Anthony L. Piro, Ehud Nakar
Ongoing transient surveys are presenting an unprecedented account of the rising lightcurves of Type Ia supernovae (SNe Ia). This early emission probes the shallowest layers of the exploding white dwarf, which can provide constraints on the progenitor star and the properties of the explosive burning. We use semi-analytic models of radioactively-powered rising lightcurves to analyze these observations. As we have summarized in previous work, the main limiting factor in determining the surface distribution of 56Ni is the lack of an unambiguously identified time of explosion, as would be provided by detection of shock breakout or shock-heated cooling. Without this the SN may in principle exhibit a "dark phase" for a few hours to days, where the only emission is from shock-heated cooling that is too dim to be detected. Nevertheless, by considering the time-dependent velocity evolution, the explosion time can be better constrained, albeit with considerable uncertainty. This technique is used to infer the surface 56Ni distribution of three recent SNe Ia that were caught especially early in their rise. Although we cannot constrain the explosion times to better than ~1 day, in all three we find fairly similar 56Ni distributions. Observations of SN 2011fe and SN 2012cg probe shallower depths than in SN 2009ig, and in these two cases 56Ni is present merely ~0.01Msun from the WD's surface. We also use our conclusions about the explosion times to reassess radius constraints for the progenitor of SN 2011fe, as well as discuss the roughly t^2 power law that is inferred for many observed rising lightcurves.
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http://arxiv.org/abs/1211.6438
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