# No plateau observed in late-time near-infrared observations of the
underluminous Type Ia supernova 2021qvv

O. Graur,1,2 E. Padilla Gonzalez,3,4 J. Burke,3,4 M. Deckers,5 S. W. Jha,6 L.
Galbany,7,8 E. Karamehmetoglu,9 M. D. Stritzinger,9 K. Maguire,5 D. A.
Howell,3,4 R. Fisher,10 A. G. Fullard,11 R. Handberg,9 D. Hiramatsu,12,13 G.
Hosseinzadeh,14 W. E. Kerzendorf,11 C. McCully,3,4 M. Newsome,3,4 C.
Pellegrino,3,4 A. Rest,15,16 A. G. Riess,15,16 I. R. Seitenzahl,17 M. M.
Shara,2 K. J. Shen,18 G. Terreran,3,4 and D. R. Zurek2
1Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth,
PO1 3FX, UK
2Department of Astrophysics, American Museum of Natural History, Central Park
West and 79th Street, New York, NY 10024-5192, USA
3Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575,
USA
4Department of Physics, University of California, Santa Barbara, CA
93106-9530, USA
5School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
6Department of Physics and Astronomy, Rutgers, the State University of New
Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA
7Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans,
sn, E-08193 Barcelona, Spain
8Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
9Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120,
DK-8000 Aarhus C, Denmark
10Department of Physics, University of Massachusetts Dartmouth, 285 Old
Westport Road, North Dartmouth, MA 02740, USA
11Department of Physics and Astronomy, Michigan State University, East
Lansing, MI 48824, USA
12Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
13The NSF AI Institute for Artificial Intelligence and Fundamental
Interactions, USA
14Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson,
AZ 85721-0065, USA
15Space Telescope Science Institute, Baltimore, MD 21218, USA
16Department of Physics and Astronomy, The Johns Hopkins University,
Baltimore, MD 21218, USA
17School of Science, University of New South Wales Canberra, The Australian
Defence Force Academy, Canberra, ACT 2600, Australia
18Department of Astronomy and Theoretical Astrophysics Center, University of
California, Berkeley, CA 94720, USA E-mail<EMAIL_ADDRESS>0000-0002-4391-6137 0000-0003-0209-9246 0000-0001-8857-9843
0000-0001-8738-6011 0000-0002-1296-6887 0000-0001-6209-838X
0000-0002-5571-1833 0000-0002-9770-3508 0000-0003-4253-656X
0000-0001-8077-7255 0000-0001-7343-1678 0000-0001-8725-4502
0000-0002-1125-9187 0000-0002-0832-2974 0000-0002-0479-7235
0000-0001-5807-7893 0000-0001-9570-0584 0000-0002-7472-1279
0000-0002-4410-5387 0000-0002-6124-1196 0000-0002-5044-2988
0000-0003-0155-2539 0000-0002-9632-6106 0000-0003-0794-5982

(Accepted 2023 September 26. Received 2023 September 14; in original form 2023
June 22)

###### Abstract

Near-infrared (NIR) observations of normal Type Ia supernovae (SNe Ia)
obtained between 150 to 500 d past maximum light reveal the existence of an
extended plateau. Here, we present observations of the underluminous, 1991bg-
like SN 2021qvv. Early, ground-based optical and NIR observations show that SN
2021qvv is similar to SN 2006mr, making it one of the dimmest, fastest-
evolving 1991bg-like SNe to date. Late-time (170–250 d) Hubble Space Telescope
observations of SN 2021qvv reveal no sign of a plateau. An extrapolation of
these observations backwards to earlier-phase NIR observations of SN 2006mr
suggests the complete absence of a NIR plateau, at least out to 250 d. This
absence may be due to a higher ionization state of the ejecta, as predicted by
certain sub-Chandrasekhar-mass detonation models, or to the lower temperatures
of the ejecta of 1991bg-like SNe, relative to normal SNe Ia, which might
preclude their becoming fluorescent and shifting ultraviolet light into the
NIR. This suggestion can be tested by acquiring NIR imaging of a sample of
1991bg-like SNe that covers the entire range from slowly-evolving to fast-
evolving events ($0.2\lesssim s_{\mathrm{BV}}\lesssim 0.6$). A detection of
the NIR plateau in slower-evolving, hotter 1991bg-like SNe would provide
further evidence that these SNe exist along a continuum with normal SNe Ia.
Theoretical progenitor and explosion scenarios would then have to match the
observed properties of both SN Ia subtypes.

###### keywords:

methods: observational – supernovae: general – supernovae: individual:
SN2021qvv – white dwarfs

††pubyear: 2023††pagerange: No plateau observed in late-time near-infrared
observations of the underluminous Type Ia supernova 2021qvv–LABEL:lastpage
Figure 1: Multicolored HST WFC3 image of SN 2021qvv (in the white reticle) and
its lenticular host galaxy, NGC 4442. The cutouts show the SN declining in
brightness in the WFC3 F160W band over a time span of four months, $\sim
170$–$250$ d post maximum light.

##  1 Introduction

Though Type Ia supernovae (SNe Ia) are widely used as standardisable candles
in cosmology, we still do not know what type of star (or star system) explodes
as this type of SN and how the explosion occurs (see reviews by Wang & Han
2012; Maoz et al. 2014; Jha et al. 2019; Soker 2019; Liu et al. 2023; for a
general introduction to SNe, see Graur 2022). This effort is compounded by the
heterogeneity of SNe Ia. So-called ‘normal’ SNe Ia share the Universe with
overluminous 1991T-like SNe (Filippenko et al., 1992b), underluminous 1991bg-
like SNe (Filippenko et al., 1992a), transitional iPTF13ebh-like SNe (Hsiao et
al., 2015), peculiar SNe Iax (Jha et al., 2006; Foley et al., 2013), and other
SN Ia subtypes with their own peculiarities.

Late-time observations of SNe Ia taken $\sim 150$–$2000$ d past maximum
$B$-band light have come forth as a probe of the progenitor and explosion
mechanism of these SNe. Ground-based, optical observations out to $\approx
640$ d have shown a continuous decline (Sollerman et al., 2004; Lair et al.,
2006; Leloudas et al., 2009), as opposed to the steep drop predicted by the
so-called ‘infrared catastrophe’ (Axelrod, 1980). Optical observations taken
with the Hubble Space Telescope (HST) starting at $\approx 800$ d have further
revealed a flattening of the SN Ia light curve that cannot be attributed to
contaminating light echoes or contribution from a surviving companion (Graur
et al. 2016; Shappee et al. 2017; Shen & Schwab 2017; Yang et al. 2018a; Li et
al. 2019; Tucker et al. 2022; though see Yang et al. 2018b regarding the use
of imaging polarimetry to discover light echoes unresolvable in regular
imaging).

Graur et al. (2018a, b) and Graur (2019) have further claimed that the shape
of the late-time light curves may be correlated with the peak luminosities of
the SNe (though see Jacobson-Galán et al. 2018). This is reminiscent of the
peak width-luminosity relation used to standardize SNe Ia for use as standard
candles (see review by Phillips & Burns 2017). If real, a second, late-time
width-luminosity correlation could be used to further standardise SNe Ia and
lower the systematic uncertainties associated with their use as cosmology
probes. However, a larger sample of SNe is required to test the significance
of this correlation.

Near-infrared (NIR) observations of SNe Ia between 150 and 500 d have revealed
a year-long plateau in the $J$\- and $H$-band light curves (Sollerman et al.,
2004; Spyromilio et al., 2004; Lair et al., 2006; Stritzinger & Sollerman,
2007; Graur et al., 2020; Deckers et al., 2023). This plateau is thought to be
the result of ultraviolet (UV) light being shifted into the optical and NIR
(Sollerman et al., 2004; Fransson & Jerkstrand, 2015; Graur et al., 2020). To
date, only normal SNe Ia have been observed long enough to detect this
plateau, and it is unclear whether other SN Ia subtypes exhibit a similar NIR
plateau or not. In this paper, we present the first late-time observations of
a 1991bg-like SN Ia, SN 2021qvv, which show—at least out to 250 d—no evidence
of a NIR plateau.

In Section 2, we describe our observations of SN 2021qvv, which consist of
ground-based imaging and spectroscopy around peak, along with HST NIR imaging
at 170–250 d past maximum light. We only have one epoch of NIR observations
around peak light, so we combine literature NIR data of SN 2006mr with our
late-time HST data. This is not a simple operation, as 1991bg-like SNe are a
heterogeneous family. Hence, in Section 3, we first demonstrate that SN
2021qvv is a bona fide 1991bg-like SN, before showing that SN 2006mr is its
closest relative. With SN 2021qvv stitched on to SN 2006mr, we find no
indication of a late-time NIR plateau out to 250 d.

