o | $19.63\pm 0.42$
2459185.89 | r | $19.86\pm 0.17$
2459185.95 | g | $20.57\pm 0.32$
2459190.90 | i | $20.25\pm 0.26$
2459192.03 | o | $19.80\pm 0.49$
2459193.90 | r | $19.73\pm 0.08$
2459194.94 | g | $21.05\pm 0.20$
2459196.99 | r | $19.70\pm 0.10$
2459198.91 | g | $20.85\pm 0.20$
2459202.91 | r | $19.76\pm 0.09$
2459202.95 | i | $20.63\pm 0.35$
2459204.96 | g | $21.10\pm 0.18$
2459206.92 | r | $19.62\pm 0.16$

Observation | filter | mag
---|---|---
Date (JD) |  |
2459209.34 | o | $20.33\pm 0.83$
2459217.84 | r | $19.59\pm 0.10$
2459217.91 | g | $20.97\pm 0.35$
2459218.86 | o | $20.08\pm 0.64$
2459220.89 | i | $20.08\pm 0.26$
2459221.84 | g | $21.05\pm 0.18$
2459221.86 | r | $19.87\pm 0.09$
2459222.08 | o | $20.23\pm 0.75$
2459223.92 | i | $20.66\pm 0.32$
2459223.96 | c | $20.21\pm 0.73$
2459225.85 | g | $20.97\pm 0.19$
2459225.86 | r | $19.81\pm 0.08$
2459230.68 | c | $20.88\pm 1.41$
2459231.83 | r | $19.85\pm 0.12$
2459233.84 | g | $21.29\pm 0.28$
2459233.95 | c | $20.45\pm 0.93$
2459245.99 | o | $20.06\pm 0.63$
2459248.72 | g | $21.41\pm 0.29$
2459248.80 | i | $20.92\pm 0.34$
2459248.87 | r | $19.93\pm 0.09$
2459251.77 | g | $21.36\pm 0.23$
2459251.95 | r | $19.83\pm 0.11$
2459254.50 | c | $21.12\pm 1.76$
2459254.81 | g | $21.43\pm 0.23$
2459257.77 | r | $20.02\pm 0.07$
2459257.78 | i | $20.90\pm 0.34$
2459257.86 | g | $20.81\pm 0.39$
2459262.72 | r | $20.00\pm 0.11$
2459262.80 | g | $21.29\pm 0.25$
2459266.69 | i | $20.32\pm 0.35$
2459266.76 | r | $19.98\pm 0.15$
2459267.37 | o | $19.99\pm 0.59$
2459273.74 | r | $20.26\pm 0.34$
2459275.67 | g | $21.64\pm 0.40$
2459277.87 | c | $20.98\pm 1.54$
2459278.68 | r | $20.03\pm 0.11$
2459290.75 | r | $20.10\pm 0.09$
2459290.78 | i | $21.24\pm 0.40$
2459292.78 | g | $20.96\pm 0.19$
2459293.41 | o | $20.17\pm 0.71$
2459294.72 | r | $20.08\pm 0.16$
2459301.79 | r | $20.19\pm 0.33$
2459305.92 | o | $20.10\pm 0.66$
2459306.74 | r | $20.11\pm 0.10$
2459306.79 | g | $21.60\pm 0.37$
2459307.68 | i | $20.95\pm 0.39$
2459309.81 | c | $20.43\pm 0.91$
2459310.70 | r | $20.20\pm 0.12$
2459312.73 | g | $21.55\pm 0.30$
2459313.67 | i | $20.76\pm 0.32$
2459314.68 | r | $20.13\pm 0.10$
2459316.72 | g | $21.56\pm 0.29$
2459319.70 | r | $20.46\pm 0.17$
2459320.66 | o | $20.46\pm 0.94$
2459321.68 | g | $21.57\pm 0.33$
2459323.70 | r | $20.17\pm 0.20$

Table 4: Summary of ZTF $gri$-band and ATLAS $co$-band observations of SN
2020jfv. ATLAS and ZTF data are binned in 3 and 0.5 days (respectively). The
full table will be made available in machine-readable format.

