# Peculiar Spectral Evolution of the Type I Supernova 2019eix: A Possible
Double Detonation from a Helium Shell on a Sub-Chandrasekhar-mass White Dwarf

E. Padilla Gonzalez Department of Physics, University of California, Santa
Barbara, CA 93106-9530, USA  Las Cumbres Observatory, 6740 Cortona Dr, Suite
102, Goleta, CA 93117-5575, USA  D.Andrew Howell Department of Physics,
University of California, Santa Barbara, CA 93106-9530, USA  Las Cumbres
Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575, USA  J. Burke
Department of Physics, University of California, Santa Barbara, CA 93106-9530,
USA  Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA
93117-5575, USA  Yize Dong (董一泽) Department of Physics and Astronomy,
University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA  D.
Hiramatsu Department of Physics, University of California, Santa Barbara, CA
93106-9530, USA  Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta,
CA 93117-5575, USA  C.McCully Department of Physics, University of California,
Santa Barbara, CA 93106-9530, USA  Las Cumbres Observatory, 6740 Cortona Dr,
Suite 102, Goleta, CA 93117-5575, USA  C.Pellegrino Department of Physics,
University of California, Santa Barbara, CA 93106-9530, USA  Las Cumbres
Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575, USA  W.
Kerzendorf Department of Physics and Astronomy, Michigan State University,
East Lansing, MI 48824, USA  Department of Computational Mathematics, Science,
and Engineering, Michigan State University, East Lansing, MI 48824, USA  M.
Modjaz Department of Astronomy, University of Virginia, Charlottesville, VA
22904, USA  G. Terreran Las Cumbres Observatory, 6740 Cortona Dr, Suite 102,
Goleta, CA 93117-5575, USA  M. Williamson Department of Physics, New York
University, New York, NY 10003, USA

(Received Soon; Revised Soon; Accepted Soon)

###### Abstract

We present photometric and spectroscopic data for the nearby Type I supernova
(SN Ia) 2019eix (originally classified as a SN Ic), from its discovery day up
to 100 days after maximum brightness. Before maximum light SN 2019eix
resembles a typical SN Ic, albeit lacking the usual O I feature. Its
lightcurve is similar to the typical SN Ic with decline rates of ($\Delta
M_{15,V}=0.84$) and absolute magnitude of $M_{V}=-18.35$. However, after
maximum light this SN has unusual spectroscopic features, a large degree of
line blending, significant line blanketing in the blue ($\lambda<5000$Å), and
strong Ca II absorption features during and after peak brightness. These
unusual spectral features are similar to models of sub-luminous thermonuclear
explosions, specifically double-detonation models of SNe Ia. Photometrically
SN 2019eix appears to be somewhat brighter with slower decline rates than
other double detonation candidates. We modeled the spectra using the radiative
transfer code TARDIS using SN 1994I (a SN Ic) as a base model to see whether
we could reproduce the unusual features of SN 2019eix and found them to be
consistent with the exception of the O I feature. We also compared SN 2019eix
with double detonation models and found them to match the observations of SN
2019eix best, but failed to reproduce its full photometric and spectroscopic
evolution.

supernovae, SN 2019eix - supernovae

††journal: ApJ††facilities: Las Cumbres Observatory (Sinistro), FTN (FLOYDS),
Zwicky Transient Facility (ZTF)††software: astropy (Astropy Collaboration et
al., 2013, 2018), TARDIS (Kerzendorf & Sim, 2014), SESNtemplate:
https://github.com/nyusngroup/SESNtemple, Nugent’s Spectral Templates:
https://c3.lbl.gov/nugent/nugent_templates.html

2212-

##  1 Introduction

As the number of supernova discoveries has grown over time, previously rare or
unseen supernovae are being found in increasing numbers. Often their
classification can be unclear and their properties may overlap between
different classes, making these SNe unique. Consequently, new discoveries have
shed light on the diversity of supernovae, and all-sky surveys have played a
crucial role in this. For instance, all-sky surveys have discovered
interacting SNe with different Circumstellar Material (CSM) compositions,
including SNe Ibn (Hosseinzadeh et al., 2017) and SNe Icn (Gal-Yam et al.,
2022; Pellegrino et al., 2022). Similarly, even the “well understood” SNe Ia
observations have shwown peculiarity in observations that point to diverse
explosion mechanisms. These include “.Ia” explosions of a helium flash on a
white dwarf (Kasliwal & Kulkarni, 2012; Poznaski et al., 2010), “Sub-
Chadrashekar” explosion of He burning followed by second detonation in the
core via converging shocks (Fink et al., 2010; De et al., 2019), “Super-
Chandrasekhar” explosions of rapidly spinning WD whose limiting mass could
exceed the $1.4\rm M\odot$ (Howell et al., 2006; Contreras et al., 2010;
Chakradhari et al., 2014; Parrent et al., 2016), and SNe Ia CSM, explosions of
SN Ia whose material interacts with the CSM (Raskin & Kasen, 2013; Shen et
al., 2013).

The progenitor channel that gives rise to SNe Ia is a subject of controversy,
with the two primary competing scenarios being the single-degenerate and
double-degenerate models. In the single degenerate channel, a White Dwarf (WD)
accretes matter from a main sequence or neutron star (Whelan & Iben, 1973). As
the WD accretes matter from its companion star, it grows in mass until it
approaches a critical limit known as the Chandrasekhar mass. At this point, a
runaway nuclear fusion reaction is triggered in the carbon and oxygen-rich
material, leading to a powerful thermonuclear explosion (Arnett, 1969; Nomoto
et al., 1984).

In the double degenerate channel two white dwarfs merge and explode (Iben &
Tutukov, 1984). Whether or not supernovae are triggered by approaching the
Chandrasekhar mass to ignite carbon is debated. Additionally, there exists an
alternative explosion mechanisms for WDs by which a WD accretes sufficient He
mass (from a He-rich companion) and undergoes ignition, leading to a
detonation from the He layer. The detonation of a pure He-shell produces fast
and faint transients and they have been used to explain these Calcium-rich
transients (Perets et al., 2010). Various concerns have been studied,
including the conditions for the detonation of the He shell and whether the He
detonation can trigger a detonation in the underlying Carbon and Oxygen (CO)
core (referred to as the double-detonation scenario, see e.g. Bildsten & Shen,
2007; Fink et al., 2010; Sim et al., 2012; Shen & Bildsten, 2014; Shen et al.,
2018).

Subsequently, various studies have also explored the characteristics of these
double detonation events by varying the mass of the He-shell and the CO WD
mass (Kromer et al., 2010; Sim & Röpke, 2010; Sim et al., 2012; Polin et al.,
2019). In double-detonation simulations, the models which approach the
limiting case of a bare CO WD detonation are capable of roughly reproducing
observations of SNe Ia (Sim & Röpke, 2010; Shen et al., 2018). In contrast,
the observables are much different in models which have a thicker He-shell due
to the He-rich burning products in the outer ejecta (Sim & Röpke, 2010; Shen
et al., 2010; Kromer et al., 2010; Polin et al., 2019). As a result, the main
spectroscopic observable for these events is a strong suppression in the blue
regime. OGLE-2013-SN-079, SN 2016hnk111although whether it is a double
detonation is still debated (Galbany et al., 2019), SN 2016dsg, and 2018byg
are all of the double detonation candidates available in literature and from
the sample all candidates display such depletion (Inserra et al., 2015;
Jacobson-Galan et al., 2020; De et al., 2019; Dong et al., 2022).
Interestingly, these handful of candidates with the exception of SN 2016hnk
have all been found in the outskirts of their host galaxies (with a host type
of E or S0; De et al., 2019; Dong et al., 2022; Inserra et al., 2015),
suggesting an older stellar population progenitor for these events.

Subluminous thermonuclear explosions, a subclass of SNe Ia that are 1 to 2
magnitudes dimmer than normal Type Ia’s, have been more difficult to
understand. Hachinger et al. (2009) confirmed that nuclear burning in dim,
91bg-like SNe Ia stops at less advanced stages compared to normal SNe Ia.
Despite this finding, the explosion mechanism of subluminous supernovae
remains a topic of debate. Some proposed models for subluminous Type Ia
supernovae include explosions of sub-Chandrasekhar mass WDs (Fink et al.,
2010) or WDs that undergo a partial deflagration followed by a delayed
detonation (Hoeflich et al., 2002; Mazzali et al., 2007).

