Early UV-optical observations of supernovae and what they tell us about the last moments in stellar evolution

In my talk, I’ll discuss how UV-optical observation of supernovae very early (<3d) in their evolution can inform us about their progenitor stars and the explosion itself. I’ll show how many calcium rich supernovae and stripped-envelope supernovae have a prominent early time behavior, probably related to the CSM they ejected weeks or months before the explosion, informing us about the explosion mechanism. I’ll also show how in Type II supernovae, the early UV-optical light curve can be used to map out the density profile of the progenitor prior to explosion. I’ll show this for the very well observed case of the M101 SN2023ixf, and discuss what can be learned from a sample of SNe II from the ZTF survey with early UV data. Using this sample, I will argue that while most SNe II have a shell of dense CSM affecting the UV-optical light curve, this is due to a luminosity bias, and up to 80% of red supergiant stars result in a supernova shock breakout from the stellar envelope itself. If time permits, I’ll outline our plans to observe such supernovae with the ULTRASAT survey, and what we can expect to learn.