In Section 4, we pin the absence of a NIR plateau on the temperature of the
ejecta of 1991bg-like SNe, which is known to be appreciably lower than in
normal SNe Ia. If this is the case, then the lack of a plateau in SN 2021qvv
does not necessarily mean that _all_ 1991bg-like SNe lack a NIR plateau. SNe
2006mr and 2021qvv are two of the fastest-evolving and dimmest 1991bg-like SNe
observed to date, and 1991bg-like SNe exist on a continuum, with slower-
evolving, more luminous members of this family exhibiting hotter ejecta and
NIR features progressively more similar to those of normal SNe Ia. Hence, it
is plausible that slower, hotter 1991bg-like SNe will exhibit late-time NIR
plateaus. Finally, we summarise our conclusions in Section 5, where we
encourage further late-time observations of 1991bg-like SNe.

##  2 Observations

SN 2021qvv was discovered by the Asteroid Terrestrial-impact Last Alert System
(ATLAS; Tonry et al. 2018) on 2021 June 23 (Tonry et al., 2021). It is located
$14\aas@@fstack{\prime\prime}3$ west and $3\aas@@fstack{\prime\prime}3$ south
of the center of NGC 4442, a nearby lenticular galaxy. The SN and its host
galaxy are shown in Figure 1. We obtained optical and NIR imaging of SN
2021qvv with the Las Cumbres Observatory, the 3.5-m telescope at the Calar
Alto Observatory, and HST (Section 2.1), as well as three optical spectra
around peak (Section 2.2).

Table 1: Las Cumbres Observatory observation log of SN 2021qvv Date | MJD | Phase | $U$ | $B$ | $V$ | $g$ | $r$ | $i$
---|---|---|---|---|---|---|---|---
| (d) | (d) | (Vega mag) | (Vega mag) | (Vega mag) | (AB mag) | (AB mag) | (AB mag)
24 Jun. 2021 | 59390.3 | $-5.0$ | $\cdots$ | 15.337(0.021) | 14.969(0.025) | $\cdots$ | 15.197(0.015) | 15.307(0.019)
26 Jun. 2021 | 59391.9 | $-3.4$ | 14.470(0.058) | 14.787(0.010) | 14.512(0.014) | 14.502(0.004) | 14.518(0.004) | $\cdots$
27 Jun. 2021 | 59393.6 | $-1.7$ | $\cdots$ | $\cdots$ | 14.249(0.020) | $\cdots$ | 14.316(0.006) | $\cdots$
28 Jun. 2021 | 59394.5 | $-0.8$ | $\cdots$ | 14.549(0.014) | 14.138(0.020) | $\cdots$ | 14.235(0.006) | 14.440(0.007)
29 Jun. 2021 | 59395.5 | $0.2$ | $\cdots$ | 14.512(0.015) | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$
30 Jun. 2021 | 59396.5 | $1.2$ | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | 13.892(0.007) | $\cdots$
1 Jul. 2021 | 59397.5 | $2.2$ | $\cdots$ | 14.592(0.017) | 13.803(0.022) | $\cdots$ | 13.848(0.008) | 14.074(0.013)
2 Jul. 2021 | 59398.5 | $3.2$ | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | 13.90(0.11)
3 Jul. 2021 | 59399.4 | $4.1$ | 15.372(0.059) | 15.019(0.011) | 13.923(0.014) | 14.498(0.004) | 13.768(0.004) | 13.845(0.004)
4 Jul. 2021 | 59400.5 | $5.2$ | 15.657(0.061) | 15.182(0.011) | 14.044(0.014) | 14.654(0.004) | 13.793(0.004) | 13.913(0.005)
6 Jul. 2021 | 59402.3 | $7.0$ | 15.802(0.066) | 15.607(0.012) | 14.240(0.014) | 15.005(0.005) | 13.938(0.004) | 13.981(0.005)
7 Jul. 2021 | 59403.5 | $8.2$ | 16.243(0.064) | 15.783(0.012) | 14.474(0.014) | 15.244(0.005) | 14.135(0.004) | 14.156(0.005)
9 Jul. 2021 | 59405.5 | $10.2$ | 16.387(0.065) | 16.072(0.013) | 14.754(0.014) | 15.551(0.005) | 14.378(0.004) | 14.310(0.005)
11 Jul. 2021 | 59407.2 | $11.9$ | 16.476(0.098) | 16.249(0.016) | 14.885(0.015) | 15.671(0.007) | 14.564(0.005) | 14.442(0.006)
13 Jul. 2021 | 59409.5 | $14.2$ | 16.61(0.11) | 16.533(0.019) | 15.154(0.015) | $\cdots$ | 14.820(0.006) | 14.720(0.007)
17 Jul. 2021 | 59413.5 | $18.2$ | 16.60(0.10) | 16.644(0.041) | 15.246(0.018) | $\cdots$ | 15.053(0.010) | 14.945(0.009)
19 Jul. 2021 | 59415.2 | $19.5$ | $\cdots$ | 16.905(0.080) | 15.495(0.054) | $\cdots$ | $\cdots$ | $\cdots$
20 Jul. 2021 | 59416.5 | $21.2$ | 16.94(0.18) | 16.710(0.023) | 15.536(0.020) | $\cdots$ | 15.278(0.011) | 15.256(0.012)
21 Jul. 2021 | 59418.5 | $23.2$ | $\cdots$ | 17.021(0.038) | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$
23 Jul. 2021 | 59419.5 | $24.2$ | 16.74(0.27) | 16.950(0.120) | 15.684(0.025) | $\cdots$ | 15.610(0.016) | 15.448(0.015)
26 Jul. 2021 | 59422.2 | $26.9$ | 16.94(0.14) | 17.122(0.023) | 15.897(0.022) | $\cdots$ | 15.835(0.015) | 15.615(0.017)
30 Jul. 2021 | 59425.5 | $30.2$ | 17.11(0.10) | $\cdots$ | 16.124(0.022) | $\cdots$ | 16.097(0.017) | 15.881(0.016)
Table 2: HST observation log of SN 2021qvv Date | MJD | Phase | F350LP | F125W | F160W
---|---|---|---|---|---
| (d) | (d) | (Vega mag) | (Vega mag) | (Vega mag)
16 Dec. 2021 | 59564.8 | 169.5 | 20.381(0.011) | 20.529(0.014) | 19.939(0.011)
14 Jan. 2022 | 59594.0 | 198.7 | 20.989(0.016) | 21.069(0.014) | 20.386(0.017)
10 Feb. 2022 | 59621.0 | 225.7 | 21.658(0.023) | 21.457(0.019) | 20.947(0.026)
08 Mar. 2022 | 59647.0 | 251.7 | 22.110(0.028) | 21.920(0.220) | 21.554(0.045)

###  2.1 Imaging

We observed SN 2021qvv between $-5.0$ and $30.2$ d past $B$-band maximum light
using Las Cumbres Observatory’s network of 1-m telescopes (Brown et al., 2013)
as part of the Global Supernova Project and the Aarhus-Barcelona cosmic FLOWS
project.111https://flows.phys.au.dk/ Due to the object’s proximity to its
host, we performed image subtraction using HOTPANTS (Becker, 2015) and
template images taken by Las Cumbres Observatory on 2022 May 26 after the
object had faded. Photometry of the SN was then obtained from the difference
images using lcogtsnpipe (Valenti et al., 2016), a PyRAF-based pipeline.

UBV observations are reported in Vega mags. We calculate zeropoints for
Landolt filters using same-night, same-telescope observations of Landolt
standard fields (Stetson, 2000). gri observations are reported in AB mags. The
zeropoints in these bands were calculated using Sloan catalog stars in the
fields of the SN (Albareti et al., 2017). The final light curves are presented
in Table 1 and Figure 2.

