Program: GN-2018A-Q-122

The supernova (SN) community is preparing for an extraordinary experiment. For 5 months, the Kepler telescope (K2) will perform a SN survey. Monitoring ~20,000 galaxies with a 30-minute cadence, K2 will detect ~50 SNe within hours - perhaps even minutes - of explosion. Such data have proven to be a unique window to the details of the SN explosion, progenitor, and circumstellar (CS) environment. We are devoting significant ground-based telescopic resources to search for and follow these SNe. We have been awared 19 orbits of HST time and a single "ultra-rapid" ToO to take advantage of these emergent SNe and exquisite K2 light curves. For the first few days after a SN explosion, one can potentially see signs of the SN interacting with its CS environment (e.g., a wind, accretion disk, companion star) that are not present later in its evolution. For instance, the large UV flux from a SN shock breakout will ionize CS gas. As the gas recombines over the following days, it produces excess broad-band flux and reveals the CSM (and thus progenitor) composition through emission lines. However, UV spectra can greatly enhance our understanding of SN progenitor systems over just optical data, including progenitor composition, CS environment, and the existence of a binary companion. Our program will observe a single K2 SN that shows signs of early interaction. Because of the ephemeral nature of the interaction signatures, this program requires an ultra-rapid ToO. We were awarded 18 hours of Gemini time in support of this program (17B-0914), and this is the accompanying proposal, where we request Rapid Gemini ToO observations. The combination of K2 photometry, ground-based data, and HST UV spectra will be a completely unique and defining data set. As Kepler will soon be retired, this is our only opportunity for such a program.