0 Assigned ID: 1600 ----- Attempting Submission 1 (Thu Apr 08 06:57:46 GMT 2021) ----- HST Phase I Proposal 1600 successfully submitted. Receipt: # 1600-1 ----- Attempting Submission 2 (Fri Apr 09 07:44:13 GMT 2021) ----- 0 ----- Attempting Submission 1 (Wed Jul 21 14:43:03 GMT 2021) ----- Radioactive Stars: Bound Remnants from White Dwarf Supernovae Type Ia supernovae (SN Ia) have enormous importance to cosmology and astrophysics, but their progenitors and explosion mechanisms are not understood in detail. Recently, observations and theoretical models have suggested that not all thermonuclear white-dwarf supernova explosions are normal SN Ia. In particular, type Iax supernovae (SN Iax, peculiar cousins to SN Ia), are thought to be exploding white dwarfs that are not completely disrupted, leaving behind a bound remnant. We propose late-time HST WFC3 observations of three recent SN Iax to directly observe if they each harbor a bound remnant, with a radiation-driven wind. Our program is designed to discover a new category of stars, powered by radioactivity, and to show that some thermonuclear supernovae are not terminal events in the life of their progenitor white dwarfs (unlike the complete disruption thought to occur in normal SN Ia). We will obtain late-time HST photometry of three nearby type Iax supernovae (SN Iax): SN 2019gsc, 2019muj, and 2020kyg. Each object will be observed in one three-orbit visit, with WFC3/UVIS F555W, F814W, and WFC3/IR F125W using one orbit per filter. We will use a 4 point dither in each orbit. For the WFC3/UVIS data, we will mitigate CTE losses by placing the target near the readout amplifier using one of the CTE apertures. The exposure times (about 600 seconds per dither position) give a sky background count level approaching or exceeding the recommended 20 e- and the fields also contain light from the supernova host galaxies, so we do not think post-flash illumination will be necessary. For SN 2019muj, we include a timing constraint to observe late in the cycle to allow for more contrast between the expectations of SN ejecta flux and the bound remant flux we are searching for. Dr. Saurabh W. Jha (Rutgers) has extensive experience observing and analyzing type Iax supernovae (SN Iax) in the optical and infrared, including with HST. Rutgers graduate students Ms. Yssavo Camacho-Neves and Ms. Lindsey Kwok will assist in the data reduction and analysis. Dr. Barnabas Barna and Dr. Tamas Szalai (U of Szeged) have experience with spectroscopic analysis and modeling of SN Iax, and have led an observational campaign on SN 2019muj. Prof. Ryan Foley (UCSC) also has extensive experience observing and analyzing type Iax supernovae (SN Iax) in the optical and infrared, including with HST. Dr. Or Graur (Portsmouth) has experience with late-time infrared photometry of white dwarf supernovae. Dr. Kate Maguire (Dublin) and Dr. Mark Magee (Dublin) are experts on spectroscopic analysis and modeling of SN Ia and SN Iax. Dr. Curtis McCully (Las Cumbres) has experience observing SN Iax and their progenitors with HST. Dr. Yen-Chen Pan (NAOJ) has performed spectroscopic analysis of SN Iax, including SN 2019muj. Dr. Ken Shen (Berkeley) has led the development of models of SN Iax bound remnants. Dr. Lina Tomasella (Padova) has led observations and analysis of SN Iax, including SN 2019gsc. 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