Observation | filter | mag
---|---|---
Date (JD) |  |
2458975.11 | o | $17.59\pm 0.06$
2458999.09 | c | $18.53\pm 0.14$
2459003.10 | c | $18.71\pm 0.17$
2459005.06 | o | $18.06\pm 0.09$
2459018.89 | g | $19.45\pm 0.07$
2459018.96 | r | $18.33\pm 0.02$
2459021.05 | o | $18.44\pm 0.13$
2459023.06 | c | $18.82\pm 0.19$
2459024.88 | g | $19.21\pm 0.08$
2459024.98 | r | $18.47\pm 0.04$
2459025.05 | o | $18.51\pm 0.14$
2459027.87 | g | $19.48\pm 0.11$
2459027.98 | r | $18.60\pm 0.05$
2459029.04 | o | $18.49\pm 0.14$
2459030.86 | g | $19.48\pm 0.18$
2459030.97 | r | $18.63\pm 0.05$
2459031.04 | c | $19.01\pm 0.23$
2459033.05 | o | $18.47\pm 0.14$
2459033.86 | r | $18.61\pm 0.07$
2459033.95 | g | $19.48\pm 0.12$
2459036.88 | r | $18.57\pm 0.07$
2459036.98 | g | $19.65\pm 0.21$
2459036.99 | o | $18.54\pm 0.14$
2459038.05 | o | $18.49\pm 0.14$
2459042.96 | r | $18.61\pm 0.04$
2459043.09 | o | $18.55\pm 0.15$
2459045.04 | o | $18.84\pm 0.19$
2459045.86 | r | $18.73\pm 0.04$
2459045.95 | g | $19.59\pm 0.07$
2459048.90 | r | $18.78\pm 0.03$
2459049.04 | o | $18.86\pm 0.20$
2459051.03 | c | $19.39\pm 0.33$
2459051.92 | r | $18.82\pm 0.03$
2459051.96 | g | $19.76\pm 0.07$
2459053.03 | o | $19.03\pm 0.23$
2459054.96 | g | $19.67\pm 0.06$
2459057.95 | r | $18.85\pm 0.04$
2459059.00 | c | $19.25\pm 0.29$
2459060.92 | r | $18.95\pm 0.03$
2459060.96 | o | $18.97\pm 0.22$
2459061.85 | r | $18.87\pm 0.05$
2459062.47 | o | $18.90\pm 0.21$
2459062.89 | g | $19.84\pm 0.17$
2459064.93 | r | $19.00\pm 0.10$
2459064.97 | o | $19.12\pm 0.25$
2459064.98 | g | $19.66\pm 0.24$
2459069.88 | g | $19.92\pm 0.15$
2459069.90 | r | $18.93\pm 0.06$
2459071.72 | o | $19.06\pm 0.24$
2459072.86 | g | $19.83\pm 0.11$
2459075.02 | o | $19.19\pm 0.27$
2459075.81 | r | $19.12\pm 0.05$
2459075.86 | g | $19.71\pm 0.07$
2459077.06 | o | $19.07\pm 0.24$
2459077.91 | g | $19.80\pm 0.08$
2459078.85 | r | $19.10\pm 0.04$
2459078.86 | g | $19.82\pm 0.08$
2459078.97 | c | $19.31\pm 0.31$
2459079.91 | r | $19.02\pm 0.04$
2459080.98 | o | $19.14\pm 0.26$
2459081.84 | g | $19.92\pm 0.10$
2459081.88 | r | $19.12\pm 0.05$
2459082.89 | g | $19.13\pm 0.15$
2459082.94 | c | $19.73\pm 0.46$

