The early UV/optical emission from core-collapse supernovae

We derive a simple approximate model describing the early, hours to days, UV/optical (UV/O) supernova emission, which is produced by the expansion of the outer ≲10^-2 M_⊙ part of the shock-heated envelope, and precedes optical emission driven by radioactive decay. Our model includes an approximate description of the time dependence of the opacity (mainly due to recombination), and of the deviation of the emitted spectrum from a blackbody spectrum. We show that the characteristics of the early UV/O emission constrain the radius of the progenitor star, R_*, its envelope composition, and the ratio of the ejecta energy to its mass, E/M. For He envelopes, neglecting the effect of recombination may lead to an overestimate of R_* by more than an order of magnitude. We also show that the relative extinction at different wavelengths (A_λ-A _V) may be inferred from the light curves at these wavelengths, removing the uncertainty in the estimate of R_* due to reddening (but not the uncertainty in E/M due to uncertainty in absolute extinction). The early UV/O observations of the types Ib SN 2008D and IIp SNLS-04D2dc are consistent with our model predictions. For SN 2008D, we find R_* ≈ 10¹¹ cm, and an indication that the He envelope contains a significant C/O fraction.