Figure 2: Ground-based optical and NIR light curves of SN 2021qvv obtained with the Las Cumbres Observatory’s network of 1-m telescopes and the 3.5-m telescope at the Calar Alto Observatory, respectively. The solid curves are SNooPy fits, while the dashed curves and patches are the GPR fits to the observations and their $1\sigma$ uncertainty regions. The SNooPy $1\sigma$ uncertainty regions are too small to show up in the figure. |
---|---
Figure 3: A spectroscopic time series of SN 2021qvv (purple), compared to the
1991bg-like SNe 2005ke and 2006mr (left) and the normal SN Ia 2011fe (right).
The spectra of the comparison SNe have been normalized to the spectra of SN
2021qvv at the 635.5 nm Si ii absorption feature. The resemblance of SN
2021qvv to SN 2006mr is apparent, as is the marked difference between SNe
2021qvv and 2011fe. Note, especially, the Ti ii band at 400–440 nm, which is
characteristic of 1991bg-like SNe Ia. Also note the narrow Na ID line in the
pre-maximum spectrum of SN 2006mr, which is due to host-galaxy reddening. This
feature is absent from the spectra of SN 2021qvv, which we attribute to an
absence of dust at the location of the SN.

A single epoch of imaging in the $J$ and $H$ bands was obtained on 2021 June
30 (MJD 59395.9, $0.6$ d after $B$-band maximum light) with the Omega2000
camera mounted on the 3.5-m telescope at the Calar Alto Observatory. Ten and
15 images, each 60 s long, were obtained in the $J$ and $H$ bands,
respectively. The images were dark and flat corrected, sky subtracted, and
combined using our own IRAF-based pipeline. Point-spread function (PSF)
fitting photometry was then computed using the FLOWS project’s automated
pipeline,222https://github.com/SNflows/flows and calibrated using stars from
the Two Micron All Sky Survey (Skrutskie et al., 2006). We note the SN is at a
location with a bright galaxy background and that host-galaxy contribution is
not excluded due to the lack of host-galaxy templates. While it is possible to
attempt to remove the galaxy background by, e.g., fitting the galaxy’s light
profile, this was not done here. However, as we show in Section 3.3, we
estimate the galaxy background to have a minimal effect on our photometry. The
resultant photometry, shown in Figure 2, is $J=13.32\pm 0.04$ mag and
$H=13.16\pm 0.06$ mag (Vega).

Late-time images of SN 2021qvv were obtained with the HST Wide Field Camera 3
(WFC3) wide-band filters F350LP (white light), F125W ($J$), and F160W ($H$)
under HST program GO–16884 (PI: O. Graur) on four separate occasions between
2021 December 16 and 2022 March 8. During these visits, the SN was $\sim
170$–250 d past maximum $B$-band light. These observations are presented in
Table 2 and can be downloaded from the Mikulski Archive for Space Telescopes
(MAST) at doi:10.17909/psf9-7d70.

During each visit, we aligned and stacked the dithered HST exposures in each
filter using the TWEAKREG and ASTRODRIZZLE tasks in the DRIZZLEPAC Python
Package (Gonzaga et al., 2012).333http://drizzlepac.stsci.edu/ This process
also removes cosmic rays and bad pixels. The location of SN 2021qvv was
verified by comparing our observations to pre-explosion images taken with the
HST Advanced Camera for Surveys on 2003 September 9 (GO-9401; PI: P. Cote). As
expected, the SN does not appear in either the F475W ($B$) or F850LP ($z$)
pre-explosion images.

Next, we performed PSF-fitting photometry using DOLPHOT (Dolphin, 2000,
2016),444http://americano.dolphinsim.com/dolphot/ which is specifically
designed to extract photometry from HST images. Because the SN fades over
time, we used forced photometry on the centroid of the SN as derived by
DOLPHOT in the first, brightest, visit. The resultant photometry, in Vega
mags, is presented in Table 2.

Analysis of the DOLPHOT photometry revealed the first F125W visit to be an
outlier. In this visit, DOLPHOT recorded crowding of the SN by a second point
source. In the first subexposure, DOLPHOT measured a magnitude of $20.029\pm
0.009$ mag, with a crowding of $0.8$ mag. In the second subexposure, the
measured photometry is $20.296\pm 0.018$ mag and a crowding of $0.3$ mag. Yet
a visual inspection of the subexposures revealed no hint of a second point
source at or close to the location of the SN.

None of the subexposures from the other F125W or any of the F160W visits
suffer from crowding. DOLPHOT reports some crowding in all of the F350LP
visits. However, a visual inspection of the SN images reveals no other point
source at the location of the SN. Moreover, the crowding reported in F350LP is
ten times smaller than that reported in the first F125W image and has a
negligible effect on the final photometry.

To address this issue, we allowed DOLPHOT to derive the centroid of the SN in
the second visit, and then used forced photometry on the other visits. The
photometry of the second and third visits was consistent with that derived
initially, but the photometry of the first and fourth visits was fainter. No
crowding was reported, but there was a difference of $\sim 0.6$ mag between
the two subexposures of the first visit. We attribute this difference to
masking of a single pixel in the second subexposure, and take the value
derived from the first subexposure, $20.529\pm 0.014$ mag, as the F125W
magnitude of the SN during the first visit. The subexposures of the fourth
visit were different by $\sim 0.2$ mag, which we attribute to the faintness of
the SN at that phase; we adopt this difference as a more conservative estimate
of the photometry uncertainty than the formal value derived by DOLPHOT. As a
final check, we repeated the process described here with the third visit used
to centre the SN. The results were consistent with those derived before.

###  2.2 Spectroscopy

A sequence of three optical spectra were taken with the FLOYDS spectrograph
mounted on the Las Cumbres Observatory’s 2-m telescope on Haleakalā, Hawai’i.
They were reduced with
floyds_pipeline,555https://github.com/lcogt/floyds_pipeline a PyRAF-based
pipeline, using standard longslit spectral reduction methods as described by
Valenti et al. (2014). These spectra are shown in Figure 3 and are available
through the Weizmann Interactive Supernova Data Repository
(WISeREP;666https://www.wiserep.org/ Yaron & Gal-Yam 2012).

##  3 Analysis

With only one epoch of early-phase NIR observations of SN 2021qvv, we must
find a suitable literature 1991bg-like SN with extensively sampled early-phase
NIR light curves to connect to our late-time HST observations. Below, we show
that the spectra (Section 3.1) and light curves (Section 3.2) of SN 2021qvv
are most similar to those of SN 2006mr, a well-studied 1991bg-like SN. A
comparison of this object’s NIR light curves with our late-time HST data
reveals no evidence of a NIR plateau out to 250 d (Section 3.3).

###  3.1 Spectroscopic classification

Based on a spectrum from 2021 June 23, SN 2021qvv was first classified as a
young SN Ia by the Young Supernova Experiment (Jones et al., 2021; Angus &
Rojas-Bravo, 2021). On 2021 July 27, the Zwicky Transient Facility (ZTF; Bellm
et al. 2019) classified the SN as a 1991bg-like object (Chu et al., 2021).

In Figure 3, the three spectra of SN 2021qvv are compared to three well-
studied SNe at similar phases, namely the 1991bg-like SNe 2005ke (Silverman et
al., 2012; Folatelli et al., 2013) and 2006mr (Stritzinger et al., 2010), as
well as the normal SN 2011fe (Parrent et al., 2012; Pereira et al., 2013;
Stahl et al., 2020). The spectra of the comparison SNe have been normalized to
match the Si ii absorption feature at 635.5 nm.

While the spectra of SN 2021qvv are clearly dissimilar to those of the normal
SN 2011fe, they are nearly identical to those of SNe 2005ke and 2006mr. The
conspicuous 400–440 nm Ti ii band that sets 1991bg-like spectra apart from
those of normal SNe Ia (Filippenko et al., 1992a) is apparent in the spectra
of both SNe starting immediately after peak. This comparison confirms the ZTF
classification of SN 2021qvv as a 1991bg-like SN Ia.

Of the two 1991bg-like SNe, SN 2021qvv more closely resembles SN 2006mr, even
though SN 2021qvv is brighter than SN 2006mr at wavelengths bluewards of
$\approx 550$ nm. We attribute this difference to host-galaxy reddening that
afflicts SN 2006mr (Section 3.2.2). There are some notable differences between
SN 2021qvv and SN 2005ke, such as the deeper Ti ii trough in its pre-maximum
spectrum and several features within the Fe-group complex in the +5 d
spectrum. For more on the similarities and differences between these SNe, see
Sections 3.2.3–3.2.4, below.

###  3.2 Photometric classification

We continue the exercise of classifying SN 2021qvv by studying its light
curves and comparing them to those of other 1991bg-like SNe.

