0 Assigned ID: 3855 ----- Attempting Submission 1 (Fri Oct 21 21:44:30 GMT 2022) ----- HST Phase I Proposal 3855 successfully submitted. Receipt: # 3855-1 Added visit 02 to repeat failed orbit in visit 01. Setting an orient link between 02 and 01 seems to push earliest schedulable block to July 2024, which is not ideal so I did not include orient link. Thank you for scheduling the repeat! 62484078 ----- Attempting Submission 1 (Thu Dec 29 17:34:29 GMT 2022) ----- ----- Attempting Submission 2 (Tue Jun 06 15:03:56 GMT 2023) ----- ----- Attempting Submission 3 (Tue Sep 19 15:12:33 GMT 2023) ----- Probing the nature of the UV-bright transient emission from AT2018cow more than 4 years post-explosion Recent advances in high-cadence surveys have revealed a new class of luminous and rapidly-evolving transients, which are not intrinsically rare but pose significant challenges to conventional supernova models. In 2018, an extreme member of this class, AT2018cow, was identified. AT2018cow was the first rapidly-evolving transient discovered promptly at a small distance, facilitating intensive multiwavelength follow-up. However, despite being the most extensively studied transient of its class, its true nature is still debated, with possibilities ranging from tidal disruption events to the failed explosion of a massive star. Late-time observations of rapidly-evolving transients offer a means to break this degeneracy, and HST images taken during 2020 (more than 700 days post-explosion) revealed a UV-bright point source at the precise position of AT2018cow. The nature of this source was initially ambiguous, as it could be interpreted as either emission from the transient or an underlying stellar population. However, new observations obtained in June 2022 now definitively show that the source is fading in the UV---and hence that the emission is due to AT2018cow itself. Here we request a single epoch of HST/UVIS imaging in order to further trace the decline rate and change in spectral shape of the late-time emission, necessary to constrain the power source. In particular, extrapolations based on the 2020 and 2022 data indicate the peak of the SED may shift into the HST band during Mid-Cycle 30. We emphasize that with its current UV-optical brightness of 25-27 mag (AB), HST is the only current instrument capable of probing this source. We request five orbits of observations with WFC3/UVIS to constrain the evolution of a transient underlying source at the position of AT2018cow more than 4 years post-explosion. For the observations, we request the use of two UV filters (F225W and F336W), three optical filters (F438W, F555W, and F814W), and one narrow Halpha filter (F665N). Two orbits are used to observe in the F814W and F665N filters with a box dither: each orbit contains four exposures (two for each filter). Three orbits are used to observe in the F225W, F336W, F438W, and F555W filters with a 3-point line dither: each orbit contains four exposures (one for each filter). The UVIS2-C1K1C-SUB aperture is used for all exposures to reduce overheads. We are aware of the recommendation of 1-3 orbits per visit for Cycle 30, but for our proposal, we do request a single visit that includes all five orbits. The reason is that our objective is to obtain an accurate spectral energy distribution of an evolving/transient source at a single epoch, which can only be achieved if all filters are observed at approximately the same time (i.e., consecutively). Finally, as indicated in the phase I proposal, we request the observation to be taken during the second half of Cycle 30 (after the start of April) when the predicted spectral evolution becomes significant enough to be discernible. All investigators on this project are experts in various aspects of transient astrophysics. PI Yuyang Chen is a PhD student at the University of Toronto who has analyzed all previous HST observations of AT2018cow. Co-I Drout is an expert in time domain astronomy and massive star populations, with a focus on the nature of peculiar and rapidly-evolving transients. Co-Is Foley and Kilpatrick are experts in a wide-array of supernova science and have been PIs of numerous previous HST proposals, including 2018 observations of AT2018cow. Finally, Co-I Piro is an expert in theoretical models of explosive transients with a focus on observational signatures. Stellar Physics and Stellar Types 3 false false false false false true false false SUPPORTED EXT-STAR SUPERNOVA Magnitude is a rough estimate from extrapolating previous HST observations. J2000 J2000 26.5 AB mag in F555W band WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F665N WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 3 false The scheduling data was generated by CASM 31.1. CASM was run in Phase II mode. 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CASM was run in Phase II mode. Repeat of failed second orbit of visit 01. (Tic-Visit :proposal-id 17290 :visit-id "02" :proposal-version -1 :trans-version "20230626" :date "19-Sep-2023 11:08:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 71312 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17290 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17290 :visit-id "02" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2756 :al-overhead-at-end 83 :internal-dur-at-end 114 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3139 :pcs-gap 0 ) true 1 0