Added request for the use of BETWEEN timing constraints. 18492989 Assigned ID: 2567 ----- Attempting Submission 1 (Tue Sep 28 10:03:41 GMT 2021) ----- HST Phase I Proposal 2567 successfully submitted. Receipt: # 2567-1 ----- Attempting Submission 2 (Tue Sep 28 15:06:38 GMT 2021) ----- 0 ----- Attempting Submission 1 (Mon Nov 22 15:47:02 GMT 2021) ----- Do subluminous Type Ia supernovae experience a near-infrared plateau? We ask for 4 WFC3 orbits to observe the subluminous 1991bg-like Type Ia supernova (SN Ia) SN 2021qvv when it is 150-250 days past maximum light. With observations in F125W (J), F160W (H), and F350LP (optical) in each of four visits, we will determine whether this SN Ia subtype undergoes a similar near-infrared (NIR) plateau as the one recently discovered in so-called "normal" SNe Ia. The latter have long been used as standard candles in cosmology, yet the nature of their progenitors and explosion mechanism remains elusive. The abundance of SN Ia subtypes, including 1991bg-like SNe Ia, only complicates matters. It is still unclear whether several (or all) SN Ia subtypes share the same progenitor and explosion method as normal SNe Ia, or whether each subtype originates in a different type of star and explosion physics. The first option ties into the nagging question of how standard SNe Ia really are as standard candles. Revealing the progenitors of normal SNe Ia and the other SN Ia subtypes would go some way to answering this question. The recently discovered NIR plateau at 150-500 days offers a new way to constrain the progenitors and explosion physics. In normal SNe Ia, this plateau could be a third NIR maximum, similar to the second NIR maximum seen shortly after the first peak. These additional peaks could be due to the recombination of iron in the expanding ejecta. These secondary NIR peaks are not seen in 1991bg-like SNe Ia, a fact used to constrain the mass, temperature, and chemical composition of the progenitor. The presence or absence of the late-time NIR plateau in 1991bg-like SNe would place further constraints on the progenitor and the recombination wave model. In this program, we will be observing a 1991bg-like supernova called SN 2021qvv. In each visit, one orbit will be split between F350LP, F125W, and F160W, with most of the time spent on the near-IR filters. The first visit is set to occur sometime in the first half of December, as I assume that's the earliest date we could schedule it. The next visit should occur 25-35 days after the first visit. This will also be the cadence for visits 3 and 4. The timing of visit 4 will be flexible. Once visits 1 and 2 are taken, we will analyze the data, locate the supernova, and decide whether we underestimated the background light from the host galaxy. If that is the case, visit 4 will be moved to the end of Cycle 30, when the supernova has faded away, and be used as a template to subtract from visits 1-3. This will allow us to subtract the galaxy and isolate the supernova. Throughout the program, we may ask to remove certain filters or change exposure times, based on our ongoing analysis of the incoming data. We will always allow plenty of time for such changes to be scheduled. Drs. Lluis Galbany, Or Graur, Griffin Hosseinzadeh, Andy Howell, Saurabh Jha, Curtis McCully, and Adam G. Riess are all expert supernova (SN) observers. Dr. Graur has used HST+WFC3 to lead several experiments similar to the one proposed here, with SN 2012cg (Graur et al. 2016), SN 2015F (Graur et al. 2018a), ASASSN-14lp (Graur et al. 2018b), SN 2014J (Graur 2019), and several SNe Ia in the near-infrared (Graur et al. 2020). Dr. Riess has a long record of observing SNe with HST. Drs. McCully and Jha are experts in the study of peculiar, subluminous SNe Ia. Drs. McCully, Hosseinzadeh, and Howell form the core of the Las Cumbres Observatory's Global Supernova Project, which has provided early-time observations of the SN in this proposal. Dr. Michael M. Shara is an expert in HST observations of various types of transients. Dr. David R. Zurek is an expert in the processing of HST images. Drs. Robert Fisher, Andrew Fullard, Wolfgang Kerzendorf, Ken Shen, and Ivo R. Seitenzahl are experts in the theory of SN explosions and nucleosynthesis; Drs. Fisher, Kerzendorf, and Seitenzahl specifically have expertise in the theory of the late-time behavior of SNe Ia. Finally, Dr. Armin Rest is a world leader in the detection and analysis of light echoes. Dr. Or Graur will serve as PI of this program, and Dr. Saurabh Jha will serve as its US administrative Co-I. 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