Observation | filter | mag
---|---|---
Date (JD) |  |
2459084.88 | g | $19.96\pm 0.09$
2459084.94 | o | $19.14\pm 0.26$
2459085.92 | g | $19.83\pm 0.08$
2459086.79 | g | $19.88\pm 0.08$
2459086.87 | r | $19.14\pm 0.05$
2459087.04 | c | $19.83\pm 0.51$
2459089.83 | g | $20.23\pm 0.20$
2459089.99 | o | $19.21\pm 0.28$
2459090.85 | g | $20.06\pm 0.22$
2459091.82 | g | $20.20\pm 0.25$
2459092.90 | g | $19.55\pm 0.18$
2459098.83 | r | $19.12\pm 0.08$
2459099.01 | o | $19.29\pm 0.30$
2459099.85 | r | $19.22\pm 0.08$
2459102.10 | o | $19.17\pm 0.27$
2459104.94 | o | $19.29\pm 0.30$
2459105.89 | c | $19.78\pm 0.48$
2459106.93 | c | $19.65\pm 0.43$
2459107.80 | r | $19.17\pm 0.07$
2459107.82 | g | $20.02\pm 0.13$
2459108.90 | o | $19.39\pm 0.33$
2459110.78 | r | $19.21\pm 0.10$
2459110.82 | g | $19.92\pm 0.12$
2459110.88 | c | $20.10\pm 0.66$
2459112.87 | o | $19.34\pm 0.31$
2459113.76 | r | $19.15\pm 0.05$
2459114.89 | c | $19.80\pm 0.49$
2459116.80 | g | $19.94\pm 0.08$
2459116.82 | r | $19.23\pm 0.06$
2459117.80 | g | $19.81\pm 0.08$
2459117.87 | o | $19.33\pm 0.31$
2459118.77 | r | $19.15\pm 0.07$
2459118.86 | o | $19.11\pm 0.25$
2459120.84 | g | $20.31\pm 0.35$
2459124.75 | r | $19.40\pm 0.17$
2459126.92 | o | $19.35\pm 0.32$
2459128.77 | r | $19.29\pm 0.08$
2459128.78 | g | $19.94\pm 0.17$
2459129.38 | o | $19.40\pm 0.33$
2459130.74 | g | $19.98\pm 0.10$
2459130.76 | r | $19.20\pm 0.06$
2459131.75 | o | $19.50\pm 0.37$
2459134.71 | r | $19.38\pm 0.06$
2459134.73 | g | $20.21\pm 0.12$
2459135.02 | o | $19.25\pm 0.29$
2459136.69 | r | $19.21\pm 0.05$
2459136.75 | g | $20.10\pm 0.10$
2459136.89 | o | $19.47\pm 0.36$
2459138.78 | r | $19.33\pm 0.08$
2459138.88 | c | $20.04\pm 0.62$
2459140.69 | g | $20.10\pm 0.10$
2459140.73 | r | $19.28\pm 0.05$
2459140.84 | o | $19.44\pm 0.35$
2459142.84 | c | $19.85\pm 0.52$
2459144.82 | o | $19.24\pm 0.28$
2459145.68 | r | $19.34\pm 0.06$
2459145.73 | g | $20.02\pm 0.10$
2459146.87 | o | $19.47\pm 0.36$
2459147.77 | r | $19.17\pm 0.30$
2459151.71 | r | $19.39\pm 0.13$
2459153.61 | r | $19.34\pm 0.12$
2459153.69 | g | $19.74\pm 0.21$
2459155.67 | g | $20.25\pm 0.22$
2459155.75 | r | $19.15\pm 0.08$

Observation | filter | mag
---|---|---
Date (JD) |  |
2459155.86 | o | $19.31\pm 0.31$
2459156.70 | r | $19.43\pm 0.13$
2459158.64 | r | $19.23\pm 0.06$
2459158.67 | g | $20.26\pm 0.18$
2459158.79 | o | $19.06\pm 0.24$
2459160.85 | o | $19.22\pm 0.28$
2459164.71 | g | $19.99\pm 0.17$
2459167.67 | g | $19.99\pm 0.12$
2459167.71 | r | $19.19\pm 0.06$
2459169.65 | r | $19.19\pm 0.08$
2459169.71 | g | $20.24\pm 0.16$
2459170.79 | c | $20.42\pm 0.90$
2459171.65 | g | $20.01\pm 0.16$
2459173.61 | r | $19.16\pm 0.06$
2459173.63 | o | $19.29\pm 0.30$
2459173.75 | g | $20.03\pm 0.13$
2459178.69 | r | $18.97\pm 0.08$
2459180.62 | r | $19.00\pm 0.13$
2459180.77 | o | $19.17\pm 0.27$
2459182.61 | r | $19.08\pm 0.09$
2459182.65 | g | $19.82\pm 0.20$
2459182.78 | o | $19.21\pm 0.28$
2459184.61 | r | $19.08\pm 0.08$
2459184.66 | g | $20.43\pm 0.34$
2459185.81 | o | $19.21\pm 0.28$
2459186.65 | g | $19.95\pm 0.24$
2459186.72 | i | $19.08\pm 0.21$
2459189.61 | g | $20.09\pm 0.12$
2459189.70 | r | $19.02\pm 0.07$
2459190.75 | c | $19.93\pm 0.56$
2459192.63 | r | $19.18\pm 0.35$
2459192.70 | g | $19.68\pm 0.30$
2459192.78 | o | $19.25\pm 0.29$
2459194.64 | r | $19.08\pm 0.06$
2459194.69 | g | $20.20\pm 0.19$
2459194.76 | c | $19.86\pm 0.52$
2459195.65 | g | $20.21\pm 0.26$
2459198.73 | c | $20.03\pm 0.62$
2459199.61 | r | $19.13\pm 0.07$
2459199.69 | g | $20.21\pm 0.23$
2459202.65 | g | $20.04\pm 0.22$
2459206.71 | o | $19.53\pm 0.38$
2459208.73 | o | $19.27\pm 0.29$
2459210.60 | r | $19.17\pm 0.12$
2459211.67 | i | $19.90\pm 0.34$
2459212.71 | o | $19.37\pm 0.32$
2459215.58 | r | $19.29\pm 0.12$
2459216.07 | o | $19.15\pm 0.26$
2459217.61 | r | $19.13\pm 0.26$
2459218.75 | c | $19.71\pm 0.45$
2459219.61 | g | $20.24\pm 0.15$
2459219.63 | r | $19.05\pm 0.06$
2459219.66 | i | $20.09\pm 0.35$
2459222.24 | c | $19.96\pm 0.58$
2459223.63 | r | $19.03\pm 0.07$
2459223.63 | g | $20.17\pm 0.19$
2459224.64 | i | $19.93\pm 0.34$
2459224.74 | c | $20.28\pm 0.79$
2459227.63 | r | $19.02\pm 0.10$
2459230.60 | r | $19.08\pm 0.09$
2459231.60 | r | $18.98\pm 0.12$
2459232.60 | r | $18.92\pm 0.07$
2459233.60 | r | $19.13\pm 0.09$
2459234.72 | o | $19.30\pm 0.30$