Table 3: Properties of SN 2021qvv and its host galaxy, NGC 4442 Property | Value | Ref.
---|---|---
SN 2021qvv
$\alpha$ (J2000) | $12^{\mathrm{h}}28^{\mathrm{m}}02\aas@@fstack{s}98$ | (1)
$\delta$ (J2000) | $+09\degr 48\arcmin 09\aas@@fstack{\prime\prime}9$ | (1)
$t(U)_{\mathrm{max}}$ |  $59388.5\pm 2.2$ d | GPR
$59393.1\pm 0.6\pm 0.3$ (sys) d | SNooPy
$t(B)_{\mathrm{max}}$ |  $59395.3\pm 0.1$ d | GPR
$t(V)_{\mathrm{max}}$ |  $59397.4\pm 0.1$ d | GPR
$t(g)_{\mathrm{max}}$ |  $59396.3\pm 0.2$ d | GPR
$59395.1\pm 0.6\pm 0.3$ (sys) d | SNooPy
$t(r)_{\mathrm{max}}$ |  $59398.6\pm 0.1$ d | GPR
$t(i)_{\mathrm{max}}$ |  $59399.7\pm 0.1$ d | GPR
$t(J)_{\mathrm{max}}$ |  $59398.1\pm 0.6\pm 0.3$ (sys) d | SNooPy
$t(H)_{\mathrm{max}}$ |  $59399.1\pm 0.6\pm 0.3$ (sys) d | SNooPy
$m(U)_{\mathrm{max}}$ |  $14.3\pm 0.2$ mag | GPR
$14.42\pm 0.15\pm 0.03$ (sys) mag | SNooPy
$m(B)_{\mathrm{max}}$ |  $14.51\pm 0.01$ mag | GPR
$m(V)_{\mathrm{max}}$ |  $13.86\pm 0.01$ mag | GPR
$m(g)_{\mathrm{max}}$ |  $14.31\pm 0.02$ mag | GPR
$14.16\pm 0.045\pm 0.014$ (sys) mag | SNooPy
$m(r)_{\mathrm{max}}$ |  $13.78\pm 0.01$ mag | GPR
$m(i)_{\mathrm{max}}$ |  $13.91\pm 0.02$ mag | GPR
$m(J)_{\mathrm{max}}$ |  $13.22\pm 0.30\pm 0.04$ (sys) mag | SNooPy
$m(H)_{\mathrm{max}}$ |  $13.07\pm 0.35\pm 0.03$ (sys) mag | SNooPy
$\Delta m_{15}(B)$ |  $2.05\pm 0.03$ mag | GPR
$s_{\mathrm{BV}}$ | $0.28\pm 0.05$ | GPR
$0.27\pm 0.02\pm 0.03$ (sys) | SNooPy
$E(B-V)_{\mathrm{MW}}$ | 0.019 mag | (2)
$E(B-V)_{\mathrm{Host}}$ |  $0.0\pm 0.0$ mag | This work
$M(B)_{\mathrm{max}}$ |  $-16.42\pm 0.07$ mag | GPR
$M(^{56}\mathrm{Ni})$ |  $0.04$–$0.06~{}M_{\odot}$ | UVOIR
NGC 4442
Type | Lenticular | (3)
$\alpha$ (J2000) | $12^{\mathrm{h}}28^{\mathrm{m}}03\aas@@fstack{s}883$ | (4)
$\delta$ (J2000) | $+09\degr 48\arcmin 13\aas@@fstack{\prime\prime}36$ | (4)
$z_{\mathrm{helio}}$ | $0.00183\pm 0.00002$ | (5)
$v_{\mathrm{helio}}$ | $547\pm 5~{}\rm{km~{}s^{-1}}$ | (5)
$\mu$ |  $30.85\pm 0.07$ mag | This work
$d$ |  $14.8\pm 0.5$ Mpc | This work
Notes. Peak magnitude uncertainties formally lower than 0.01 mag
have been rounded up.
References: (1) Tonry et al. (2021); (2) Schlafly & Finkbeiner (2011);
(3) de Vaucouleurs et al. (1991); (4) Rines & Geller (2008);
(5) Cappellari et al. (2011).
---
Figure 4: Determining $s_{\mathrm{BV}}$. Top: The $B-V$ colour curve of SN
2021qvv (purple squares) is similar to that of SN 2005ke (circles). The data
for SN 1999by were taken from Ganeshalingam et al. (2010) while the data for
SNe 2005ke, 2006mr, and 2007ba were taken from Krisciunas et al. (2017).
$s_{\mathrm{B}V}$ values for these SNe were taken from Burns et al. (2014) and
Dhawan et al. (2017). Bottom: The correlation between $s_{\mathrm{BV}}$ and
$\langle f_{\lambda}(i)\rangle_{\mathrm{20-40}}$ provides an $s_{\mathrm{BV}}$
value similar to that of SN 2006mr. The shaded area denotes the $1\sigma$
confidence region around the best-fitting first-order polynomial curve.

####  3.2.1 Light-curve fits

Standard light-curve fitters, such as MLCS2k2 and SNooPy (Jha et al., 2007;
Burns et al., 2011), classify SN 2021qvv as a 1991bg-like SN but disagree on
the values of the various light-curve parameters, including the date and
magnitude of $B$-band maximum light. MLCS2k2 is optimised to fit normal SNe Ia
and does not contain many templates of 1991bg-like SNe. SNooPy is trained on a
wider range of templates, including those of underluminous SNe Ia, but
provides questionable fits to our $V$\- and $r$-band light curves (Figure 2).

As a consequence, we fit our early, ground-based observations using Gaussian
process regression (GPR). This is done using the Matlab R2023a fitrgp routine
with the default parameters. The resultant fits provide the date and magnitude
of maximum light in each filter. To estimate the uncertainties on these
parameters, we repeat the fit in each filter 100 times, each time varying our
photometry randomly according to the measurement uncertainties. The mean and
standard deviation of the results are then taken as the estimated peak date
and its $1\sigma$ uncertainty. The same method is applied to estimating the
apparent peak magnitude in each filter. Magnitude uncertainties smaller than
$0.01$ mag are rounded up. The GPR fits are shown in Figure 2, and the
resultant parameters are summarised in Table 3.

The ‘max model’ used in the SNooPy fits provides a single date of maximum
light ($59395.1\pm 0.6\pm 0.3$ (sys) d). As a result, the reported peak
apparent magnitude in each band is the magnitude at the time of $B$-band
maximum light, not the magnitude on the date in which the SN peaks in each
filter (for details of the ‘max model,’ see Stritzinger et al. 2010 and Burns
et al. 2011). This assumption does not hold for 1991bg-like SNe, in which the
light curves in successively redder filters peak at progressively later times.
Here, we extract the peak data and magnitude in each filter from the fits
produced by SNooPy and assume that the formal uncertainties reported by SNooPy
hold for these values as well. Because the SNooPy fits are overall similar to
our GPR fits, we only report the peak date and apparent magnitude in $U$, $g$,
$J$, and $H$, where the SNooPy fits offer additional information to that
provided by the GPR fits.

According to our GPR fits, SN 2021qvv reached $B$-band maximum light on MJD
$59395.3\pm 0.1$ d, with $m(B)_{\mathrm{max}}=14.51\pm 0.01$ mag. As seen in
other 1991bg-like SNe, the date of maximum light arrives progressively later
in redder filters. In our analysis below, we adopt these and other GPR-derived
values. We caution not to place too much confidence in our $U$-, $g$-, $J$-,
and $H$-band fits (from both the GPR and SNooPy fits), as the light curves do
not provide good coverage of the peaks in those filters.

####  3.2.2 Reddening

The Galactic line-of-sight reddening towards SN 2021qvv is
$E(B-V)_{\mathrm{MW}}=0.019$ mag (Schlafly & Finkbeiner, 2011). Based on its
colors, location within a lenticular galaxy, and lack of narrow Na ID features
(Figure 3), we estimate no host-galaxy reddening. This is consistent with
several other 1991bg-like SNe found in lenticular and elliptical galaxies
(e.g., Hoogendam et al. 2022).

####  3.2.3 Colour stretch

Burns et al. (2014) have shown that the shape of the $B-V$ colour curve of
normal and underluminous SNe Ia is correlated with other light-curve
parameters, such as $t(i)_{\mathrm{max}}$ or the strength of the second
maximum in the $i$ band. They parameterised the time of $B-V$ maximum, divided
by 30 d, as the ‘colour stretch’ parameter $s_{\mathrm{BV}}$. The top panel of
Figure 4 shows the apparent $B-V$ colour curve of SN 2021qvv along with a
selection of colour curves of 1991bg-like SNe Ia from the literature. The
$B$\- and $V$-band light curves were not corrected for Galactic or host-galaxy
extinction, since Burns et al. (2014) have shown that the $B-V$ colour curves
are insensitive to reddening, with extinctions up to $A_{V}=3$ mag resulting
in changes of $<1$ per cent in $s_{\mathrm{BV}}$ (see their section 3.2).

