Early Ultraviolet Spectroscopy of a Nearby Supernova

We propose for an ultra-rapid, disruptive ultraviolet (UV) spectroscopic sequence of one nearby SN discovered within ~12 hours of explosion and within the TESS footprint. We will combine these observations with a rapid response, high cadence Swift UV light curve, along with a comprehensive optical+NIR ground-based campaign. This data set will offer an unique view of the explosive shock breakout of the SN explosion, companion star interaction, and the last phases of massive star evolution. At early times, our understanding of SN progenitors usually comes from extrapolations from the optical, where less than 1% of the light is. Only UV spectroscopy will allow us to put firm constraints on the luminosity, the temperature and the metallicity of the progenitor. For core collapse SNe, the early temperature evolution can constrain the progenitor radius or extended envelope. The UV light, or the SN ejecta, can ionize the surrounding circumstellar medium (CSM) and can constrain the progenitor star's metallicity, wind speed and CSM extent. For type Ia SNe, recent very early light curve excesses point to shocking with a normal companion star, CSM interaction or an unusual nickel distribution, but models cannot reproduce UV light curves, and only spectroscopy will reveal the progenitors of these essential cosmological tools. A concurrent TESS light curve for this UV spectroscopic sequence will pinpoint the moment of explosion and act as an additional probe of CSM/companion star interaction. Very early UV spectroscopy of a nearby SN is a completely unique observation that only HST can provide, and the SN community may not have this opportunity again for the foreseeable future.