Table 5: Summary of PGIR $J$-band observations of SN 2020jfo.a Observation Date | Rest-Frame Phase | $J$
---|---|---
(JD) | (days) | (mag)
2458635.71 | -328.22 | $>14.87$
2458640.73 | -323.28 | $>15.17$
2458648.71 | -315.44 | $>15.12$
2458669.68 | -294.82 | $>14.84$
2458676.68 | -287.94 | $>14.87$
2458682.67 | -282.05 | $>14.84$
2458866.99 | -100.81 | $>14.82$
2458896.06 | -72.22 | $>14.98$
2458913.02 | -55.55 | $>14.82$
2458964.87 | -4.56 | $>14.98$
2458967.87 | -1.61 | $>14.88$
2458976.81 | 7.18 | $15.10\pm 0.23$
2458977.83 | 8.18 | $14.64\pm 0.18$
2458980.74 | 11.04 | $14.29\pm 0.10$
2458986.74 | 16.94 | $14.15\pm 0.10$
2458993.75 | 23.83 | $14.02\pm 0.10$
2458995.68 | 25.73 | $13.96\pm 0.08$
2458997.23 | 27.26 | $14.16\pm 0.09$
2459004.77 | 34.67 | $14.15\pm 0.19$
2459011.75 | 41.53 | $14.19\pm 0.27$
2459019.72 | 49.37 | $14.10\pm 0.23$
2459177.06 | 204.08 | $>14.79$
2459223.00 | 249.25 | $>14.18$
2459249.93 | 275.73 | $>14.49$
2459269.87 | 295.34 | $>14.86$
2459280.84 | 306.13 | $>15.17$