Similar to SNe Ia, SNe Ib/c also show a wide range of physical properties.
They are core-collapse explosions of massive stars $(>8M\odot)$ whose
outermost layers have been partly or completely stripped of Hydrogen and
Helium (Clocchiatti et al., 1997). A wide variety of SNe Ib/c exist (for a
review on CCSNe, see Filippenko 1997 and Modjaz et al. 2019). Their luminosity
and kinetic energy can range more than an order of magnitude, as seen from the
overluminous SN 1998bw to the subluminous SN 2004aw (Drout et al., 2011;
Mazzali et al., 2009). In addition to the significant luminosity range, the
light curves and spectra also are considerably diverse (Liu et al., 2016;
Fremling et al., 2018; Williamson et al., 2019; Barbarino et al., 2021). SN
1994I is an example of a normal Type Ic spectroscopically, but stands out due
to its fast decline in the R-filter magnitude from maximum time to 15 days
later ($\Delta m_{\rm 15,R}=1.5$ mag, Drout et al., 2011; Bianco et al., 2014;
Prentice et al., 2016). Furthermore, some SNe Ic are characterized by broad
features (Type Ic-BL). This type is sometimes associated with gamma-ray bursts
(GRBs); see Woosley & Bloom (2006) for a review. SNe Ic-BL are
spectroscopically distinct in terms of their extreme velocity values that
cause the spectral features to blend (Modjaz et al., 2016).

In this paper, we discuss the peculiar object SN 2019eix. This object is
unique because it has many similarities to two distinct classes (e.g.,
classes: Type Ic and peculiar Type Ia). We will model this object to attempt
to find its progenitor and explosion mechanism, more specifically whether it
is a core-collapse or a thermonuclear event. The paper is arranged as follows.
In Section 2, we discuss the discovery and observations. In Section 3, we
discuss reddening, light curve and spectral analysis in detail. In Section 4,
we model the spectra in two different ways: to replicate the core-collapse SN
Ic scenario we directly model the spectra using the radiative transfer code
TARDIS (Kerzendorf & Sim, 2014; Kerzendorf et al., 2022a) and to replicate the
double-detonation SN Ia scenario we compare the spectra and light curve models
from Kromer et al. (2010); Sim et al. (2012); Polin et al. (2019) to SN
2019eix. Additionally, we use Arnett’s model to infer masses of ejecta and
56Ni, to better understand the progenitor of the supernova. Finally, in
Section 5 we discuss the possible physical origins and classification of SN
2019eix and conclude in Section 6.

##  2 Discovery and Observations

###  2.1 Discovery

SN 2019eix was discovered by ATLAS on 2019, May 1.59 UT, using the instrument
ACAM1 at Haleakalā with a discovery magnitude of 17.0 in the cyan filter
(Tonry et al., 2019). The last non-detection of the same object was on 2019,
April 21.63 UT with a magnitude of 18.39 (orange filter) from the same
telescope. A classification report was provided about a week after its
discovery (2019, May 7th), with a classification spectrum taken on 2019, May
5th with the 3 meter Lick Shane reflector telescope suggesting that SN 2019eix
was a Type Ic supernova. Due to the lack of early photometry the explosion
epoch 2458600 JD was adopted throughout the paper assuming a rise time of 16
days to its maximum light in the r band. A typical Ic rise time falls between
13-16 days (Taddia et al., 2017) and SN 2019eix appears to be on the wide side
for SNe Ic as shown in Figure 6 thus the choice for 16 days (and the fact that
SN 2019eix has similar lightcurves to SNe Ic). The classification spectrum was
downloaded from the Transient Name Server (TNS)222https://www.wis-tns.org/ and
was included in our analysis. The supernova is located at right ascension
$18^{h}42^{m}42^{s}.89$ and declination $+40^{\circ}22^{\prime}07".8$, and is
situated in the host galaxy NGC 6695 with a redshift of $z=0.018303$
(Corrales, 2015). In Figure 1, we show an image of the field of view,
location, and host galaxy of SN 2019eix in the gri bands taken with the 1-m
telecope of Las Cumbres Observatory (LCO; Brown et al., 2013) on 2019, May
4th. The distance of the galaxy NGC 6695 is $\approx$83.80 Mpc according to
the NASA Extragalactic Database (NED)333https://ned.ipac.caltech.edu/ where
the Tully-Fisher method was used to calculate the luminosity distance
(Theureau et al., 2007). The Milky Way extinction value $E(B-V)=0.0604$ was
adopted from the Schlafly & Finkbeiner (2011) calibration of the Schlegel &
Finkbeiner (1998) dust maps.

Figure 1: gri band staked image of the host of SN 2019eix. The location of the
SN is marked in red crosshair.

###  2.2 Photometry

After the discovery, the Global Supernova Project obtained photometry in BVgri
bands using the 1-m telescopes of LCO . We reduced the data using lcogtsnpipe
(Valenti et al., 2016). PSF-fitting photometry was performed and the zero
points were calculated from the Landolt standard catalogue (Landolt, 1992) for
the BV filters. Additonally, the gri filter zeropoints were calculated by
incorporating the Sloan magnitudes of field stars (Albareti et al., 2017).
Besides LCO data, the public Zwicky Transient Facility
(ZTF)444https://alerce.online data including their upper limits in the g and r
bands, were used for comparison and are plotted in Figure 2.

Additional photometric information is presented in Table 1, where it shows the
time of maximum, the peak absolute magnitude, and the decline rates for SN
2019eix. We measured the peak magnitude by fitting a polynomial through the
data points and used their maximum value. For the B filter where rising data
is not available, we used the peak magnitude from the B band data as an
estimate of the maximum light. A comparison of $\Delta m_{15}$ with typical
supernovae against SN 2019eix is shown in Table 2, where we can see that SN
2019eix’s decline rates fall within the average values for SNe Ic but are
somewhat faster in the r and i band (but not unprecedented, see Taddia et al.,
2017).

Figure 2: Milky Way extinction corrected photometry using $E(B-V)_{\rm{MW}}=0.0604$ as discussed in Section 2. These data are not corrected for host extinction (see Section 3.1 for details on host extinction). The data was obtained from LCO and ZTF, where the data points marked with downward arrows represent the upper limits obtained from ZTF.  Filter | $t_{\rm{max}}(\rm{JD})$ | $M_{\rm{max}}$ | $\Delta m_{15}$
---|---|---|---
B | 2458609.8 | -17.2 | 1.2
g | 2458607.8 | -18.12 | 1.0
V | 2458612.9 | -18.35 | 0.84
r | 2458615.5 | -18.63 | 0.95
i | 2458616.35 | -18.33 | 0.72
Table 1:  Time of maximum light, absolute magnitude, and decline rate ($\Delta m_{15}$, the drop in magnitudes between peak and 15 days after peak) for SN 2019eix in each filter of its lightcurve.  Filter | 19eix | IIb | Ib | Ic
---|---|---|---|---
B | – | 1.37(0.18) | 1.52(0.19) | 1.23(0.59)
g | 1.0 | 1.10(0.10) | 1.25(0.19) | 1.11(0.45)
V | 0.84 | 0.93(0.08) | 1.03(0.19) | 0.90(0.22)
r | 0.95 | 0.67(0.08) | 0.75(0.21) | 0.62(0.24)
i | 0.72 | 0.53(0.05) | 0.57(0.16) | 0.53(0.17)
Table 2:  $\Delta m_{15}$ per filter for SN 2019eix compared to average values
for different SN types, the values in parenthesis correspond to the error.
Values were obtained from Taddia et al. (2017).

###  2.3 Spectroscopy

As part of the Global Supernova Project follow-up, LCO obtained a
spectroscopic series using the FLOYDS spectrograph mounted on the 2m Faulkes
Telescope North in Haleakalā, Hawai’i. The spectra were reduced as described
in Valenti et al. (2013). We also use the classification spectrum posted in
TNS (described above); the KAST 3m Lick Shane Reflector took the spectrograph
on 2019, May 5.0 UT. All spectra were corrected for Milky Way reddening
($E(B-V)=0.0604$) and host corrected by measuring the equivalent width of the
NaID doublet as described in section 3.1 and shown in Figure 3.

Figure 3:  Spectral time series of SN 2019eix. Epochs with respect to V band
maximum are included on the right. The dashed lines correspond to the
absorption features listed on the top of the figure. The spectra have been
corrected for both Milky Way and host extinction (see Section 3.1 for details
of host extinction). The last spectrum in the series has an unusual flat-
bottomed Ca feature, discussed more in Section 3.2.

##  3 Analysis

###  3.1 Reddening and Light Curves

We investigated two techniques for determining host galaxy reddening which
produced different results as shown in Table 3. In the first method, the
equivalent width of the sodium line Na ID was measured to be $EW_{\rm
NaID}=0.139[\rm\AA]$ from IRAF’s555https://iraf.net/ task splot, using the
equation $A_{\rm V}^{\rm host}$ = 0.78($\pm$ 0.15)$\times$ $EW_{\rm
NaID}[\rm\AA]$ from Stringer et al. (2018). This gives $A_{\rm V}^{\rm
host}=0.04\pm 0.15$ and an excess value of E(B-V) of 0.013 $\pm$ 0.002
assuming a reddening law of $R_{\rm v}$ = 3.1.

