The early-time light curves of type II and type IIb supernovae from the ATLAS survey

During the last decade, the increase in survey depth, cadence, and sky coverage has led to a significant increase in the number of discovered supernovae (SNe) within days of explosion. Discovering SNe at such epochs enables investigation into the outer density profile of progenitor stars. Understanding the origin of this progenitor material can lead to strong constraints on late stages of stellar evolution. Here, I will discuss the early time light-curves of both Hydrogen-rich SNe type II (SNe~II) and Hydrogen-poor SNe~IIb observed by the ATLAS survey. For SNe~II, we compare light-curve properties with the presence or absence of early-time 'flash' ionisation features, finding evidence that those SNe~II with such spectra features show distinct light curves. We also find that those SNe~II displaying flash features show much lower absorption of H-alpha during the later plateau. These results suggest a link between early-time interaction and later-phase transient properties of SNe~II. In the case of SNe~IIb, we characterise the presence and frequency of early-excess in SNe~IIb light curves - a feature that has generally been attributed to 'shock cooling' in the literature. We find a frequency of early-excess light curves of SN~IIb of between ~30 and 40%. In addition, we find that those SNe~IIb displaying an early-excess light curve appear to have longer-duration light curves. I end discussing of the implications of these results on the progenitors of SNe~II and SNe~IIb.