We follow Burns et al. (2014) and fit the $B-V$ colour curve of SN 2021qvv
with the model:

$y(t)=\frac{(s_{\mathrm{0}}+s_{\mathrm{1}})}{2}+\frac{(s_{\mathrm{1}}-s_{\mathrm{0}})\tau}{2}\mathrm{log}\left[\mathrm{cosh}\left(\frac{t-t_{\mathrm{1}}}{\tau}\right)\right]+c+f_{n}(t,t_{\mathrm{0}}),$
(1)

where $t$ is time; $s_{\mathrm{0}}$ and $s_{\mathrm{1}}$ are the slopes before
and after the time at which the function curves, $t_{\mathrm{1}}$,
respectively; $\tau$ is the timescale over which the curving occurs; $c$ is a
constant; and $f_{n}(t,t_{\mathrm{0}})$ is a $n$-order polynomial for
$t<t_{\mathrm{0}}$ and 0 otherwise. We note that the original equation in
Burns et al. (2014) contained two typos, which have been corrected here.

In the limit $\tau\to 0$ and $s_{\mathrm{0}}\to s_{\mathrm{1}}$, or just
$s_{\mathrm{0}}\to s_{\mathrm{1}}$, the time at which the $B-V$ colour curve
reaches maximum, $t_{\mathrm{max}}$, equals $t_{\mathrm{1}}$. Otherwise,

$t_{\mathrm{max}}=t_{\mathrm{1}}+\frac{\tau}{2}\mathrm{log}\left(\frac{-s_{\mathrm{0}}}{s_{\mathrm{1}}}\right).$
(2)

In either case, $s_{\mathrm{BV}}=t_{\mathrm{max}}/30$.

Given the scatter of our $B-V$ measurements, we dispense with the
$f_{n}(t,t_{\mathrm{0}})$ component of the fit and find best-fitting values of
$s_{0}=0.09\pm 0.01$, $s_{1}=-0.010\pm 0.003$, $\tau=3.9\pm 1.3$ d,
$t_{1}=8.2\pm 1.2$ d, and $c=0.98\pm 0.05$ mag. We note that, in order to
converge, the fit requires the addition of an extra $\pm 0.1$ mag to the
uncertainties of our $B-V$ measurements to properly account for their scatter.
These best-fit parameters result in $t_{\mathrm{max}}=12.3\pm 1.5$ d, which
translates to $s_{\mathrm{BV}}=0.41\pm 0.05$.

A different way to estimate the colour stretch of SN 2021qvv is to study the
correlation between $s_{\mathrm{BV}}$ and $\langle
f_{\lambda}(i)\rangle_{20-40}$, which parameterises the strength of the second
maximum in the $i$ band (Krisciunas et al., 2001; Papadogiannakis et al.,
2019). While slower-evolving 1991bg-like SNe Ia exhibit a second peak in the
$i$ band similar to that seen in normal SNe Ia, that bump gradually merges
with the primary peak in SNe with faster-evolving light curves.

We use GPR to fit the peaks and the 20–40 d range of the $i$-band light curves
of 16 1991bg-like SNe Ia observed by the Carnegie Supernova Project (Contreras
et al., 2010; Stritzinger et al., 2011; Krisciunas et al., 2017). Seven
objects lack pre-maximum observations and are discarded, leaving us with nine
SNe with $\langle f_{\lambda}(i)\rangle_{\mathrm{20-40}}$ values (namely, SNe
2005bl, 2005ke, 2006mr, 2007N, 2007ax, 2009F, 2006bd, 2007al, and 2008bt). The
$s_{\mathrm{BV}}$ values of these SNe are taken from table 3 in Krisciunas et
al. (2017). We find $s_{\mathrm{BV}}=(2.4\pm 0.8)\langle
f_{\lambda}(i)\rangle_{\mathrm{20-40}}-(0.2\pm 0.2)$ with
$\chi^{2}/\mathrm{DOF}=1.3/7$. For SN 2021qvv, we measure $\langle
f_{\lambda}(i)\rangle_{\mathrm{20-40}}=0.209\pm 0.004$. As shown in the bottom
panel of Figure 4, this correlation provides an $s_{\mathrm{BV}}$ value of
$0.28\pm 0.05$. This value is consistent with the value of $0.27\pm 0.02\pm
0.03$ (sys) derived by SNooPy.

An $s_{\mathrm{BV}}$ value of $0.41\pm 0.05$ would put SN 2021qvv in the
company of SN 2005ke ($s_{\mathrm{BV}}=0.419\pm 0.003$), while a value of
$0.28\pm 0.05$ would make it more similar to SN 2006mr
($s_{\mathrm{BV}}=0.260\pm 0.004$). Both SNe were fit by Burns et al. (2014).

A comparison of the $BVri$ light curves of SNe 2005ke, 2006mr, and 2021qvv
reveals that SN 2021qvv evolves appreciably faster than SN 2005ke. Instead, as
shown in Figure 5, SN 2021qvv is nearly a doppelgänger of SN 2006mr. The only
difference between these SNe lies in the $B$-band light curve, which then
produces a wider $B-V$ colour curve than that of SN 2006mr.

Figure 5: Comparison of the $B,~{}V,~{}r,$\- and $i$-band light curves of SNe 2005ke (circles), 2006mr (diamonds), and 2021qvv (squares). Except in the $B$ band, the light curves of SN 2021qvv are more similar to those of SN 2006mr than those of SN 2005ke. The differences in magnitude between SNe 2006mr and 2021qvv are within the uncertainties of their peak $B$-band absolute magnitudes. Table 4: Comparison between SNe 2005ke, 2006mr, and 2021qvv | 2005ke | 2006mr | 2021qvv
---|---|---|---
$m(B)_{\mathrm{max}}$ (mag) | $14.90\pm 0.01$ | $15.43\pm 0.03$ | $14.51\pm 0.01$
$\mu$ (mag) | $31.94\pm 0.13$ | $31.36\pm 0.04$ | $30.85\pm 0.07$
$E(B-V)_{\mathrm{MW}}$ (mag) | 0.020 | 0.018 | 0.019
$E(B-V)_{\mathrm{Host}}$ (mag) | $0.263\pm 0.033$ | $0.089\pm 0.039$ | $\cdots$
$R_{V}$ | 1.0 | 2.9 | $\cdots$
$M(B)_{\mathrm{max}}$ (mag) | $-17.68\pm 0.13$ | $-16.36\pm 0.12$ | $-16.42\pm 0.07$
$s_{\mathrm{BV}}$ | $0.419\pm 0.003$ | $0.260\pm 0.004$ | $0.28\pm 0.05$

####  3.2.4 Peak absolute magnitude

Another way to settle whether SN 2021qvv is more akin to SN 2005ke or to SN
2006mr is to compare their peak absolute $B$-band magnitudes. We use the same
GPR technique described above to measure the peak apparent $B$-band magnitudes
of SNe 2005ke and 2006mr from observations published by Krisciunas et al.
(2017). Table 4 presents these values, along with distance moduli calculated
below and Galactic (Schlafly & Finkbeiner, 2011) and host-galaxy $E(B-V)$ and
$R_{V}$ values (Burns et al., 2014).

Distance moduli of the host galaxy of SN 2021qvv, NGC 4442, have been measured
using the Tully-Fisher (Theureau et al., 2007), surface-brightness
fluctuations (Mei et al., 2007; Blakeslee et al., 2009), and globular-cluster
radius and luminosity function techniques (Jordán et al., 2005, 2007; Villegas
et al., 2010). We calculate a weighted mean of the measurements reported by
the works listed above and derive $\mu_{\mathrm{21qvv}}=30.85\pm 0.07$ mag.

In the same manner, we calculate weighted means of the distance moduli
measured for the host galaxies of SNe 2005ke and 2006mr and find
$\mu_{\mathrm{05ke}}=31.94\pm 0.13$ mag (using values from Tonry et al. 2001;
Terry et al. 2002; Tully et al. 2013; Sorce et al. 2014; Tully et al. 2016)
and $\mu_{\mathrm{06mr}}=31.36\pm 0.04$ mag (based on post-2010 values from
Cantiello et al. 2013; Neill et al. 2014; Sorce et al. 2013; Sorce et al.
2014; Tully et al. 2016).