The second method dealt with comparing the color curves with a template of a
typical Type Ic SN (Taddia et al., 2017). We considered using this method due
to the similarities with Type Ic. Host-galaxy reddening values are estimated
through the comparison of observed optical and NIR colors to intrinsic color-
curve templates constructed from sub-samples of minimally-reddened CSP-I
Stripped Envelope SNe (Stringer et al., 2018). The templates can be found on
the Carnegie Supernova Project (CSP) page
666https://csp.obs.carnegiescience.edu/data/. Figure 4 shows the color curve
B-V of SN 2019eix and the SN Type Ic template against time. The template is
not an appropriate template for the data as it does not match the shape of
curve due to its peculiarity, it is easier shown when the template is raised
by four standard deviations to “match” the SN 2019eix color data. In Figure 5,
we see that the color evolution of SN 2019eix is inconsistent with SN Ic,
meanwhile it exhibits a notable agreement with SN 2016hnk (a double detonation
candidate).

The color excess from the weighted difference between the two, is E(B-V) =
0.96. This value is inconsistent with the one measured from the equivalent
width of the NaID absorption feature E(B-V) = 0.013. In addition, studies have
shown that the V-R for a SNe Ic is about $(V−R)_{\rm V10}$ = 0.26 $\pm$ 0.06
mag where the $V_{\rm 10}$ corresponds to 10 days after V band maximum (Drout
et al., 2011) in contrast SN 2019eix has a value of $(V−R)_{\rm V10}$ =
0.3612. This points to the possibility that SN 2019eix could be intrinsically
more red than a normal SN Ic. This supernova is inconsistent with a reddened
normal Type Ic, so we conclude that the red color is primarily from the
intrinsic red color of this peculiar event. We therefore adopted the
extinction value from the sodium line to correct for reddening.

Figure 6 shows the lightcurves of SNe Ib, Ic, and Ia (with sub-types such as
91bg and double-detonation candidates) along with SN 2019eix in the B , r, and
i band. All SNe are Milky Way and host extinction corrected. We observe that
the peak absolute magnitude and shape is most similar to Type Ic’s including
SN 1999ex (Stritzinger et al., 2002) and various others in gray. SN 2016dsg (a
thermonuclear He-shell detonation candidate, Dong et al., 2022) and SN 1991bg
(a subluminous SN Ia, Leibundgut et al., 1993) are the closest in peak
magnitude, particularly in the i band (although fainter), with faster evolving
light curves than SN 2019eix. For instance, SN 2016dsg has an i band decline
rate around 0.077(0.003) mag/day (or $\Delta m_{15}$ = 1.15), as opposed to
$\Delta m_{15}$ = 0.72 for SN 2019eix. Generally, from Figure 6 we see that
SNe Ib/c appear to be a better match to SN 2019eix than the thermonuclear SNe
in terms of their lightcurve.

Figure 4: Comparison between the color evolution of B-V of SN 2019eix and the Type Ic template. This comparison shows that the template is not a good fit for the color evolution of SN 2019eix. The blue line represents a less likely template which describes the data better, but is still a poor fit.  Figure 5: The color evolution for SN 2019eix in black along with many SNe Ic (1994I, 2004aw, 2007gr,2004fe, 2004gt) in pink, and sub-luminous Type Ia SNe (91bg, 2005bl, 2016hnk, and 2016dsg) in turquoise were plotted for comparison. Note the difference between SN 2019eix and the SNe Ic. SN 2019eix is much redder than the typical Ic and seems to be more comparable to SN 2016hnk (a double-dentonation candidate).  Method | E(B-V) | $\sigma_{E(B-V)}$
---|---|---
Na ID | 0.013 | 0.002
Color | 0.96 | 0.06
Table 3: The color excess values for the host using two methods, the
equivalent width of the Na ID line listed as “Na ID” or the weighted
subtraction from the color curve of the data and the template listed on the
table as “template”. Figure 6: Absolute magnitude of SN 2019eix and other Type
Ib, Type Ic, and Sub-luminous Type Ia SNe are plotted in the B, r, and i
bands. All SNe have been extinction corrected. In this figure the gray (we
chose to make some of these SNe in gray for visibility purposes obtained from
the Open Supernova Catalogue) and the red/pink lines represent SNe Type Ib/c.
We can see how SN 2019eix declines faster than the cobalt decay line and is
most similar to Type Ib/c. The double-detonation candidates SN 2016hnk
(labeled with “?” as it is highly debated whether it’s a double detonation;
Gal-Yam, 2016), OGLE-2013-SN079 and SN 2016dsg, in the bluer colors appear to
be fainter and decline faster than SN 2019eix along with SN 91bg (a
sublumnious Type Ia). Generally we notice that SN 2019eix is more similar to
Type Ib/c than the double-detonation candidates in terms of light curves.

###  3.2 Spectral Analysis

The striking features of the spectral sequence of SN 2019eix are concentrated
in the suppression of the blue color $\lambda<5000$Å, the absence of O I
feature (This is peculiar for SNe Ic which have been shown to generally have a
strong O I 7774 feature, however an exception has been observed; Fremling et
al., 2018; Williamson et al., 2022), and the strong evolution of Ca II
feature. In Figure 7, we plot SN 2019eix along with various types of SNe that
match spectroscopically at different epochs. We see that the early spectrum at
-5 days before maximum matches SNe Ic (e.g. SN 1994I and SN 2007gr; Gal-Yam,
2016) with the exception of the lack of an O I absorption feature. We also
compared SN 2019eix with SN 2019ewu, a peculiar Type Ic supernova that, like
SN 2019eix, lacks an O I absorption feature. However, SN 2019ewu displays
distinct characteristics at this epoch, including an extreme blue continuum
and high absorption velocities, which are not typically observed in SNe Ic
(Williamson et al., 2022). These peculiarities are outlined and discussed in
greater detail in Section 3.4. We refrain from comparing SN 2019ewu at later
phases to SN 2019eix due to the significant observational differences already
mentioned and the reappearance of the O I feature in the SN 2019ewu spectra.

At -5 days we see strong emission features in the blue between 4000 to 5000
$\rm\AA$ and a less pronounced Si II feature, much less pronounced than the
91bg-like SN 2005bl. Moreover, as demonstrated in Figure 7, the
characteristics observed between 5500 and 6500 $\rm\AA$ do not align with
those of SNe Ic. Furthermore, as indicated in Figure 12, this particular
feature is primarily attributed to Si II. Generally, before maximum SN 2019eix
looks like SN 2005bl, but the similarities are not as apparent as they are to
SNe Ic.

At maximum light, the 3500 to 5000 $\rm\AA$ region becomes suppressed making
it peculiar compared to the average Ic. SN 2019eix appears to transition in
appearance to SN 2018byg a double-detonation candidate and to the fast moving
Type Ic PTF12gzk (although the photospheric velocity is much slower than
PTF12gzk, having photospheric velocities of $\rm v_{ph}$ $\approx$ 10,500 km/s
and $\rm v_{ph}$ $\approx$ 15,300 km/s at peak, respectively; Ben-Ami et al.,
2012). Around 13 days after maximum, the suppression in the blue along with
the absence of O I feature persists. We observe PTF12gzk to evolve like SN
2019eix up until this epoch in terms its general shape (initially resembling a
Type Ic and evolving with suppressed features in the blue). Nevertheless, we
also noticed key differences including a O I feature, a smoother profile, and
a large blueshift most apparent in the Ca II and Si II absorption features in
PTF12gzk. Similarly, the double-detonation candidate SN 2016hnk and SN 2005bl
(a 91bg-like) also appear to have strong resemblance (however, SN 2005bl does
not have as strong of a suppression in the blue), with the exception of the O
I feature.

The epochs 22 and 32 days after maximum of SN 2019eix seem to resemble 91bg-
like and double-detonation Ia’s as shown in Figure 7, but with even stronger
and more blended features. Additionally, the Ca II feature becomes so broad
and large that it even appears to be saturated at 33 days after maximum. We
also notice a weak O I feature previously not observed in the earlier spectra.

The spectral evolution presented in Figure 7 does not fully match a typical
Type Ic. This is illustrated further when plotting SN 2019eix spectra with the
mean spectra of SN Ic illustrated in Figure 8. The templates were used from
the NYU SN group github
page777https://github.com/nyusngroup/SESNtemple/tree/master/MeanSpec/ (Modjaz
et al., 2016). The grey shaded area represents one standard deviation from the
mean at various epochs. These standard deviation regions quantify the spectral
diversity within the SN Ic class and facilitate the identification of
anomalous features in SN 2019eix. From Figure 8, the early spectrum is near
the mean Type Ic but not at later epochs. SN 2019eix reaches the upper limits
of the standard deviation from the mean template and in some cases falls
outside the one standard deviation regime. This along with the lack of O I
7774 feature shows that SN 2019eix is not a typical SN Ic spectroscopically.

