0 Assigned ID: 3890 ----- Attempting Submission 1 (Fri Feb 17 18:37:10 GMT 2023) ----- HST Phase I Proposal 3890 successfully submitted. Receipt: # 3890-1 0 ----- Attempting Submission 1 (Tue Apr 25 16:23:57 GMT 2023) ----- Supernova (SN) 2021gno in NGC 4165 (D=30.5 Mpc) is the second closest known Calcium-rich (Ca-rich) transient and only the second object in this class with an X-ray detection. Prompt, high-cadence follow-up of this transient across the EM spectrum indicated that the progenitor star was likely low mass (e.g., a white dwarf or a <10 Msun star) and surrounded by a dense shell of circumstellar material (CSM) whose geometry/density was capable of producing luminous X-ray emission as well as a double-peaked optical light curve. The close proximity of SN 2021gno provided only the second opportunity to obtain multi-color HST observations out to ~450 days post explosion, which indicate a pure radioactive decay power source. Ca-rich transients typically decrease in magnitude rapidly and therefore their extremely late-time decline rate and power source at t>700 days is completely unknown. Here we propose multi-color imaging of SN 2021gno in order to search for emission from a bound remnant white dwarf as predicted by a favored progenitor model for this explosion. Detection and characterization of such a remnant will be the most substantial proof of a WD origin for a Ca-rich transient to date and will provide a novel window into the explosion mechanism. Additionally, these observations will provide one of the best constraints to date on a potential non-degenerate companion star in any thermonuclear SN progenitor system, such as Ca-rich SNe. We propose to observe nearby supernova 2021gno with WFC3/UVIS in filters F555W and F814W in the first of our two allocated orbits. In the second orbit, we will observe SN 2021gno with WFC3/IR in filter F160W. Our optimal observing window is between 07-May-2023 and 18-June-2023, however, 22-June-2023 to 27-July-2023 is another acceptable scheduling window. We have requested that IR imaging be taken within 3 days of the UVIS imaging orbit. Below is our observing plan for each filter in each orbit: Orbit 1: F555W: Total Exposure time = 1500s; Split into two 750s exposures using WFC3-UVIS-DITHER-LINE pattern F814W: Total Exposure time = 900s; Split into two 450s exposures using WFC3-UVIS-DITHER-LINE pattern Orbit 2: F160W: Eight ~300s individual exposures for each filter using the WFC3-IR-DILTER-BOX-MIN pattern. Mr. Wynn Jacobson-Galan has extensive experience analyzing and reducing WFC3/UVIS data, the instrument proposed for this program. Mr. Jacobson-Galan is a graduate student with burgeoning expertise on the late-time light curves of thermonuclear supernovae observed with HST (2 published papers) and calcium-rich transients (4 published papers). Mr. Jacobson-Galan has completed analysis on the early-time, multi-wavelength observations of the proposed target, SN 2021gno, and all data on this object is included in journal article Jacobson-Galan+ 2022b, published in ApJ. Prof. Raffaella Margutti is a leading expert on x-ray and radio observations of exotic transients such as SN 2021gno. Dr. Charles Kilpatrick has significant experience in reduction and analysis of HST/WFC3, ACS, and WFPC2 imaging data, including the prime instruments used in these proposed observations. Prof. Ryan Foley is an expert in transient surveys and follow up of all types of explosive transients. Dr. Armin Rest is an expert in transient science, in particular SN Ia cosmology, light echoes of historic SN, and exotic transients. His technical expertise lies in data reduction, image analysis, photometry, and their application to wide-field time-domain surveys. Drs. Ken Shen and Yossef Zenati have extensive experience with theoretical modeling and interpretation of Ca-rich and other thermonuclear transient varieties, including modeling of bound remnant systems. Huei Sears has extensive experience with HST reductions and will assist in the data analysis. Prof. Ashley Villar, Prof. Katie Auchettl and Mr. David Coulter are experts in photometric analysis and modeling. Stellar Physics and Stellar Types 3 false false false false false true false false SUPPORTED STAR SUPERNOVA J2000 J2000 g ~ 26-27, r ~ 25, i/z ~ 24 WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W WFC3/IR Imaging No No F160W POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 1 false POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 2 false 1682694639842 The scheduling data was generated by CASM 31.0. CASM was run in Phase II mode. /private/var/folders/z3/xy_g1v_s5fz9yrhzb8thz4cr0001qc/T/OPVisit16717556105421611822.xml Orbit 1: Filters: F555W, F814W Orbit 2: Filters: F160W (Tic-Visit :proposal-id 17300 :visit-id "01" :proposal-version -1 :trans-version "20220630" :date "28-Apr-2023 11:10:39.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 3104800 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17300 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17300 :visit-id "01" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2763 :al-overhead-at-end 116 :internal-dur-at-end 383 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17300 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2736 :al-overhead-at-end 0 :internal-dur-at-end 614 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) true 2 0