The host galaxies of SNe 2005ke and 2006mr also have distance moduli derived
by fitting the light curves of the SNe themselves. However, as we have noted
above, SN light-curve fitters are not optimized for 1991bg-like events. Thus,
it is not surprising that these fits produce a wide range of values. Fits to
SN 2005ke have produced distance moduli in the range 31.92–32.06 mag (Wood-
Vasey et al., 2008; Brown et al., 2010), which are consistent with our
weighted average, as well as $31.09\pm 0.11$ mag (Folatelli et al., 2013),
which is nearly a whole magnitude brighter. There is also a range of $\approx
1$ mag in the distance moduli derived from the light curves of SN 2006mr.
Stritzinger et al. (2010) used SNooPy to measure a distance modulus of
$31.25\pm 0.07$ mag from the light curves of three normal SNe Ia that exploded
in the same galaxy (consistent with our weighted average), but $31.84\pm 0.15$
mag from the light curves of SN 2006mr. Folatelli et al. (2013) measured an
even fainter $32.58\pm 0.34$ mag. Because of this wide range of values, we
have excluded SN-based measurements from our weighted averages.

After applying the Cardelli et al. (1989) extinction law to the reddening
towards each SN and subtracting the resultant $A_{B}$ values and distance
moduli from the $B$-band apparent magnitudes measured here, we estimate peak
absolute $B$-band magnitudes of $M(B)_{\mathrm{05ke}}=-17.68\pm 0.13$ mag,
$M(B)_{\mathrm{06mr}}=-16.37\pm 0.15$ mag, and
$M(B)_{\mathrm{21qvv}}=-16.42\pm 0.07$ mag. The uncertainties of these
measurements are the root-mean-squares of the uncertainties on the apparent
magnitudes, distance moduli, host-galaxy reddening, and $R_{V}$ values. The
luminosity we measure for SN 2021qvv, which is consistent with that of SN
2006mr, makes it one of the dimmest SNe Ia observed to date. SN 2005ke, on the
other hand, is roughly three times as luminous.

In sum, we find that SN 2021qvv is most similar to SN 2006mr. The two SNe
share a consistent peak absolute $B$-band brightness, fast-evolving light
curves, and similar spectra. The only deviation appears in the $B$ band
starting at $\approx 10$ d past maximum light, where SN 2021qvv seems to be
fainter than SN 2006mr. This difference, which might simply be due to the
scatter in our measurements at that phase, dictates the shape of the $B-V$
colour curve, which then appears to be more similar to that of the slower-
evolving, more luminous SN 2005ke.

Our analysis has shown that, in order to place a given SN within the continuum
of 1991bg-like SNe, it is not enough to measure its colour stretch,
$s_{\mathrm{BV}}$. Instead, a complete analysis of its spectra, light curves,
and absolute magnitude is required.

Figure 6: Comparison between the late-time HST light curves of SN 2021qvv
(squares), the early light curves of SN 2006mr (diamonds), and the NIR plateau
phase of the normal SNe Ia 2021jad (pentagrams) and 2021pit (hexagrams). The
solid curves are best-fitting polynomials to the late-time data, and the
shaded regions around them represent the $2\sigma$ uncertainty regions of the
fits. In the case of the F125W light curve, the first measurement was excluded
from the fit. Extrapolations of the fits show that, once we account for the
similarity between SNe 2006mr and 2021qvv, there is no need for a late-time
NIR plateau to connect between the early and late phases of the light curves.
The late-time HST observations of SN 2021qvv differ markedly from the
observations of SNe 2021jad and 2021pit (scaled to match the light curves of
SN 2021qvv at 169.5 d). Whereas the normal SNe Ia transition onto the NIR
plateau, SN 2021qvv continues to decline in magnitude. Figure 7 presents a
closer look at the late-time phase. Figure 7: Comparison between the late-time
NIR light curves of SN 2021qvv (squares) and normal SNe Ia 2021jad
(pentagrams) and 2021pit (hexagrams). The light curves of SNe 2021jad and
2021pit have been shifted to match the light curves of SN 2021qvv at 169.5 d.
While the normal SNe Ia transition onto the NIR plateau, SN 2021qvv continues
to fade. By our final observation, SN 2021qvv is $\sim 1.4$ mag and $\sim 1$
mag fainter in F125W and F160W, respectively, than expected had it also
plateaued.

###  3.3 No sign of a NIR plateau

Now that we have established the similarity between SN 2021qvv and SN 2006mr,
we can compare the late-time NIR observations of the former to the early NIR
light curves of the latter. We begin by determining the late-time decline rate
in each HST band by fitting first-order polynomials to our measurements at
170–250 d. We find decline rates of $2.16\pm 0.03$ mag/$100$ d in F350LP
($\chi^{2}/\mathrm{DOF}=12/2$), $1.44\pm 0.09$ mag/$100$ d in F125W
($\chi^{2}/\mathrm{DOF}=0.16/2$), and $1.63\pm 0.05$ mag/$100$ d in F160W
($\chi^{2}/\mathrm{DOF}=27/2$). The high $\chi^{2}/\mathrm{DOF}$ values
indicate that the formal uncertainties of our observations are underestimated.

In Figure 6, we show the absolute $VJH$ light curves of SNe 2006mr and
2021qvv. We have not applied $S$ corrections, as we have no overlapping
observations in similar ground-based and HST filters, but these are expected
to be $<0.5$ mag (Deckers et al., 2023). Our comparison of the F350LP
observations of SN 2021qvv to the $V$-band observations of SN 2006mr is based
on the finding of Milne et al. (2001) that, at $50<t<600$ d, a constant
fraction of the luminosity of normal SNe Ia is emitted in the $V$ band, which
dominates the 350–970 nm wavelength range spanned by the F350LP band (Graur et
al., 2016). We assume that the same is true of 1991bg-like SNe.

The early-phase $V$-band observations of SN 2021qvv are fully consistent with
those of SN 2006mr. The $J$\- and $H$-band observations at $0.6$ d, however,
are $\sim 0.5$ mag brighter than those of SN 2006mr at the same phase. This
difference could be attributed either to contamination from host-galaxy light
or to the lack of $S$ corrections (see above). However, as we show below, it
does not affect our analysis of the late-time HST data.

A comparison between the early-phase light curves and extrapolations of the
fits to the late-time HST observations show a smooth transition in the $V$ and
$H$ bands. SN 2006mr is not sampled in the $J$ band beyond 36 d, so we cannot
connect its observations in this band to our late-time F125W observations of
SN 2021qvv.

Normal SNe Ia exhibit plateaus in the $J$ (F125W) and $H$ (F160W) bands
between $\sim 150$–$500$ d with decline rates of $0.4\pm 0.1$ mag$/100$ d and
$0.5\pm 0.1$ mag$/100$ d, respectively (Deckers et al., 2023). In Figures 6
and 7, we compare our HST observations to ground-based NIR observations of the
normal SNe 2021jad and 2021pit taken at similar phases (Deckers et al., 2023).
The $JH$ light curves of these SNe have been shifted in magnitude to match the
F125W and F160W light curves of SN 2021qvv at 169.5 d. Even as the light
curves of SNe 2021jad and 2021pit transition onto the NIR plateau, SN 2021qvv
continues to fade. At 251.7 d, SN 2021qvv is $\sim 3.6$ and $\sim 2.5$ times
dimmer in F125W and F160W, respectively, than it would have been had it
plateaued.

No statistical test is required to see that the decline rates of SN 2021qvv in
the NIR are markedly different from those of normal SNe Ia. We conclude that,
at least in the range 170–250 d, SN 2021qvv exhibited no measurable plateau.
Furthermore, based on the agreement between the extrapolations of the late-
time observations of SN 2021qvv and the early observations of SN 2006mr, we
propose that SN 2021qvv did not go through a NIR plateau prior to 170 d,
either.

###  3.4 UVOIR light curve and 56Ni mass

Our $UBVgriJK$ observations of SN 2021qvv make it possible to construct the a
UVOIR (or ‘pseudo-bolometric’) light curve which, in turn, will yield an
estimate of the mass of 56Ni produced by the explosion, $M(^{56}\mathrm{Ni})$.
Since the SN was not observed with the full range of filters at each phase, we
use our GPR fits to the $UBVgri$ filters and the SNooPy fits to the $J$\- and
$K$-band measurements presented in Figure 2. At each phase, we convert our
photometry to flux and construct an artificial, low-resolution spectrum that
spans the wavelength range of our filters, 3100–18200 Å. The artificial
spectrum is then integrated and the result multiplied by $4\pi d^{2}$, where
$d$ is the distance to the SN as derived from the distance modulus: $14.8\pm
0.5$ Mpc. The resultant UVOIR light curve is shown in Fig. 8.

