0 Assigned ID: null ----- Attempting Submission 1 (Wed Jul 18 08:27:28 GMT 2018) ----- Submission failed due to error communicating with the submission server. Please retry in a few minutes. If this error persists please contact the help desk at help@stsci.edu Assigned ID: 20 ----- Attempting Submission 1 (Wed Jul 18 08:28:12 GMT 2018) ----- slight change to POS-TARG 488006573 ----- Attempting Submission 1 (Thu Jul 19 16:25:44 GMT 2018) ----- HST Phase II Proposal 15600 (thecow.apt) successfully submitted. Receipt: # 15600-1 ----- Attempting Submission 2 (Fri Jul 20 00:16:08 GMT 2018) ----- HST Phase II Proposal 15600 (thecow.apt) successfully submitted. Receipt: # 15600-2 ----- Attempting Submission 3 (Fri Jul 20 14:20:32 GMT 2018) ----- Seeing The Cow in Ultraviolet Light With an exponentially increasing number of explosive transient discoveries, we are finding increasingly diverse and spectacular ways for stars to die. In the last few years, we identified a class of rapidly evolving transients - objects with luminosities similar to supernovae (SN), but fade ten times faster. They are distinct from normal SN, and represent a new variety of stellar explosion. We recently discovered a truly unique member, AT 2018cow ("The Cow"). It is similar to other rapidly-evolving transients, but represents both the most luminous and fastest event discovered to date. In addition, it exhibits a significant non-thermal component, manifesting as both strong radio emission and particularly luminous, long-lasting, and variable X-ray emission. Such a transient was not predicted and no physical model to-date explains all observations. The prompt discovery, high luminosity, and close distance (60 Mpc) of The Cow offer an unique opportunity to probe the nature of these explosions. Currently, the best models are either a magnetar-powered explosion or a fallback supernova. While current observations cannot distinguish between these models, continued photometric monitoring in the UV with HST can. Multiple imaging epochs can also provide a link between the thermal and non-thermal components if the X-ray decline changes significantly during this campaign. A UV spectrum will reveal unique spectral species, constraining the nature of the progenitor star. Here we request HST UV spectra and photometry to determine the progenitor and explosion scenario for this extraordinary event and to provide a legacy data set for future "bovine" transients. We plan to image AT 2018cow in 4 UV filters over 3 epochs. 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