Figure 7: Spectral evolution of SN 2019eix compared with other types of
supernovae, including Type Ic and subluminous Ia. The light blue dash lines
show the absorption features created by the respective line labeled at the top
of the figure. Note the similarities of SN 2019eix at early epochs to Type
Ic’s and how it transitions to looking more like a sub-luminous Ia (including
91bg-likes, and He-shell double-detonations (DD) candidates at later times.
The top left panel shows SN 2019eix at early epochs before maximum along with
various Type Ic. The bottom left panel shows SN 2019eix at maximum light and
both of the right panels are after maximum light showing a better match to
subluminous SNe Ia.  Figure 8: Spectral evolution of SN 2019eix compared with
the mean spectra of Type Ic on the left and Ic-BL on the right. The light grey
corresponds to one standard deviation from the mean. Although SN 2019eix is
not a good match to the templates at later epochs, we do see that before
maximum it agrees with the mean Type Ic (on the left panels). After maximum
light the shape of SN 2019eix agrees slightly better with Ic-BL than a Ic, but
the features for SN 2019eix are sigificantly stronger and more suppressed in
the blue. In general, we observed that SN 2019eix is well out of the norm as
it is much redder in comparison to both mean spectra for Type Ic an Ic-BL.

###  3.3 Pseudo-Bolometric Luminosity

To estimate the pseudo-bolometric luminosity we used the technique detailed in
Howell et al. (2009). We warped spectral templates for Type Ic SN found from
Peter Nugent’s page888https://c3.lbl.gov/nugent/nugent_templates.html so that
it matched our photometry. We integrated the warped template to get the pseudo
bolometric flux at a given epoch where we had data. A bolometric factor was
applied in the end to account for the missing flux for those filters with no
data and a bolometric luminosity was computed. We also made an effort to
determine the bolometric luminosity using a subluminous Type Ia template.
Nonetheless, the resulting luminosity showed only a slight alteration,
increasing from $3.5\times 10^{42}$ to $3.6\times 10^{42}$. As a result, we
decided that the estimate obtained using the Type Ic template was more
appropriate and opted to use it in our analysis.

The bolometric luminosity of SN 2019eix is plotted along other SNe Types as
shown in Figure 9. The luminosity for Type Ib/c SNe was acquired from the
Carnegie supernova project’s
website999https://csp.obs.carnegiescience.edu/data. For the subluminous Type
Ia comparison, we used the double-detonation candidates including SN 2016hnk
from Jacobson-Galan et al. (2020) and OGLE13-079 from Inserra et al. (2015);
although whether SN 2016hnk it’s a double detonation or a 91bg-like it is
still debated Galbany et al. (2019). For the other subluminous Type Ia case we
used SN 2005bl, a 91bg-like from Taubenberger et al. (2008). From Figure 9, we
can see that the maximum bolometric luminosity for SN 2019eix is about
$3.5\times 10^{42}$ erg/s/$\rm cm^{2}$/$\AA$. Comparing SN 2019eix with the
various types of SNe in this plot, we see that it is a better match to Type
Ic. However, SN 2019eix is more luminous than most Type Ic; while much
brighter than a subluminous SN Ia.

Figure 9: Bolometric luminosity of SN 2019eix (in black) plotted with various
core-collapse SNe and thermonuclear events. We plotted Type Ic, Ib, Ic-BL, and
Sub-luminous Ia in colors pink, coral, dark blue, and turquoise respectively.
We note that SN 2019eix is a better match to the bolometric luminosities of
SNe Ib/c.

###  3.4 Photospheric Velocities

An essential property of the physics of the ejecta used to measure important
physical parameters such as the ejecta and 56Ni mass, is the photospheric
velocity. One approach to estimate the photospheric velocity is by measuring
the Fe II 5018 line in the spectra. Because the Fe II in our spectra was
blended and difficult to identify as shown in Figure 10, we used the Si II
6355 line instead. Figure 11 shows a comparison of the velocity with other
types of SNe, including Ic, Ic-BL, and subluminous SNe Ia. SN 2019eix seems to
have velocities slightly above a typical SN Ic and be slower evolving. The
data from Figure 11 was taken from Ben-Ami et al. (2012), Gutiérrez et al.
(2021), Taddia et al. (2016) for the SN Ic and SN Ic-BL. For the sub-luminous
SN Ia, we used SN 2016hnk (Calcium-rich transient from a proposed He
detonation; Jacobson-Galan et al., 2020), SN 2018byg (double-detonation He-
shell Type Ia candidate; Jacobson-Galan et al., 2020), and SN 2005bl a 91bg-
like (Taubenberger et al., 2008). It is important to notice that different
absorption features can give different velocities and in some cases the
identification of Si II 6355 line can be problematic (Parrent et al., 2016).
Nonetheless, from Figure 11 we can see that the Si II 6355 velocities seem to
decline at similar rates to the sub-luminous SNe. We also notice that Type Ic
SNe decline faster and seem to have lower velocities than SN 2019eix.

Figure 10: Fe II and Si II feature identification, we referred SN 1994I (in
orange) from Hachinger et al. (2012) to identify the Fe II feature for SN
2019eix (in black). Note that the feature in that region is broad and not
obvious to identify. Due to the blending, we used the Si II line to measure
the photospheric velocity instead. We also plotted sub-luminous SN 1991bg (in
blue) for comparison.  Figure 11:  The photospheric velocity is plotted
against time for SN 2019eix (in black) and various core-collapse and
thermonuclear SNe. We plotted Type Ic, Ic-BL, and sub-luminous Ia in colors
pink, dark blue, and turquoise respectively. The velocities were measured
using the Si II feature. Note how SN 2019eix decline rates are most similar to
the sub-luminous Ia suggesting similar evolution, but still within range of
Type Ic.

##  4 Modeling

###  4.1 Arnett models

Using the pseudo-bolometric luminosity calculated from Section 3.3, we fit
Arnett’s model presented by Arnett (1982) and Cano (2013), see Cano’s equation
4.1. We attempt to estimate physical parameters such as the mass of the ejecta
and the mass of 56Ni synthesized in the explosion.

where,

$A(z)=2z\exp^{-2zy+z^{2}},B(z)=2ze^{-2zy+2zs+z^{2}}$ (3)

$\tau_{m}\approx\left(\frac{\kappa}{\beta c}\right)^{1/2}\left(\frac{{M_{\rm
ej}}}{{v_{\rm ph}}}\right)^{1/2}$ (4)

and $x\equiv$ $t/\tau_{m}$, $y\equiv$ $\tau_{m}$/(2$\tau_{Ni}$) and s $\equiv$
($\tau_{m}$($\tau_{Co}$ − $\tau_{Ni}$)/(2$\tau_{Co}$ $\tau_{Ni}$)).

The assumptions used here are the same as the stripped envelope case from Cano
(2013). The constant opacity $\kappa$ = 0.07 $\rm cm^{2}g^{-1}$ and the
constant of integration $\beta$ was set to 13.8. The energy released in one
second per gram of 56Ni and 56Co are $\epsilon_{\rm Ni}=3.90\times 10^{10}$
erg $\rm s^{−1}g^{−1}$ and $\epsilon_{\rm Co}=6.78\times 10^{9}$ erg $\rm
s^{−1}g^{−1}$ (Capperallo et al., 1997), respectively with decay times of
$\tau_{Ni}$ = 8.77 d and $\tau_{Co}$ = 111.3 d (Taubenger et al., 2006, and
references therein).

The fit is done using data prior to thirty days after explosion (the
bolometric luminosity used is shown in Figure 9) using $\rm v_{ph}\approx
10,500$ km/s measured from the Si II feature and an assumed time of maximum of
16 days. From the relationship between $\rm v_{ph}$ and the ejecta energy, the
kinetic energy was calculated using $\rm
E_{K}/M_{ej}=\frac{3}{10}v_{ph}(t_{max})^{2}$ (Wheeler & Clocchiatti, 2014).
Table 4 shows the resulting parameters of SN 2019eix from the Arnett’s fit,
where the estimated ejecta mass is $\approx$ 2.5 $\rm M_{\odot}$ and the
nickel mass 56Ni is $\approx$ 0.17 $\rm M_{\odot}$. The rest of the values are
taken from Table 8 (Taddia et al., 2017) for comparison, and we notice they
agree with a Type Ic supernovae. In contrast, some of the Type Ic SNe with
similar spectra had values that were more diverse, showing the extreme range
of ejecta masses a Type Ic/Ic-BL can have. SN 1994I Ic had ejecta mass of $\rm
M_{\rm ej}$ = 0.5 ± 0.2$\rm M_{\odot}$ (Nicholl et al., 2015), PTF10vgv Ic-Pec
had $\rm M_{\rm ej}$ = 1.5 $\rm M_{\odot}$, and lastly PTF12gzk Ic-pec had
$\rm M_{\rm ej}$ = 7.5 $\rm M_{\odot}$ (Ben-Ami et al., 2012). SN 1994I had
fast rise times and therefore less massive ejecta. On the other hand PTF12gzk
had long rise times, larger ejecta masses and high ejecta velocities of about
30,000 km/s days after explosion (Ben-Ami et al., 2012).

