UV/Optical Emission from the Expanding Envelopes of Type II Supernovae

Abstract

The early part of a supernova (SN) light-curve is dominated by radiation escaping from the expanding shockheated progenitor envelope. For polytropic Hydrogen envelopes, the properties of the emitted radiation are described by simple analytic expressions and are nearly independent of the polytropic index, n. This analytic description holds at early time, t < few days, during which radiation escapes from shells initially lying near the stellar surface. We use numerical solutions to address two issues. First, we show that the analytic description holds at early time also for non-polytropic density profiles. Second, we extend the solutions to later times, when the emission emerges from deep within the envelope and depends on the progenitor's density profile. Examining the late time behavior of polytropic envelopes with a wide range of core to envelope mass and radius ratios, 0.1 ≤ M c /M env ≤ 10 and 10 −3 ≤ R c /R ≤ 10 −1 , we find that the effective temperature is well described by the analytic solution also at late time, while the luminosity L is suppressed by a factor, which may be approximated to better than 20[30]% accuracy up to t = t tr /a by A exp[−(at/t tr ) α ] with. This description holds as long as the opacity is approximately that of a fully ionized gas, i.e. for T > 0.7 eV, t < 14(R/10 13.5 cm) 0.55 d. The suppression of L at t tr /a obtained for standard polytropic envelopes may account for the first optical peak of double-peaked SN light curves, with first peak at a few days for M env < 1M ⊙ . Subject headings: radiation hydrodynamics -shock waves -supernovae: general 1. INTRODUCTION During a supernova (SN) explosion, a strong radiation mediated shock wave propagates through and ejects the stellar envelope. As the shock expands outwards, the optical depth of the material lying ahead of it decreases. When the optical depth drops below ≈ c/v sh , where v sh is the shock velocity, radiation escapes ahead of the shock and the shock dissolves. In the absence of an optically thick circum-stellar material, this breakout takes place once the shock reaches the edge of the star, producing an X-ray/UV flash on a time scale of R/c (seconds to a fraction of an hour), where R is the stellar radius. The relatively short breakout is followed by UV/optical emission from the expanding cooling envelope on a day timescale. As the envelope expands its optical depth decreases, and radiation escapes from deeper shells. The properties of the breakout and post-breakout cooling emission carry unique information on the structure of the progenitor star (e.g. its radius and surface composition) and on its pre-explosion evolution, which cannot be directly inferred from observations at later time. The detection of SNe on a time scale of a day following the explosion, which was enabled recently by the progress of wide-field optical transient surveys, yielded important constraints on the progenitors of SNe of type Ia, Ib/c and II. For a recent comprehensive review of the subject see Waxman & Katz (2016).At radii r close to ...

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