0 Assigned ID: 2684 ----- Attempting Submission 1 (Mon Jan 31 19:42:50 GMT 2022) ----- 0 ----- Attempting Submission 1 (Tue Mar 22 00:08:48 GMT 2022) ----- 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) reveal a UV-bright point source at the precise position of AT 2018cow. However, the nature of this underlying source is ambiguous as it can be interpreted as either the emission from the transient or an underlying stellar population--both of which would place significant constraints on the origin of AT2018cow. Recently, a shallow HST SNAP observation in 2021 showed tentative (2-sigma) evidence for fading in the optical (where the source is faint). Here we request a single epoch of HST/UVIS imaging to confirm the transient nature of the source and measure its rate-of-decline, necessary to constrain the power source. Extrapolations based on physically and observationally motivated models show that the source may be detected during this Mid-Cycle at 25.5-26.0 mag (AB). We emphasize that HST is the only current instrument with the resolution and depth capable of probing this source. We request three orbits of observations with WFC3/UVIS to measure the properties of the source identified at the position of AT2018cow and determine whether it is constant or evolving with time. For the observations, we request the use of two UV filters (F225W and F336W) and two optical filters (F555W and F814W). Each filter will have three exposures with a 3-point line dither. Each orbit will contain four exposures - one exposure for each filter - and the three orbits will complete exactly the desired twelve exposures for all filters (three exposures x four filters). The UVIS2-C1K1C-SUB aperture is used for all exposures to reduce overheads. 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 SUPPORTED EXT-STAR SUPERNOVA Magnitude is a rough estimate from extrapolating the brightness assuming the same spectral shape. J2000 J2000 26.0 ABmag in F555W WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false 1647893741205 C:\Users\frank\AppData\Local\Temp\OPVisit17311612047713675920.xml The scheduling data was generated by CASM 30.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16925 :visit-id "01" :proposal-version -1 :trans-version "20211223" :date "21-Mar-2022 16:15:40.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 213904 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16925 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16925 :visit-id "01" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2765 :al-overhead-at-end 74 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16925 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2765 :al-overhead-at-end 74 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16925 :visit-id "01" :orbit-number 3 :pcs-time 392 :si-vis-utilization 2765 :al-overhead-at-end 74 :internal-dur-at-end 270 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3157 :pcs-gap 0 ) true 3 0