The early UV/Optical emission from core-collapse supernovae
  We derive a simple approximate model describing the early, hours to days,
UV/optical supernova emission, which is produced by the expansion of the outer
<~0.01 solar mass part of the shock-heated envelope, and precedes the optical
emission driven by radioactive decay. Our model includes an approximate
description of the time dependence of the opacity (due mainly to
recombination), and of the deviation of the emitted spectrum from a black body
spectrum. We show that the characteristics of the early UV/O emission constrain
the radius of the progenitor star, 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 over estimate of progenitor radius by more than an
order of magnitude. We also show that the relative extinction at different
wavelengths may be inferred from the light-curves at these wave-lengths,
removing the uncertainty in the estimate of progenitor radius due to reddening
(but not the uncertainty in E/M due to uncertainty in absolute extinction). The
early UV/O observations of the type Ib SN2008D and of the type IIp SNLS-04D2dc
are consistent with our model predictions. For SN2008D we find progenitor
radius to be approx. 10^11 cm, and an indication that the He envelope contains
a significant C/O fraction.
