0 Assigned ID: 771 ----- Attempting Submission 1 (Fri Apr 05 21:11:56 GMT 2019) ----- Please find attached our submission of GO 15974. Best Wishes, Andrew -1160691342 ----- Attempting Submission 1 (Wed Jul 24 20:18:21 GMT 2019) ----- Unmasking the cow: Distinguishing models for the unique transient AT2018cow AT2018 is a transient unlike any seen before, unprecedented in its rapid rise time and high luminosity. It may well be a manifestation of one of two long sought after astrophysical phenomena: either an intermediate mass black hole, or a direct view to the central engine in a collapsing star. Here we propose a modest set of HST imaging observations to answer a vital question relating to AT2018cow -- where was it born? If it was created in the tidal disruption of a white dwarf star by an intermediate-mass black hole, we might expect to uncover a globular cluster at its location, since it is in these locations that IMBHs may exist with a sufficiently high stellar density for white dwarfs to be scattered into their loss cone. Alternatively, if the source of AT2018cow is a massive star, we should see signs of ongoing star formation. Our observations in H-alpha and the UV will probe very young, extremely massive (tens of solar masses) stars and distinguish them from somewhat older, less massive (~10 Msol) stars. Deep optical observations will uncover any underlying globular cluster directly. Through these observations, we will measure the environment in which AT2018cow was born, and through that environment, we will determine its origin. We propose to obtain three orbits of observations of AT2018cow after is has faded with the aim of understandings its environment. We will obtain images in two UV filters (F225W and F606W), two optical filters (F555W and F814W) and two narrow band filters (F665N and F657N, the first of these will be off-band at the redshift of AT2018cow, while the second will match with H-alpha). For each filter we will obtain 3 exposures along a standard line dither with an observation time of half an orbit. In order to fit within the orbit our UV and narrow band observations will use the UVIS2-C1K1C-SUB aperture to reduce overheads. For F555W and F814W it is possible to fit the observations within a single orbit while still using the full chip. We have requested that the observations be scheduled approximately within 1 month of eachother since if there is any residual transient flux the data will be most useful if they are approximately taken at the same time. We have extensive experience in observations of transients and their host galaxies with the Hubble Space Telescope. Levan has led a total of 34 HST proposals (347 orbits). the majority of which have focussed on this science area. Perley has led a further two proposals (63 orbits) specifically focussed on the host galaxies of gamma-ray bursts. In addition Tanvir has led 23 HST proposals and Fruchter 28 science proposals (STScI staff also lead calibration and other proposals). We have published our HST data extensively, with HST data appearing in well over 100 papers from the CoIs of this proposal. These papers have been cited over 10000 times (ADS numbers). We continue to have both the experience and resources to commit to the analysis and publication of these data. 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