0 Assigned ID: 2032 ----- Attempting Submission 1 (Fri Apr 09 17:10:28 GMT 2021) ----- HST Phase I Proposal 2032 successfully submitted. Receipt: # 2032-1 ----- Attempting Submission 2 (Fri Apr 09 18:44:53 GMT 2021) ----- 0 ----- Attempting Submission 1 (Wed Jul 28 18:55:14 GMT 2021) ----- Supernova 2020wnt: a pair-instability explosion in our backyard? We propose for one orbit of WFC3 UVIS/IR imaging, and one non-disruptive ToO orbit of WFC3 IR grism spectroscopy of a nearby superluminous supernova (SLSN) 2020wnt to discern whether it is a pair-instability (PI) SN. PISNe are the theoretical death of a 140-260 solar mass stars, and they may be a dominant contributor to nucleosynthesis in the early universe. A direct observation of a bona fide PISN does not yet exist in the literature. SN 2020wnt is the best candidate for a PISN yet discovered. It has a long rise from a large ejecta mass (~60 Msun) to a peak luminosity of -20.5 mags; both consistent with some PISN models. The light curve declines post-maximum with a rate of 0.01 mag/day, consistent with the radioactive decay of Co-56. Lastly, latest near-IR spectrum shows shows features associated with Fe-group elements. These features point to an explosion that produces a copious amount of nickel, like a PISN. Our imaging observations will (i) determine whether the SN is still bright enough for the grism observations and (ii) probe the host properties local to the SN site. If the SN is brighter than 22 mags in the F140W filter, we will trigger a non-disruptive ToO to obtain grism spectroscopy. The particular lines we aim to observe are the [S I], [Si I] and [Fe II] line complexes around 1.1, 1.25, and 1.6 micron. The nebular spectrum will measure the nucleosynthetic yield of the SN, and unambiguously determine whether SN 2020wnt is a PISN or other more common deaths of a massive star. We plan to observe SN 2020wnt in two epochs. The first epoch contains one orbit and focuses on imaging the SN and the host using WFC3 UVIS and IR with the F275W, F475W, F110W, F140W, and F160W. For the second epoch, we observe SN 2020wnt with the WFC3 IR Grisms. We will obtain direct imaging in the F105W and F140W filters. Then we will observe the source with both the G102 and G141 grisms. Dr. Samaporn Tinyanont specializes in near-infrared observations of astronomical transients associated with the death of massive stars. Prof. Ryan Foley is an expert transient observer who has extensively used HST and other space-based observatories. His previous HST programs include an ultra rapid ToO to observe interacting SN in the TESS field; a snapshot program to observe late-time emissions from nearby SNe; and a ToO program to observe SNe Ia in the UV at early time. He has preferred access to telescopes at Lick and Keck observatory, with which he will support this program. Dr. Kirsty Taggart is an expert in SLSNe and their host environment. She led a comprehensive and comparative host property studies between host galaxies of SLSNe, CCSNe, and other astronomical transients. Dr. Lin Yan has led several SLSN projects, and is experienced with analyses of SLSN LC and spectroscopic data, at the UV, optical and near-IR wavelength, including HST observations of SLSNe, e.g, Gaia16apd. Dr. Ragnhild Lunnan is an expert on observations of superluminous supernovae, and has led several studies of objects with PISN or PPI SN characteristics. She is currently leading the SLSN discovery and follow-up efforts within the ZTF collaboration. 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