From the measured parameters in Table 4 we observe that these masses (56Ni and
ejecta) and energies of SN 2019eix are more consistent with those of a typical
Type Ic, since any thermonuclear event would require a lower mass than the 1.4
M⊙ limit. Another method we used to estimate the mass of 56Ni, is from the
tail of the light curve. We used equations 1 and 2 from Terreran et al. (2016)
and fit the equations to our late photometry (past 50 days after maximum) and
estimated a 56Ni mass of $\approx$ 0.09 $\rm M_{\odot}$. This mass is lower
than the one approximated from peak. For our purposes, we will use this value
and add a 0.08 $\rm M_{\odot}$ error estimate to include the measured value
from peak for consistency. Additionally, to test for inconsistencies we also
applied the constant opacity $\kappa$ = 0.1 $\rm cm^{2}g^{-1}$ commonly used
for SNe Ia (Lyutykh et al., 2021). The resulting mass of ejecta slightly
decreased from 2.5 $\rm M_{\odot}$ to 2.1 $\rm M_{\odot}$, bringing it closer
to the Type Ia scenario, but it still remains significantly higher than the
1.4 $\rm M_{\odot}$ threshold.

It is important to note a major limitation in applying semi-analytic modeling
techniques (Arnett’s model) to stripped envelope light curves is the
assumption of a constant opacity, $\kappa$, ambiguity regarding the velocity,
and lack of early photometry. It was shown by Dessart et al. (2016), how
different values of $\kappa$ can lead to different results for the progenitor
parameters, and that ultimately the assumption of constant opacity is quite
poor for stripped envelope SNe. Parrent et al. (2016) showed that the Si II
velocity was problematic, while the Fe II 5018 can be hard to identify as
shown in Figure 10. Therefore there is no true velocity to use as both methods
come with uncertainties and the estimates of the ejecta mass highly depend on
this velocity. Earlier photometry would have allowed for a more accurate rise
time and as a result more accurate ejecta mass estimates from the lightcurve,
but lacked these early observations. Additionally, we assumed no 56Ni mixing,
if SN 2019eix is a double-detonation a significant amount of 56Ni would be
synthesized in the surface of the He shell. This is equivalent to some degree
of 56Ni mixing that could possibly affect our estimates of the ejecta and 56Ni
masses. Overall, the 56Ni mass error is dominated by the error on the SN
distance, explosion epoch estimate, the fit uncertainty, and the possible 56Ni
mixing synthesized in the He shell (if SN 2019eix is a double-detonation).

Supernova $M^{R}_{\rm max}$ $\Delta m_{15}(R)$ $M_{\rm ej}$[$M_{\odot}$]
$M_{\rm Ni}$[$M_{\odot}$] Spiral Gal 19eix -18.32 0.95 2.5(1.0) 0.09(0.05) ✓
Ic −18.3(0.6) 0.73(0.27) 2.1(1.0) 0.13(0.04) ✓ Ic-BL −19.0(1.1) 0.6(0.14)
4.1(0.9) 0.96(1.09) ✗ 91bg like -16.7 to -17.7(B) 1.8-2.1 (B) 0.5 0.05-0.1 ✗
Ultra stripped (2005ek) -17.26(0.15) 2.88(0.05) 0.3 0.03 ✗ Calcium-rich
transient -15.5 to -16.5 ? 0.4-0.7 $\approx$ 0.01 ✗ Ia-DD (2018byg) -18.2 1.23
$<$ 1.4 0.11 ✗

Table 4:  This table summarizes our findings and we check the similarities
between SN 2019eix in the R band (we converted r to R using the vega - AB
magnitude conversion, Blanton & Roweis, 2007) and the different classes, from
this table we can see that Type Ic shares more similarities than the other
classes. The inputs with an “(B)” indicates the value in the B band as the R
band is not provided and the values in parentheses are the errors.

###  4.2 TARDIS models

Due to the complex nature of the spectra of SN 2019eix, we experimented with
whether it was possible to reproduce some of the unusual features using a SN
Ic as a base model (as originally classified). By employing TARDIS, a Monte
Carlo radiative transfer code that tracks the number of photons that propagate
through the supernova ejecta, we simulated the spectra of SN 2019eix. The
version of TARDIS utilized in this study is TARDIS 2022.11.21 (Kerzendorf &
Sim, 2014), which is based on previous works (Abbott & Lucy, 1985; Lucy &
Abbott, 1993; Mazzali & Lucy, 1993; Lucy, 1999, 2002, 2003). The user must
input the density structure, velocity, and abundance of elements. TARDIS then
solves the ionization and excitation states of the plasma assuming a
homologous expansion. The evolution in the early photospheric phase is such
that the photosphere can be approximated to be optically thick and thus the
black body approximation is appropriate. Additionally, TARDIS approximates the
photosphere by assuming it’s position in velocity space. For the simulations
presented in this paper, we use the Kurucz atomic dataset version 1.0 (i.e.
line list) for calculating the bound-bound transitions. We also used the
initial parameters listed in Table 1. from Williamson et al. (2020) and their
input files for SN 1994I that can be found in the Github page
101010https://github.com/tardis-sn/tardis-
setups/tree/master/2020/2020_williamson_94I.

Given that SN 2019eix is inconsistent with the spectral evolution of typical
Type Ic, we modeled SN 2019eix spectra using a SN 1994I (SN Ic) spectral model
at day 16 as our base model. We adjusted the abundance Hachinger et al.
(2012), density, velocity, and temperature of radiation according to different
epochs and set the damping constant to 0.5. We changed these parameters in
order to see whether the spectral features of SN 2019eix could be reproduced,
as discussed below. The epochs modeled include 4 days before maximum, 1 day
after maximum, and 13 days after maximum light as shown in Figure 12. All of
our models were well converged in the the temperature of radiation space.

In the Appendix A, we plot the changes made to the temperature and density
profiles and in Figure 16, we show the changes made to the abundances per
epoch. Note in Figure 12 that the models fail to reproduce the O I region as
they are unable to attenuate it. Attempts were made to deplete the O I feature
throughout all epochs, by either raising the temperature (in attempt to ionize
the O I) or lowering the abundance in the outer layers. We found that raising
the temperature to ionize the O I was unable to fully eliminate the absorption
feature in the model. Additionally, the temperature increase caused the
spectra to gain bluer features that did not match our observations. Lowering
the abundance on the other hand (various combinations were attempted including
depleting the outer shells, reducing O I in all shells, and linearly
decreasing the O I in all shells) did not seem to affect the spectra as shown
in Figure 12.

The phase 4 days before maximum light was assumed to be 13 days after
explosion in our TARDIS model, with an accompanying photospheric velocity of
12200 km/s (chosen based on the measured Si II velocity and the velocity
values that best-matched TARDIS models to observations). We used the reference
model with a slight adjustment in the density and temperature which were all
increased. Additionally Ti was increased in the earlier shells and Si was
decreased. Although not perfect, this combination yielded the best results
that matched our observations. One issue with it was the overprediction of the
Si II feature. Attempts were made to enhance the feature including decreasing
its abundance, but failed to reproduce it.

At around maximum light (+1 day), we used a time of explosion from SN 2019eix
of 19 days and a photospheric velocity of 11700 km/s. We lowered the density
profile from our reference model as the ejecta expands and kept the same
temperature from the base model. A slight increase of Ti and Fe, gave the best
match to our data. One approach we used to determine which elements to alter
in our models was comparing the absorption and emission features from Figure
12 with the data. From this Figure, we were able to identify which elements
could be responsible for creating particular features. Therefore, altering the
abundance of these elements could help achieve the desired results and uncover
the tomography of the supernova at specific epochs. When contrasted with the
results from Hachinger et al. (2012) at 22 days after the explosion, it
becomes apparent that SN 2019eix has considerably higher concentrations of Ti
and V in its inner layers. On the other hand, the spectral model for 1994I
(Hachinger et al., 2012) at 22 days after the explosion shows more significant
amounts of Ni and Fe in the inner layers, indicating a substantially faster
evolution rate than that of SN 2019eix.

At phase 12 days after maximum light a time of 29 days after explosion was
assumed for the model and a photospheric velocity of 10370 km/s. The density
was significantly reduced along with the temperature and the Fe was increased
by a factor of 5 from our reference model. When comparing this model to SN
1994I (Hachinger et al., 2012) at day 30 after the explosion we notice it to
be significantly more evolved than ours. Their velocity at this point has
dropped by a factor of 6 from day 16 and has significantly more Iron-Group
Elements (IGE) and Intermediate-Mass Elements (IME) (such as Fe, Ni and Si, S,
respectively) in the inner shells than we observe for our SN 2019eix model. SN
1994I is a fast evolving Ic, whereas SN 2019eix appears to not change much in
terms of its abundance or photospheric velocity from before to after maximum
light.