We note that, while the UV and NIR light curves are not well sampled, each of
these bandpasses provides only $\sim 10$ per cent of the flux that goes into
the final UVOIR light curve, as seen in other 1991bg-like SNe (Scalzo et al.,
2019). We further note that, because the SNooPy-derived fits to the NIR bands
lack uncertainties, the total uncertainty of the UVOIR light curve is
underestimated.

To estimate $M(^{56}\mathrm{Ni})$, we follow Arnett’s Rule (Arnett, 1982;
Arnett et al., 1985; Pinto & Eastman, 2000b, a), according to which the
maximum luminosity of an SN Ia, $L_{\mathrm{max}}$, is proportional to the
instantaneous energy deposition rate from the radioactive decays within the
expanding ejecta, $E_{\mathrm{Ni}}(t_{R})$:

$L_{\mathrm{max}}=\alpha E_{\mathrm{Ni}}(t_{R}),$ (3)

where $t_{R}$ is the rise time to maximum light and the value of the
proportional parameter, $\alpha$, is $\approx 1$; following Stritzinger &
Leibundgut (2005), we take $\alpha=1$. The energy produced by 1 $M_{\odot}$ of
56Ni is given by the equation (Stritzinger & Leibundgut, 2005):

$E_{\mathrm{Ni}}(1~{}M_{\odot})=6.45\times
10^{43}\mathrm{e}^{-t_{R}/8.8}+1.45\times 10^{43}\mathrm{e}^{-t_{R}/111.3},$
(4)

where $t_{R}$ is the rise time to maximum light and the denominators in the
exponentials are the e-folding decay times for 56Ni and 56Co, respectively.
The 56Ni mass of a given SN is then:

$M(^{56}\mathrm{Ni})=\frac{L_{\mathrm{max}}}{E_{\mathrm{Ni}}(1~{}M_{\odot})}~{}M_{\odot}.$
(5)

For normal SNe Ia, the mean rise time is $19\pm 3$ d (Stritzinger &
Leibundgut, 2005), but underluminous SNe display faster rise times.
Taubenberger et al. (2008), for example, estimated a rise time of $14\pm 1$ d
for the underluminous SN 1999by. The UVOIR light curve derived here peaks
$\approx 2.7$ d after $B$-band maximum light, which is $\approx 10$ d after
the discovery of SN 2021qvv. This provides a rough lower limit on the rise
time of the SN.

With a maximum UVOIR luminosity of $\approx 1.4\times
10^{42}~{}\mathrm{erg~{}s^{-1}}$ (the underestimated formal uncertainties are
${}^{+0.09}_{-0.03}~{}\mathrm{erg~{}s^{-1}}$) and rise times of 10–15 d, we
find that the 56Ni mass produced by SN 2021qvv lies in the range
$0.04$–$0.06~{}M_{\odot}$. Using the same method, but a rise time of 19 d,
Stritzinger et al. (2006) estimated 56Ni masses of $0.05$ and
$0.09~{}M_{\odot}$ for SNe 1999by and 1991bg, respectively. Assuming a rise
time of 14 d converts these values to $\sim 0.04$ and $\sim 0.07~{}M_{\odot}$,
respectively. Hence, the 56Ni mass produced by SN 2021qvv is consistent with
estimates derived from similar, though more luminous, 1991bg-like SNe.

Figure 8: UVOIR luminosity curve of SN 2021qvv (solid curve) and $1\sigma$
uncertainty region (grey patch). The SN peaked at
$L_{\mathrm{max}}=2.59^{+0.17}_{-0.15}~{}\mathrm{erg~{}s^{-1}}$, $\approx$ 2.7
d after $B$-band maximum light. The SNooPy fits to the NIR data have no
uncertainties, so the formal uncertainty regions shown here are only lower
estimates. Figure 9: 1991bg-like SNe with high $s_{\mathrm{BV}}$ values, such
as SN 2007ba, exhibit second maxima in the $i$ and NIR bands similar to those
seen in normal SNe Ia. As we progress to faster-evolving 1991bg-like SNe with
lower $s_{\mathrm{BV}}$ values, such as SN 2005ke, the second maxima begin to
blend into the initial peak. Finally, the second maxima disappear completely
in the fastest-evolving SNe, such as SN 2006mr. This evolution may hint that,
although we find no evidence for a late-time NIR plateau in SN 2021qvv, such a
plateau may still exist in slower-evolving 1991bg-like SNe.

##  4 Discussion

This work is motivated by the preponderance of SN Ia subtypes and the need to
associate each subtype with a progenitor and explosion scenario. It is widely
agreed that the direct progenitor of all SNe Ia is a carbon-oxygen white dwarf
(WD). As with all stars, WDs are found in different types of systems: alone;
coupled with a second WD or a non-degenerate type of star; and in higher-order
systems. Theory suggests various methods by which WDs in different stellar
systems could be brought to explode (for the most recent review of the various
progenitor channels, see Liu et al. 2023).

The observed diversity of WD systems makes it hard to countenance that all SN
Ia subtypes would arise from a single combination of progenitor and explosion
scenario. At the same time, there is no reason to assume that each and every
SN Ia subtype requires its own boutique set of physics. Instead, it is
plausible that some SN Ia subtypes exist along a continuum, where variations
in the composition of the exploding WD, the parameters of its stellar system,
or the parameters of the explosion method (or some combination of all three)
give rise to SNe Ia with a continuous range of spectroscopic and photometric
properties.

There are reasons to assume that 1991bg-like SNe are distinct from normal SNe
Ia. These include the unique spectroscopic features of these SNe, such as
their telltale Ti ii trough (Figure 3); their apparent non-adherence to the
peak width-luminosity relation (Phillips et al. 1999; Garnavich et al. 2004;
Taubenberger et al. 2008; but see Burns et al. 2014); and their preference for
exploding in massive, passive galaxies with old stellar populations (Howell,
2001; Neill et al., 2009; Graur et al., 2017; Barkhudaryan et al., 2019;
Larison et al., 2023).

At the same time, there are also suggestions that 1991bg-like SNe Ia are but
dimmer, faster-evolving siblings of normal SNe Ia. Nugent et al. (1995) and
Heringer et al. (2017) showed that varying the luminosity and temperature of a
single ejecta structure could produce the spectroscopic properties of both
normal and underluminous SNe Ia. Blondin et al. (2013) showed that by varying
the amount of 56Ni synthesized in the delayed detonation of a Chandrasekhar-
mass WD, they could recreate the photometric and spectroscopic properties of
both subtypes. In a similar vein, Shen et al. (2018, 2021) showed that
underluminous 1991bg-like SNe, normal SNe Ia, and overluminous 1991T-like SNe
all arose from double detonations of WDs with varying initial masses between
0.85 and 1.1 $M_{\odot}$. This result was echoed by Polin et al. (2019, 2021),
who showed that double detonations of WDs with thin He shells
($0.01~{}M_{\odot}$) recreated many of the features seen in near-peak and
nebular spectra of underluminous, normal, and overluminous SNe Ia.

On the face of it, the absence of a NIR plateau in SN 2021qvv should further
set 1991bg-like SNe apart from normal SNe Ia. There is, however, an argument
to be made that this is another sign that 1991bg-like SNe Ia exist on a
continuum with normal SNe Ia.

Nebular NIR spectra of normal SNe show that, during the plateau phase, [Fe
iii] emission lines fade away while [Fe ii] lines remain at a constant
luminosity (Dhawan et al., 2018; Diamond et al., 2018; Maguire et al., 2018),
suggesting that doubly-ionized iron ions are recombining to singly-ionized
ions (Deckers et al., 2023). In this respect, the NIR plateau is reminiscent
of the secondary NIR maximum seen in SNe Ia, which is thought to be the result
of a ‘recombination wave’ that sweeps through the ejecta. Starting out in the
cooler outer layers of the ejecta, this wave recedes inwards. When the iron-
rich inner regions of the ejecta begin to recombine, the iron-rich gas becomes
fluorescent: high-energy photons scatter off the dense forest of UV iron lines
and are redistributed to the NIR (Pinto & Eastman, 2000a; Kasen, 2006;
Fransson & Jerkstrand, 2015). A similar effect is thought to be the reason for
the existence of the late-time NIR plateau (Sollerman et al., 2004; Fransson &
Jerkstrand, 2015; Graur et al., 2020).

