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 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 (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, 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 over estimate 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 wave-lengths, 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 type Ib SN2008D and of the type IIp SNLS-04D2dc are consistent with our model predictions. For SN2008D we find R * ≈ 10 11 cm, and an indication that the He envelope contains a significant C/O fraction.

Type-IIn supernovae (SNe), which are characterized by strong interaction of their ejecta with the surrounding circumstellar matter (CSM), provide a unique opportunity to study the mass-loss history of massive stars shortly before their explosive death. We present the discovery and follow-up observations of a Type IIn SN, PTF 09uj, detected by the Palomar Transient Factory (PTF). Serendipitous observations by GALEX at ultraviolet (UV) wavelengths detected the rise of the SN light curve prior to the PTF discovery. The UV light curve of the SN rose fast, with a time scale of a few days, to a UV absolute AB magnitude of about −19.5. Modeling our observations, we suggest that the fast rise of the UV light curve is due to the breakout of the SN shock through the dense CSM (n ≈ 10 10 cm −3 ). Furthermore, we find that prior to the explosion the progenitor went through a phase of high mass-loss rate (∼ 0.1 M ⊙ yr −1 ) that lasted for a few years. The decay rate of this SN was fast relative to that of other SNe IIn.

On 2011 May 31 UT a supernova (SN) exploded in the nearby galaxy M51 (the Whirlpool Galaxy). We discovered this event using small telescopes equipped with CCD cameras and also detected it with the Palomar Transient Factory survey, rapidly confirming it to be a Type II SN. Here, we present multi-color ultraviolet through infrared photometry which is used to calculate the bolometric luminosity and a series of spectra. Our early-time observations indicate that SN 2011dh resulted from the explosion of a relatively compact progenitor star. Rapid shock-breakout cooling leads to relatively low temperatures in early-time spectra, compared to explosions of red supergiant stars, as well as a rapid early light curve decline. Optical spectra of SN 2011dh are dominated by H lines out to day 10 after explosion, after which He i lines develop. This SN is likely a member of the cIIb (compact IIb) class, with progenitor radius larger than that of SN 2008ax and smaller than the eIIb (extended IIb) SN 1993J progenitor. Our data imply that the object identified in pre-explosion Hubble Space Telescope images at the SN location is possibly a companion to the progenitor or a blended source, and not the progenitor star itself, as its radius (∼10 13 cm) would be highly inconsistent with constraints from our post-explosion spectra.

We present our observations of SN 2010mb, a Type Ic SN lacking spectroscopic signatures of H and He. SN 2010mb has a slowly-declining light curve (∼ 600 days) that cannot be powered by 56 Ni/ 56 Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free CSM including a blue quasi-continuum and , uniquely, narrow oxygen emission lines that require high densities (∼ 10 9 cm −3 ). From the observed spectra and light curve we estimate that the amount of material involved in the interaction was ∼ 3M . Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.

A unique feature of deflagration-to-detonation (DDT) white dwarf explosion models of SNe of type Ia is the presence of a strong shock wave propagating through the outer envelope. We consider the early emission expected in such models, which is produced by the expanding shock-heated outer part of the ejecta and precedes the emission driven by radioactive decay. We expand on earlier analyses by considering the modification of the pre-detonation density profile by the weak-shocks generated during the deflagration phase, the time evolution of the opacity, and the deviation of the post-shock equation of state from that obtained for radiation pressure domination. A simple analytic model is presented and shown to provide an acceptable approximation to the results of 1D numerical DDT simulations. Our analysis predicts a ∼ 10 3 s long UV/optical flash with a luminosity of ∼ 1 to ∼ 3 × 10 39 erg s −1 . Lower luminosity corresponds to faster (turbulent) deflagration velocity. The predicted luminosity of the UV flash is an order of magnitude lower than that of earlier estimates, and is expected to be strongly suppressed at t > t drop ∼ 1 hr due to the deviation from pure radiation domination.