From our TARDIS models of SN 2019eix in comparison to Hachinger et al. (2012),
we note that the velocity does not evolve as fast as SN 1994I (Hachinger et
al., 2012) drops by a factor of 6 in 14 days, and in SN 2019eix models it
drops by a factor of 1.2. The temperature has the opposite effect and appears
to drop faster for SN 2019eix than for the Hachinger et al. (2012).
Additionally, SN 2019eix also seems to be denser than Hachinger et al. (2012)
throughout all the epochs. SN 2019eix seems to be a slow evolver in terms of
velocity and reaction rates. A possible explanation to these peculiar features
we observe in SN 2019eix could be a density and temperature effect, as those
were the main parameters we varied the most and appeared to successfully
reproduce the spectra as shown in Figure 12.111111All of our model input files
can be found on: https://github.com/tardis-sn/tardis-setups/2022/2019eix

Figure 12: We plot the best matched TARDIS models to SN 2019eix using SN 1994I
SN Ic as a base model. On the left we have SN 2019eix and the final product of
the TARDIS models while on the right it exhibits the elements responsible for
its features. The negative values correspond to absorption contributions to
the spectrum, while the positive values represent emission. TARDIS fails to
hide the OI feature, but appears to reproduce most of the features overall.

###  4.3 Double-Detonation from He shell models

SN 2019eix shares numerous similarities with double-detonation (DD) candidates
from He shell Type Ia as shown in Figures 6 and 7. Spectroscopicaly, SN
2019eix shows significant red colors as well as suppression in emission lines
below 5000 Å; similar to SN 2016hnk (Jacobson-Galan et al., 2020), SN 2018byg
(De et al., 2019), and SN 2016dsg starting from maximum following to 30 days
after maximum light. The strong blanketing in the DD on the He shell scenario
can be attributed to the large amount of iron group elements created in the
outer ejecta from the He burning. The substantial reddening can be explained
by the ashes produced by the He shell detonation causing redder colors
throughout its evolution. Photometrically, SN 2019eix appears to be brighter
and with a somewhat wider lightcurve than the other DD He shell candidates as
shown in Figure 6, with the exception of SN 2016hnk which shows a slower
evolution than SN 2019eix post maximum. However, in terms of the color
evolution SN 2016hnk seems to be the best match as shown in Figure 5 from the
extensive comparison with multiple SNe of various types.

Due to the similarities of SN 2019eix with this class we compare the spectra
and the light curves to the double detonation models from Kromer et al.
(2010), Sim et al. (2012), and Polin et al. (2019). Kromer et al. (2010)
explored observable properties of double detonation models. The simulations
presented were carried out in 2D using radiative transport from ARTIS. To
initialize models, they chose estimated values of temperature, central density
of the CO core, and temperature and density at the base of the He layer. They
ignited an initial He detonation in a single point at the base of the He shell
and eventually created a shock wave that propagates into the core. These
models have He shell masses ranging from 0.0035 to 0.0126 $\rm M_{\odot}$
previously considered by Fink et al. (2010).

Similar to Kromer et al. (2010), Sim et al. (2012) created their models using
ARTIS and investigated small (0.45 $\rm M_{\odot}$ WD + 0.21 $\rm M_{\odot}$
He) and large (0.58 $\rm M_{\odot}$ \+ 0.21 $\rm M_{\odot}$ He) masses for
both single and double detonation scenarios. The two methods investigated were
the He detonation wrapping around the CO WD, modeled as the convergence of
shocks in the first method, and the He detonation igniting an inward-
propagating shock at the CO core’s edge, called the Edge-lit core detonation
in the second approach. Additionally, Polin et al. (2019) examined the
explosions of WDs varying from 0.6 to 1.2 $\rm M_{\odot}$ with He shell masses
of 0.01, 0.05, 0.08 $\rm M_{\odot}$. Polin et al. (2019) found that thicker
shell models showed early time flux excess, redder colors, and higher line
blanketing in the UV through the blue regime of the spectrum. Their models
were created using the Eulerian hydrodynamics code Castro. After the SN ejecta
reaches homologous expansion, a multi-dimensional time dependent radiation
transport code (SEDONA) is used to created the synthetic spectra and light
curves.

In our study, the models we chose to compare to SN 2019eix are the double
detonation models Polin0.9+0.08-D, Polin0.76+0.15-D, Polin0.76+0.15-0.2-D,
Kromer0.81+0.126-D (model1), and Sim0.58+0.21-D (CSDD-S) as shown in Table 5.
We chose these models as they appeared to be more consistent with our
lightcurves and spectra of SN 2019eix. However, we also considered
Polin0.8+0.08, Sim 0.45+0.21-D, and the only single detonation,
Sim0.58+0.21-S. From Figure 13, we can see that photometrically, the more
massive model with the thicker He shell such as Sim0.58+0.21-D (CSDD-S)
overestimates the brightness in both bands. The other models Polin0.76+0.15-D,
Polin0.76+0.15- 0.2-D, and Kromer0.81+0.126-D (model1) seem to be a better
match to SN 2019eix, especially Polin0.76+0.15-D. Nonetheless,
Polin0.76+0.15-D (unmixed) our best model underestimates the magnitude in the
r band and declines much faster than SN 2019eix, but the i band is more
consistent from maximum light up until 10 days after. Additionally, some of
the challenges in modelling the lightcurves of SN 2019eix were attributed to
the lack of early photometry. With earlier photometry, a more detailed
comparison to double-detonation He-shell models that predict an early bump in
the light curve as illustrated in Figure 13.

As mentioned, SN 2019eix shows substantial line blanketing in the blue side of
the spectra. This is reproduced by the majority of the models at maximum and
past maximum light as shown in Figure 14. Reproducing the data from the models
consistently at various epochs was difficult. This is evident in Polin
0.9+0.08, which indicates that a reasonable result at one epoch was observed
and not the others. Nevertheless, in Sim0.58+0.21-D (CSDD-S) we observe
consistent behavior throughout the epochs, although the light curve of this
model severely overestimates the flux. However, it is a known issue that Sim
et al. (2012) are brighter due to the pure He shell assumption, but in
actuality the the He layer can be polluted by the WD core material. By
contrast, the Polin0.76+0.15-D (unmixed) light curve model is the best match,
but spectroscopically appears to be a mismatch before maximum light but
overall a reasonable fit.

Generally, it appears that the light curve models do not match SN 2019eix
consistently. However, some of this mismatch could be attributed to the degree
of mixing in the outer layer (Kromer et al., 2010; Shen et al., 2010; Sim et
al., 2012; Polin et al., 2019) in addition to the viewing angle (Kromer et
al., 2010). The models used for comparison are all from Local-Themodynamic
Equlibrum which takes into effect about 20 days before maximum light.
Therefore, it is important to compare the models to the data at early times.
In order to fully compare the light curve at all epochs, we must use non-LTE
models.

Figure 13: Lightcurve comparison between SN 2019eix and other DD candidates to
detonations of He-shell models. The phase is measured from the i/I-band
maximum. Notice there is not a light curve that matches both the r and i the
band, but Polin 0.76+0.15 unmixed is the closest match to SN 2019eix.  Figure
14:  Spectroscopic comparison between SN 2019eix and double detonation models.
The left panel shows the spectra about 4 days before maximum light, the middle
panel is at 1 day after maximum light, and the right panel is 12 days after
maximum light.

Model Original name WD Mass He Shell Mass Reference Polin0.76+0.15-0.2-D 0.76
0.15 a Polin0.76+0.15-D 0.76 0.15 a Polin0.9+0.08-D 0.9 0.08 a
Kromer0.81+0.126-D Kromer model 1 0.81 0.126 b Sim0.58+0.21-D Sim CSDD-S 0.58
0.21 c

Table 5:  This table shows the models used for comparison with SN 2019eix.
(a)Polin et al. (2019); (b) Kromer et al. (2010); (c)Sim et al. (2012). Note
that Polin0.76+0.15-0.2-D and Polin0.76+0.15-D are similar, but differ since
Polin0.76+0.15-0.2-D includes 0.2 $M_{\odot}$ mixing in the outer ejecta

##  5 Discussion

SN 2019eix is an uncommon event. It has similar features to a SNe Ic including
the lightcurve peak magnitude and decline rates, while having a strong
suppression in the blue flux, red colors, a lack of an O I feature, and a
strong Ca II feature unlike those usually seen in SNe Ic. Since it is unlike
any known SN Ic, we also investigate other SN types including Calcium-rich
transients and thermonuclear supernovae like subluminous supernovae and double
detonation SNe Ia.

###  5.1 Comparison to Type Ic and peculiar Ic

In Figure 6, we showed that the lightcurves for SN 2019eix are the most
similar to SNe Ic in terms of peak magnitude (Table 1), bolometric luminosity
(illustrated in Figure 9), and decline rates (Table 2). However, from the
spectral and color evolution standpoint we do not observe such consistency.
Spectroscopically, before maximum SN 2019eix resembles SNe Ic with the
exception of the absence of the O I feature. However, we observe significant
discrepancy in the spectra at later times as shown in Figure 8 (albeit
peculiar SNe Ic show some level of consistency as we will discuss below).
Additionally, the color evolution of SN 2019eix appeared to be much more red
than SNe Ic as depicted in Figure 5.