The recombination of [Fe iii] to [Fe ii] provides two possible explanations
for the absence of a NIR plateau in SN 2021qvv. First, one-dimensional, non-
local thermodynamic equilibrium time-dependent radiative transfer simulations
of four SN Ia ejecta models conducted by Wilk et al. (2018) showed a marked
difference in the late-time NIR spectra produced by the central detonation of
a sub-Chandrasekhar-mass WD. Artificial spectra produced $\sim 100$ and $\sim
200$ d after bolometric maximum light showed doubly-ionized Fe and Co lines
but no singly-ionized lines from these elements. The absence of these lines
was explained by a larger fraction of local radioactive heating by positrons,
which inhibits recombination. Without the recombination of [Fe iii] to [Fe
ii], a SN produced by such an explosion scenario would not experience a late-
time NIR plateau. This explanation, if borne out by spectroscopic observations
of underluminous SNe Ia at late times, would strengthen the proposition that
these SNe are the result of detonations of sub-Chandrasekhar-mass WDs.

A second explanation may be found in the observation that the NIR light curves
of 1991bg-like SNe display a marked evolution from slower- to faster-evolving
objects (Figure 9). The slowest-evolving 1991bg-like SNe, such as SN 2007ba
($s_{\mathrm{BV}}=0.55$), display secondary peaks in the $iYJ$ bands similar
to those seen in normal SNe Ia. As we progress to faster-evolving 1991bg-like
SNe, such as SN 2005ke ($s_{\mathrm{BV}}=0.42$), the secondary maxima shift to
earlier times and begin to blend with the primary maxima. These features are
completely absent from the fastest-evolving 1991bg-like SNe, such as SN 2006mr
($s_{\mathrm{BV}}=0.26$).

The evolution of the secondary NIR maximum may be related to the temperature
of the SN ejecta, which are thought to be colder in 1991bg-like SNe than in
normal SNe Ia (e.g., Mazzali et al. 1997; Doull & Baron 2011; Heringer et al.
2017). For example, the progressive narrowing of line widths seen in optical
nebular spectra from normal to 1991bg-like SNe is interpreted as evidence that
faster-evolving SNe Ia (with lower $s_{\mathrm{BV}}$ values) have
progressively colder ejecta (Graham et al., 2022; Kumar et al., 2023). In this
scenario, as we transition from hotter (high $s_{\mathrm{BV}}$) to colder (low
$s_{\mathrm{BV}}$) SNe, the iron recombination wave responsible for the second
maximum occurs at earlier times, eventually merging with the primary maximum
in the coldest 1991bg-like SNe (Taubenberger, 2017).

In the same vein, perhaps below a certain temperature the ejecta of 1991bg-
like SNe become too cold to produce the fluorescence mechanism that gives rise
to the late-time NIR plateau. Spectroscopic modeling of 1991bg-like SNe—and
especially of SNe 2006mr and 2021qvv—could reveal whether this is the case.
Such modeling could also predict whether all 1991bg-like SNe are too cold for
the fluorescence mechanism or whether there is a temperature threshold above
which flux can once more be redistributed from the UV to the NIR. Future NIR
observations of 1991bg-like SNe at 150–500 d would then reveal which is the
case. Either the NIR plateau will never be observed in underluminous SNe or it
will appear only in SNe with a certain $s_{\mathrm{BV}}$ value and above.

The results of such a test would also impact our understanding of normal SNe
Ia. A detection of the NIR plateau in hotter 1991bg-like SNe would strengthen
the suggestion that these underluminous SNe are an extension of normal SNe Ia.
This would be doubly true if the appearance of the NIR plateau becomes more
similar to that of normal SNe Ia as we progress from colder to hotter 1991bg-
like SNe. Any progenitor and explosion scenario will then be required to
recreate the observed properties of both normal and 1991bg-like SNe, lending
more credence to works that manage to accomplish this feat (e.g., Blondin et
al. 2013 and Shen et al. 2021) over works that treat 1991bg-like SNe as a
distinct subtype (e.g., Pakmor et al. 2010, 2011; Dhawan et al. 2017).

##  5 Conclusions

In this work, we have presented optical and NIR observations of SN 2021qvv at
early times ($-5$ to 30 d past $B$-band maximum) and at late times (170–250
d). The spectroscopy and early light curves of SN 2021qvv show that it belongs
to the underluminous, 1991bg-like subtype of SNe Ia. SN 2021qvv closely
resembles SN 2006mr, making it one of the dimmest
($M(B)_{\mathrm{max}}=-16.42\pm 0.07$ mag) and fastest-evolving
($s_{\mathrm{BV}}=0.28\pm 0.05$) 1991bg-like SNe to date.

A comparison of the early NIR light curves of SN 2006mr to our late-time HST
observations of SN 2021qvv reveals no indication of the late-time NIR plateau
seen in normal SNe Ia, at least out to 250 d past maximum light. The absence
of the plateau is consistent with certain sub-Chandrasekhar-mass detonation
models that inhibit the recombination of [Fe iii] to [Fe ii] that drives the
NIR plateau (Wilk et al., 2018).

We further argue that this absence does not preclude the appearance of a late-
time NIR plateau in other 1991bg-like SNe. The secondary NIR maxima routinely
observed in normal SNe Ia are also observed in slowly-evolving 1991bg-like SNe
but gradually occur earlier in faster-evolving SNe until finally merging with
the primary maximum in the fastest-evolving underluminous SNe Ia. This is
thought to be the result of lower temperatures in the ejecta of faster-
evolving 1991bg-like SNe. Since SN 2021qvv is one of the dimmest, fastest-
evolving SNe observed to date, the lack of a plateau may also be connected to
its presumably colder ejecta.

Nebular NIR spectra of 1991bg-like SNe Ia will reveal whether the absence of
the NIR plateau is due to a high ionization state of the ejecta, as predicted
by Wilk et al. (2018). In parallel, spectroscopic modeling of fast-evolving
1991bg-like SNe Ia, including SNe 2006mr and 2021qvv, could reveal whether the
fluorescence mechanism that is thought to give rise to the NIR plateau depends
on temperature, and whether below a certain ejecta temperature it would cease
to operate altogether. In parallel, a larger sample of 1991bg-like SNe
observed in the NIR out to 500 d should reveal whether the NIR plateau is
absent in all such objects, or whether there is a correlation between the
appearance of the plateau and the temperature of the SN, as encapsulated in
its colour stretch parameter, $s_{\mathrm{BV}}$. The existence of such a
correlation would strengthen the proposition that 1991bg-like SNe and normal
SNe Ia exist along a single continuum and, hence, share the same progenitor
and explosion physics.

## Acknowledgments

We thank Chris Burns for his assistance fitting the colour curve of SN 2021qvv
and the anonymous referee for their comments and suggestions. M.D. and K.M.
are funded by EU H2020 ERC grant no. 758638. L.G. acknowledges financial
support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the
Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European
Social Fund (ESF) ‘Investing in your future’ under the 2019 Ramón y Cajal
program RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project, from
Centro Superior de Investigaciones Científicas (CSIC) under the PIE project
20215AT016, and the program Unidad de Excelencia María de Maeztu
CEX2020-001058-M. The Aarhus supernova group is funded in part by an
Experiment grant (# 28021) from the Villum FONDEN, and by project grants
(#8021-00170B, 10.46540/2032-00022B) from the Independent Research Fund
Denmark (IRFD). K.J.S. was in part supported by NASA/ESA Hubble Space
Telescope programs #15871 and #15918. This work is based on data obtained with
the NASA/ESA Hubble Space Telescope through program GO-16884, as well as data
obtained with the Las Cumbres global telescope network. The Las Cumbres
Observatory group is supported by NSF grants AST-1911151 and AST-1911225, as
well as HST-GO-16884. This research has also made use of NASA’s Astrophysics
Data System and the NASA/IPAC Extragalactic Database (NED), which is funded by
the National Aeronautics and Space Administration and operated by the
California Institute of Technology. Finally, this work has made use of the
Open Supernova Catalog (Guillochon et al., 2017).

## Data Availability

The raw HST data can be found in MAST at doi:10.17909/psf9-7d70. The ground-
based images are archived by the Las Cumbres Observatory and can be accessed
by request. The reduced spectra will be made available in WISeREP at
https://www.wiserep.org/. The reduced data and derived measurements in this
article will be shared on request to the corresponding author.

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