0 Assigned ID: 4932 ----- Attempting Submission 1 (Thu Nov 16 03:13:43 GMT 2023) ----- HST Phase I Proposal 4932 successfully submitted. Receipt: # 4932-1 Resubmission of revised Phase II file to schedule refunded orbits in new Visit 03 -773723748 ----- Attempting Submission 1 (Thu Feb 22 14:52:07 GMT 2024) ----- HST Phase II Proposal 17596 successfully submitted. Receipt: # 17596-1 ----- Attempting Submission 2 (Wed Mar 06 20:46:25 GMT 2024) ----- ----- Attempting Submission 3 (Wed Jul 03 16:18:12 GMT 2024) ----- Revealing the Powering Mechanisms of Type Icn Supernovae through Late-time Observations We propose to obtain two epochs of HST imaging of supernova (SN) 2023xgo, the closest Type Icn SN to date (d = 62 Mpc), between 120 to 300 days after explosion in order to constrain its late-time powering mechanism. Type Icn SNe are a rare but astrophysically-interesting class of transients powered by interaction with H- and He-poor circumstellar material-- evidence that their uncertain progenitor stars underwent extraordinary mass loss before explosion. A better understanding of their progenitors and mass-loss mechanisms will impact theories of stellar evolution and the formation of gravitational wave sources. Modeling the light curves of these objects has surprisingly revealed that their explosions produce little, if any, radioactive 56Ni. Progenitor scenarios that permit such little 56Ni include the explosion of a massive Wolf-Rayet star undergoing significant fallback accretion, or of an ultra-stripped, lower-mass (<= 12 Msun) star. We request late-time photometric observations in order to differentiate between the proposed progenitor scenarios by estimating the explosive nucleosynthetic yields and by searching for evidence of ongoing interaction with material at larger distances from the progenitor star. These observations will further constrain radioactive decay as a powering mechanism for synthesized 56Ni masses >= 0.001 Msun. The low redshift of SN 2023xgo and its optimal phase throughout Cycle 31 offer the first possibility to observe the late-time light curve of a Type Icn SN to date. These observations will directly probe the progenitor properties of these rare and exotic transients, informing our understanding of the extremes of mass loss in massive stars. We propose two visits, each comprising of two orbits, to image the nearby supernova SN 2023xgo with the WFC3/UVIS F606W and F814W bands as well as the WFC3/IR F125W and F160W bands. We will use the first orbit in each visit to image SN 2023xgo with WFC3/UVIS in the F606W and F814W filters, and the second orbit in each visit to image SN 2023xgo with WFC3/IR in the F125W and F160W filters. We request that our first visit be scheduled on 22 March 2024 and our second visit on 20 July 2024. This window fits our criterion of having two visits separated by at most 120 days. During this time frame we expect the brightness of SN 2023xgo to be within our proposed range of V band magnitude 24.0 +/- 2.0. In particular, obtaining our first observation on 22 March is vital as the supernova will not be visible to HST again until mid July. Waiting until that time for our first visit runs the risk that the supernova will be too faint to observe by the second visit. For each visit, our observation plan is as follows. In our first orbit, we will utilize the WFC3-UVIS-DITHER-LINE pattern with the F606W and F814W filters. We request a total of 1200s of exposure time in F606W and 1432s of exposure time in F814W, each split evenly between two exposures. In our second orbit, we will utilize the WFC3-IR-DITHER-BOX-MIN pattern with the F125W and F160W filters. For each filter, we request a total of ~1312s of exposure time, split evenly into four exposures. We expect these observations to be unaffected by the possibility of reduced gyro operations during Cycle 31. These observations do not rely on utilizing HST's full field of view, nor on its normal spatial scan speed. This team comprises of experts in supernova observations, particularly observations of peculiar stripped-envelope and interacting supernovae with multi-wavelength resources, including HST. Dr. Craig Pellegrino is the P.I. and one of the forefront experts on Type Icn supernovae, having published the first sample paper of this class of objects (Pellegrino et al. 2022, ApJ, 938, 73). Prof. Maryam Modjaz is an expert on stripped-envelope and interacting supernovae and has led multiple HST studies of these objects. Dr. Tyler Pritchard is an expert on fast-evolving and stripped-envelope supernovae, particularly in the UV and optical, and will aid in the analysis of these observations. Kyle Davis is a Ph.D. student and an expert on observations of Type Icn supernovae, particularly in the infrared. Prof. Ryan Foley is an expert supernova observer who has worked Type Ibn supernovae for the last 17 years. He is PI of multiple ground-based observing programs, including those at Lick and Keck Observatories, which will observe SN 2023xgo. 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