Table 6: Summary of Swift/UVOT observations of SN 2020jfo.a Observation Date | Rest-Frame Phase | $V$ | $B$ | $U$ | $UVW1$ | $UVW2$ | $UVM2$
---|---|---|---|---|---|---|---
(MJD) | (days) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag)
2458976.62 | 1.41 | 15.00 (0.06) | 14.77 (0.04) | 14.57 (0.05) | 14.69 (0.05) | 14.79 (0.06) | 14.78 (0.05)
2458978.23 | 3.02 | 14.66 (0.06) | 14.57 (0.05) | 14.41 (0.05) | 14.65 (0.05) | 14.85 (0.06) | 14.71 (0.05)
2458978.73 | 3.51 | 14.66 (0.06) | 14.61 (0.05) | 14.43 (0.05) | 14.83 (0.05) | 15.21 (0.07) | 14.89 (0.05)
2458979.85 | 4.63 | 14.55 (0.06) | 14.51 (0.05) | 14.41 (0.05) | 14.90 (0.05) | 15.41 (0.07) | 15.12 (0.05)
2458979.92 | 4.70 | 14.63 (0.07) | 14.59 (0.05) | 14.51 (0.05) | 15.03 (0.05) | 15.51 (0.07) | 15.17 (0.05)
2458980.72 | 5.49 | 14.54 (0.05) | 14.51 (0.05) | 14.41 (0.05) | 14.98 (0.05) | 15.79 (0.07) | 15.38 (0.05)
2458982.26 | 7.02 | 14.62 (0.06) | 14.63 (0.05) | 14.57 (0.05) | 15.42 (0.05) | 16.37 (0.07) | 15.96 (0.05)
2458982.92 | 7.68 | 14.61 (0.06) | 14.55 (0.05) | 14.58 (0.05) | 15.51 (0.05) | 16.38 (0.07) | 16.09 (0.06)
2458985.17 | 9.92 | 14.77 (0.06) | 14.61 (0.05) | 14.81 (0.05) | 15.99 (0.06) | 16.88 (0.08) | 16.78 (0.06)
2458986.16 | 10.91 | 14.78 (0.08) | 14.68 (0.06) | 14.85 (0.06) | 16.21 (0.07) | 17.18 (0.09) | 17.08 (0.08)
2458994.41 | 19.11 | 14.69 (0.06) | 15.00 (0.05) | 16.27 (0.07) | 18.24 (0.12) | 19.96 (0.27) | 20.03 (0.27)
2458996.06 | 20.75 | 14.72 (0.06) | 15.09 (0.06) | 16.63 (0.08) | 18.56 (0.14) | 19.79 (0.24) | 20.41 (0.36)
2459003.26 | 27.91 | 14.82 (0.05) | 15.49 (0.05) | 17.33 (0.08) | 19.27 (0.17) |  $>$ 20.66 |  $>$ 20.80
2459007.61 | 32.24 | 14.89 (0.05) | 15.61 (0.05) | 17.70 (0.09) | 19.54 (0.19) | 20.61 (0.33) |  $>$ 20.85
2459013.59 | 38.19 | 14.97 (0.05) | 15.76 (0.05) | 18.08 (0.11) | 19.92 (0.26) |  $>$ 20.74 |  $>$ 20.84
2459018.49 | 43.07 | 14.91 (0.07) | 15.89 (0.08) | 18.55 (0.24) | 19.61 (0.32) |  $>$ 20.43 |  $>$ 20.36
2459048.91 | 73.33 | 17.45 (0.14) | 18.44 (0.19) |  $>$ 20.05 |  $>$ 20.62 |  $>$ 20.87 |  $>$ 21.00
2459059.83 | 84.19 | 17.29 (0.13) | 18.64 (0.23) |  $>$ 20.02 |  $>$ 20.58 |  $>$ 20.88 |  $>$ 20.99
2459064.91 | 89.24 | 17.55 (0.16) | 18.86 (0.27) |  $>$ 20.03 |  $>$ 20.59 |  $>$ 20.90 |  $>$ 20.95
2459070.00 | 94.31 | 17.46 (0.20) | 18.75 (0.25) |  $>$ 20.01 |  $>$ 20.61 |  $>$ 20.89 |  $>$ 20.81
2459168.08 | 191.88 |  $>$ 18.06 |  $>$ 19.08 |  $>$ 19.85 |  $>$ 20.49 |  $>$ 20.72 |  $>$ 20.77
2459195.53 | 219.19 |  $>$ 18.52 |  $>$ 19.19 |  $>$ 19.91 |  $>$ 20.54 |  $>$ 20.82 |  $>$ 20.91
2459207.91 | 231.50 |  $>$ 18.81 |  $>$ 19.43 |  $>$ 20.15 |  $>$ 20.67 |  $>$ 20.95 |  $>$ 21.02
2459217.75 | 241.29 | 18.73 (0.34) |  $>$ 19.46 |  $>$ 20.18 |  $>$ 20.64 |  $>$ 20.99 |  $>$ 21.04
2459228.02 | 251.51 |  $>$ 18.84 |  $>$ 19.45 |  $>$ 20.21 |  $>$ 20.66 |  $>$ 20.99 |  $>$ 21.07