For peculiar SN Ic PTF12gzk, the color evolution at early times seems to agree
with SN 2019eix (Figure5), but a full comparison of colors was not conducted
as as we lacked photometry at later times. Additionally, the spectrum of
PTF12gzk before and after maximum matches SN 2019eix as it starts to develop a
blue suppression after maximum light, but does not seem to be depleted of the
O I 7774 feature. PTF12gzk appears to also have more blended features than SN
2019eix suggesting higher velocities. Ben-Ami et al. (2012) reported PTF12gzk
to exhbit large expansion velocities of $\approx$ 30000 km/s measured days
after explosion from the Si II line. Additionally, the mass of ejecta was
measured to be $\rm M_{ej}$ = 7.5 $\rm M_{\odot}$, about a factor of 3 larger
than our estimates for SN 2019eix. This suggests a higher initial progenitor
mass of 25-30 $\rm M_{\odot}$ (Ben-Ami et al., 2012) and a lower-mass
progenitor for SN 2019eix.

We considered the USSNe scenario due to the lack of O I in SN 2019eix, as
these SNe can be stripped of their O in addition to H and He layers from a
compact binary (Woosley et al., 2020; Dessart et al., 2020). We dismissed this
scenario, since neither the spectra nor the lightcurves are comparable (eg.
USSNe SN 2010X and SN 2005ek show no blue suppression and have O I features
unlike SN 2019eix, as shown in Figure 15). The fast rise times of ultra-
stripped supernovae (typically between 5-10 days and magnitude of -16; Moriya
et al., 2016) and fast decline rates ($\rm\Delta m_{15}\approx$ 3; Drout et
al., 2013), suggest a smaller progenitor than SN 2019eix.

From observations we note that peculiar Type Ic’s (PTF12gzk) could roughly
produce spectra that look similar to SN 2019eix. Due to the peculiar spectral
evolution of SN 2019eix we modeled the spectra using TARDIS. From our models
we showed that spectra similar to SN 2019eix could be reproduced by using the
abundance models from SN 1994I type Ic (Hachinger et al., 2012; Iwamoto et
al., 1994), but again the O I failed to be reproduced as discussed previously
in Section 4.1. Overall, the lightcurves appear to be consistent with SN Ic,
however the spectral evolution appears to be atypical for this class, in
particular the strong reddening, lack of O I, and strong Ca II at later times.

###  5.2 Subluminous SNe Ia

As shown in Figure 7, SN 2019eix starts to resemble subluminous SNe Ia in its
later spectra, although it is notably different before maximum.
Photometrically, 91bg-likes reach peak magnitudes between -16.7 to -17.7 in
the B band similar to SN 2019eix. Overall, despite the similarities between SN
2019eix and 91bg-likes in terms of the spectra and the absolute magnitude,
they also display key differences.

Some of these differences include: bluer colors as shown in Figure 5 (91bg-
likes reach colors of $\rm(B-V)_{max}\approx 0.5-0.6$; Taubenberger et al.,
2008; Sullivan et al., 2011) being much bluer than SN 2019eix
($\rm(B-V)_{max}\approx 1.8$); faster decline rates ($\rm\Delta
m_{15}(B)\approx 1.8-2.1$) than SN 2019eix ($\rm\Delta m_{15}(B)\approx 1.2$);
and spectroscopic disagreement in some absorption features (especially the Si
II, O I, and Ca II features and generally the spectra is not as suppressed in
the blue as SN 2019eix); Thus the inferred 56Ni mass and ejecta mass for 91bg-
like SNe were found to be between $\sim 0.05$ and $\rm\sim 0.10M_{\odot}$
(Mazzali et al., 1997; Sullivan et al., 2011) and $\rm\approx 0.5M_{\odot}$
(Stritzinger et al., 2006), respectively, significantly lower than SN 2019eix.
For these reasons it is unlikely that SN 2019eix shares a similar progenitor
as the 91bg-like objects. However, it is important to note that the $\rm
M_{\rm ej}$ mass estimate could be overestimated for SN 2019eix, as it depends
on the photospheric velocity and width of the light curve.

###  5.3  Calcium-rich Transients

Calcium-rich transients are peculiar as their explosion mechanisms is not well
understood and a variety of scenarios can explain their observations. Due to
strength of the Ca II absorption feature on SN 2019eix, we briefly compare
these transients to SN 2019eix, but rule them out due to significant
incompatibilities. Calcium-rich SNe are mostly characterized by peak
magnitudes of -14 to -16.5, with fast evolving light curves (much dimmer and
faster evolving than SN 2019eix). From Figure 15 we see that SN 2005E (a
Calcium-rich transient) is a mismatch to SN 2019eix in terms of its spectral
evolution (e.g. it does not display the extreme suppression in the blue seen
in SN 2019eix), but instead develops a [Ca II] 7291, 7323 line 18 days after
peak (not observed in SN 2019eix, but a common feature for these transients).
Additionally, these transients also posses moderately red colors
$\rm(B-V)_{max}\approx$ 0.6 mag (Taubenberger, 2017), unlike the significant
red colors observed in SN 2019eix. The majority of these objects display low
56Ni and ejecta masses between 0.4-0.7 $\rm M_{\odot}$ (Kasliwal et al.,
2012). Due to the low luminosity, different color evolution, and incompatible
spectra evolution; SN 2019eix is unlikely to share the same progenitor as
these transients.

###  5.4 Double-detonation Type Ia

The observations of He-shell double detonation candidates (OGLE-2013-SN-079,
SN 2016hnk, SN 2016dsg, and 2018byg) display significant spectroscopic
resemblance to SN 2019eix. Before maximum light, we do not observe much
similarities and thus are not plotted. However, during and after maximum light
the spectra of SN 2016dsg and SN 2018byg are found to be extremely suppressed
in the blue and have a minimum to no absorption O I feature, similar to SN
2019eix. During and after maximum light the spectra continue to have
considerable line blanketing at $\lambda<5000\rm{\AA}$ and show stronger Ca II
absorption as shown in Figure 7.

An explanation for the strong line blanketing in the blue for these double-
detonation candidates is that the detonation of a helium shell on the surface
of a C/O WD pollutes the outer layers of the ejecta. These IME and IGE burning
products cause this blue blanketing and significant reddening (Kromer et al.,
2010; Shen et al., 2010; Sim et al., 2012; Polin et al., 2019). The reddening
is further illustrated in Figure 5, where we see SN 2019eix sharing impressive
similarities with SN 2016hnk as they both seem to be redder than the rest of
the SN types. DD models also explain the strong Ca II feature. This is a
result of the He-burning creating a large amount of IME in the outer layers
(Fink et al., 2010; Kromer et al., 2010). SN 2019eix develops Ca II into a
deep and high-velocity feature after maximum light, as shown in Figure 3.
Therefore, a large amount of IME created in the DD models can explain such
anomaly observed in SN 2019eix.

We compare existing DD models to SN 2019eix in Figure 13 and Figure 14. The
models are roughly able to reproduce the lightcurve and spectral features. The
best DD model of SN 2019eix is Polin 0.76+0.15 (unmixed), as it is able to be
a better match to the lightcurves (although it still declines faster in the r
band) and it’s able to reproduce the most prominent features during and after
maximum light (although it fails to match the features before maximum light).
Notice in Figure 14 how the DD spectral models reproduce the lack of O I
feature, a pronounced feature in SN 2019eix. The physics of the absence of O I
feature in the case where the He shell is thick is justified by the ash of the
He burning products created in the outermost layers of the ejecta. It is
expected to cover the unburned oxygen from the core underneath the He products
limiting them to a restricted velocity range (Kromer et al., 2010; Sim et al.,
2012; Polin et al., 2019). This could explain why the O I feature for SN
2019eix is not observed until roughly 30 days after maximum.

Figure 6 shows that DD candidates show some level of agreement with SN
2019eix, particularly in the r-band. Existing candidates seem to be dimmer and
faster declining than SN 2019eix (Figure 9), although not many of these
objects have been found in literature. Therefore, SN 2019eix remains a great
candidate for the DD He shell model (with a possible WD mass close to 0.76 M⊙
and a thick He-shell close to 0.15 M⊙), as it is able to physically explain
the abnormalities from the spectra and the color evolution unlike other
classes. Therefore, for those reasons the DD scenario is the favored scenario
for SN 2019eix.

Figure 15: A comparison between SN 2019eix and SN 2010X, SN 2005ek (ultra-
stripped supernovae), and SN 2005E (cal-rich transient) the comparisons show
the lack of compatibility between the two objects suggesting that SN 2019eix
is not an ultra-stripped spectroscopically.

##  6 Conclusion

We summarize our primary observations of SN 2019eix below.

  1. 1.

SN 2019eix is intrinsically red compared to other SNe Ic. This is indicated by
the lack of Na ID absorption in the photospheric spectrum and the color
evolution of B-V

  2. 2.

During and after maximum spectra show strong suppression in the blue

  3. 3.

Spectra show some of the strongest ever observed broad and deep Ca II
absorption and little to no O I absorption

  4. 4.