Table 7: Summary of Spectroscopic Observations

Object | UT Observation Date | Rest-Frame Phase | Telescope+Instrument
---|---|---|---
| (YYYY MM DD) | (days) |
SN 2020jfo | 2020 May 06 | 1.21 | LT+SPRAT
SN 2020jfo | 2020 May 06 | 1.22 | NOT+ALFOSC
SN 2020jfo | 2020 May 07 | 1.66 | P60+SEDM
SN 2020jfo | 2020 May 07 | 2.22 | LT+SPRAT
SN 2020jfo | 2020 May 08 | 3.22 | LT+SPRAT
SN 2020jfo | 2020 May 09 | 3.49 | P60+SEDM
SN 2020jfo | 2020 May 10 | 4.64 | P60+SEDM
SN 2020jfo | 2020 May 15 | 9.56 | P60+SEDM
SN 2020jfo | 2020 May 17 | 11.50 | Lick+Kast
SN 2020jfo | 2020 May 25 | 19.48 | Lick+Kast
SN 2020jfo | 2020 May 25 | 19.53 | P60+SEDM
SN 2020jfo | 2020 May 29 | 23.44 | Lick+Kast
SN 2020jfo | 2020 Jun 12 | 37.31 | P60+SEDM
SN 2020jfo | 2020 Jun 17 | 42.36 | P60+SEDM
SN 2020jfo | 2020 Dec 04 | 211.63 | P60+SEDM
SN 2020jfo | 2020 Dec 07 | 214.40 | NOT+ALFOSC
SN 2020jfo | 2020 Dec 22 | 229.63 | P60+SEDM
SN 2020jfo | 2021 Jan 16 | 254.17 | NOT+ALFOSC
SN 2020jfo | 2021 Feb 04 | 273.33 | P60+SEDM
SN 2020jfo | 2021 Feb 09 | 277.93 | NOT+ALFOSC
SN 2020jfo | 2021 Mar 09 | 305.77 | NOT+ALFOSC
SN 2020jfo | 2021 Apr 20 | 348.41 | NOT+ALFOSC
SN 2020amv | 2020 Jan 24 | 1.67 | P60+SEDM
SN 2020amv | 2020 Jan 28 | 5.42 | P60+SEDM
SN 2020amv | 2020 Jan 29 | 6.59 | GeminiN+GMOS
SN 2020amv | 2020 Jan 31 | 8.12 | P60+SEDM
SN 2020amv | 2020 Feb 02 | 9.84 | NTT+EFOSC2aaFluxes with S/N $<3\sigma$ are shown as upper limits. These magnitudes are Vega tied to 2MASS, and $m$(AB) $-$ $m$(Vega) = 0.91. Fluxes with S/N $<3\sigma$ are shown as upper limits.
SN 2020amv | 2020 Feb 04 | 12.03 | P60+SEDM
SN 2020amv | 2020 Feb 06 | 13.89 | P60+SEDM
SN 2020amv | 2020 Sep 19 | 230.37 | P60+SEDM
SN 2020amv | 2020 Sep 27 | 238.00 | P60+SEDM
SN 2020amv | 2020 Nov 18 | 287.67 | P200+DBSP
SN 2020amv | 2021 Jan 23 | 350.61 | NOT+ALFOSC
SN 2020amv | 2021 Feb 14 | 372.35 | NOT+ALFOSC
SN 2020amv | 2021 Apr 07 | 421.58 | Keck+LRIS
SN 2020jfv | 2020 Jun 20 | 50.55 | P60+SEDM
SN 2020jfv | 2020 Jun 24 | 54.42 | P60+SEDM
SN 2020jfv | 2020 Dec 01 | 211.58 | P60+SEDM
SN 2020jfv | 2020 Dec 06 | 217.18 | NOT+ALFOSC
SN 2020jfv | 2020 Dec 07 | 217.45 | P60+SEDM
SN 2020jfv | 2021 Jan 14 | 254.89 | Keck+LRIS
SN 2020jfv | 2021 Jul 08 | 426.89 | NOT+ALFOSC

aafootnotetext: Uploaded from TNS from ePESSTO.

Table 8: Supernova light-curve properties SN Name | $t^{\rm{rise}}_{g}$ | $t^{\rm{rise}}_{r}$ | $t^{\rm{rise}}_{i}$ | $m^{\rm{peak}}_{g}$ | $m^{\rm{peak}}_{r}$ | $m^{\rm{peak}}_{i}$ | $M^{\rm{peak}}_{g}$ | $M^{\rm{peak}}_{r}$ | $M^{\rm{peak}}_{i}$
---|---|---|---|---|---|---|---|---|---
|  | (rest-frame days past explosion) |  |  | (mag) |  |  | (mag) |
SN 2020amv | 8.99 | 14.16 | 16.46 | 17.38 | 17.57 | 17.62 | -19.23 | -19.04 | -18.99
SN 2020jfo | 4.68 | 4.87 | 5.97 | 14.38 | 14.37 | 14.65 | -16.49 | -16.50 | -16.22
SN 2020jfv | – | – | – | $<18.53$ | 17.71 | 17.28 | -15.95 | -16.77 | -17.22
  *[ZTF]: Zwicky Transient Facility