Peak absolute magnitude reaches $M_{B}=-17.3$ with a decline parameter of
$\Delta m_{15}(B)=1.2$

  5. 5.

Total ejecta and nickel masses are $2.5M_{\odot}$ and $0.09M_{\odot}$,
respectively

While the lightcurve decline rate and luminosity of SN 2019eix are consistent
with SNe Ic, the spectra were challenging to reproduce. Notably, the lack of
an O I feature is inconsistent with both observed SNe Ic and our attempts to
model it as such.

The thermonuclear double detonation scenario, on the other hand, is a better
match to the data. The light curves from Polin 0.76+0.15 models seem to
decline slightly faster than SN 2019eix and underestimate the luminosity for
SN 2019eix in the r band, but have better agreement in the i band. However,
spectroscopically we see SN 2019eix matches the spectral evolution of many DD
candiates during and after maximum light as well as models. Moreover, the
mystery of the O I is also explained by DD models along with the blueward
supression. Conversely, most DD He-shell SNe candidates appear to come from
elliptical galaxies and to be far away from the center of the galaxy, that is
inconsistent with the host spiral galaxy of SN 2019eix. However, spiral
galaxies also have an old stellar population and thus the DD scenario remains
a possibility for SN 2019eix. Table 7 summarizes the pros and cons between the
core-collapse and the double-detonation thermonuclear events, favoring the
latter.

Because SN 2019eix is unable to be fully explained by either model (core-
collapse or the DD He-shell) despite having a better physical explanation in
the DD scenario (as it shares some of the peculiar spectral features and
colors seen in SN 2019eix); there are some open questions in the DD scenario
that point to future study. The light curve width is large, possibly implying
a large ejecta mass. This could be inconsistent with a white dwarf progenitor.
We thus classify this SN as a peculiar Type I SN. Due to the lack of early
photometry (to constrain the rise time), nebular data (to observe whether it
is Fe rich down to the core), and IR data (to check if there is unburnt He
from the shell in the 10,500$\rm{\AA}$ line), we are unable to conclude with
certainty that SN 2019eix is the result of a DD He-shell explosion. To get a
better sense of the progenitor, further analysis of the SN 2019eix host galaxy
star formation rate is essential to analyze whether the progenitor could have
come from a massive star, unfortunately such analysis is beyond the scope of
this paper. Nonetheless, SN 2019eix is an exciting target as it is arguably
one of the rarest objects we have discovered so far, whether it is a double-
detonation (being one of the handful of events ever discovered) or a core-
collapse (being an extremely rare object with many unconventional features for
this kind).

We are grateful for the National Science foundation (NSF) and the University
of California, Santa Barbara (UCSB) for funding this project through the NSF-
GSP-1911225 grant and the Central Campus Fellowship.

This research makes use of observations from the Las Cumbres Observatory
network, in addition to the MARS ZTF alert broker developed by Las Cumbres
Observatory software engineers.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which
is operated by the Jet Propulsion Laboratory, California Institute of
Technology, under contract with NASA.

This research made use of tardis, a community-developed software package for
spectral synthesis in supernovae (Kerzendorf & Sim, 2014; Kerzendorf et al.,
2022b). The development of tardis received support from GitHub, the Google
Summer of Code initiative, and from ESA’s Summer of Code in Space program.
tardis is a fiscally sponsored project of NumFOCUS. tardis makes extensive use
of Astropy and Pyne.

The research by Y.D. is supported by NSF grants AST-2008108

M.W. is supported by the NASA Future Investigators in NASA Earth and Space
Science and Technology grant (80NSSC21K1849).

M.M. is supported in part from NASA under the Swift GI program 1619152 (NASA
grant No. 80NSSC21K0280), the Tess GI program G03267 (NASA grant No.
80NSSC21K0240), the ADAP program grant No. 80NSSC22K0486, and the HST GO
program HST-GO-16178.007

##  Appendix A Appendix information

The tables and plots are design to facilitate compare and contrast between the
different SN classes/models to SN 2019eix. In Table 4, we provide additional
information about the types of SNe and their various parameters for a more
obvious comparison to SN 2019eix. For a better understanding of the spectra
modeling conducted, in Figure 16, we show the the different input values we
used for our TARDIS models such as density and temperature over the velocity
space. In Table 6, we provide the changes we have made to the abundances from
the base model with respect to time from TARDIS. Finally, in Table 7, we
created a table showing the main differences between core-collapse and the
double-detonation for an easier comparison between the two.

Figure 16: The plot on the top shows the altered density profiles used for SN
2019eix for the different epochs we modeled from the base model SN 1994I. The
bottom plot shows the adjusted temperature from the base model as well.

Model time velocity X(Na) X(Mg) X(Si) X(Ti) X(Fe) X(Ni) 1994I 16.0 8900 4.0
10.0 0.7 0.08 0.03 1.8 19eix Mod 1 13.0 12250 4.0 5.0 3.0 0.4 0.16 1.8 19eix
Mod 2 19.0 11700 4.0 10.0 0.7 2.0 0.5 2.2 19eix Mod 3 29.0 10370 0.7 10.0 0.7
0.08 0.9 2.6

Table 6:  This table shows the elements changed from the base model SN 1994I,
the elements were slightly increased in Fe and Ni as time evolved.

Variables Core-collpse DD Light curves ✓ ? $M_{\rm Ni}$ = 0.17 ✓ ? $M_{\rm
ej}$ $>$ 2.5 ✓ ✗ no O I ✗ ✓ Line blanketing ✗ ✓ $\Delta m_{15}(B)$ ✓ ? Strong
Ca II ✗ ✓

Table 7:  This table summarizes the similarities and differences between SN 2019eix and the different explosion mechanisms. The entries with “?” represent measurements with not enough data or with large uncertainty. We do see that overall the double-detonation has stronger pros and weaker cons.  JD | epoch | B | V | g | i | r
---|---|---|---|---|---|---
2458608 | -5 |  | 16.53(0.02) | 16.74(0.04) | 16.93(0.02) | 16.64(0.02)
2458610 | -3 | 17.65(0.04) | 16.5(0.05) | 17.03(0.02) | 16.86(0.1) | 16.48(0.03)
2458619 | 6 | 18.37(0.19) | 16.53(0.07) | 17.38(0.02) | 16.53(0.03) | 16.23(0.02)
2458620 | 7 | 18.57(0.09) | 16.6(0.02) | 17.55(0.01) | 16.62(0.03) | 16.3(0.01)
2458621 | 8 | 18.43(0.16) | 16.61(0.04) | 17.6(0.04) | 16.62(0.01) | 16.31(0.01)
2458625 | 12 | 18.86(0.17) | 17.1(0.05) | 18.28(0.13) | 16.96(0.06) | 16.71(0.1)
2458631 | 18 | 19.16(0.05) | 17.39(0.05) | 18.41(0.03) | 17.13(0.02) | 17.0(0.03)
2458634 | 21 | 19.42(0.09) | 17.62(0.02) |  |  |
2458641 | 28 | 19.63(0.02) | 18.08(0.01) | 18.99(0.03) | 17.99(0.03) | 17.87(0.01)
2458647 | 34 | 19.81(0.08) | 18.38(0.01) | 19.17(0.05) | 18.41(0.09) | 18.23(0.01)
2458651 | 38 |  | 18.67(0.06) | 19.22(0.06) | 18.36(0.04) | 18.38(0.03)
2458655 | 42 | 19.88(0.13) | 18.63(0.04) |  |  |
2458659 | 46 |  |  | 19.41(0.04) |  |
2458670 | 57 | 19.93(0.1) | 19.1(0.08) | 19.59(0.03) | 19.13(0.14) | 19.28(0.23)
2458673 | 60 |  | 19.48(0.12) |  |  | 19.43(0.06)
2458681 | 68 | 19.97(0.09) | 19.42(0.09) | 19.63(0.08) |  | 20.06(0.16)
2458689 | 76 | 20.34(0.09) | 19.55(0.19) | 19.93(0.07) | 19.89(0.21) | 20.15(0.03)
2458699 | 86 | 20.47(0.1) | 20.05(0.03) | 20.34(0.05) | 20.41(0.09) | 20.58(0.1)
2458707 | 94 |  | 20.1(0.17) | 20.2(0.11) | 20.68(0.11) | 20.95(0.15)
2458718 | 105 | 20.91(0.2) | 20.81(0.12) | 20.64(0.15) | 21.05(0.38) |
2458726 | 113 | 20.87(0.16) | 20.95(0.26) | 20.83(0.21) |  |
Table 8: Phase with respect to the V max where the dates are rounded up. UT Date | epoch | Telescope | Instrument
---|---|---|---
2019-05-05 | -4 | Lick | KAST
2019-05-10 | +1 | FTN | FLOYDS
2019-05-14 | +5 | FTN | FLOYDS
2019-05-22 | +13 | FTN | FLOYDS
2019-06-01 | +23 | FTN | FLOYDS
2019-06-11 | +33 | FTN | FLOYDS
Table 9: Phase with respect to the V max

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