The Rise-Time of Type II Supernovae

TL;DR

This study analyzes early light-curves of 223 Type II supernovae to determine rise-times and progenitor radii, revealing smaller radii than typical red supergiants and implications for supernova explosion models.

Contribution

It provides the first large-sample analysis of supernova rise-times and constrains progenitor radii using observational data and theoretical models.

Findings

Median rise-time is 7.5 days in g-band.

Inferred progenitor radii are less than 400 solar radii.

Massive hydrogen envelopes are necessary to explain observed plateaus.

Abstract

We investigate the early-time light-curves of a large sample of 223 type II supernovae (SNe) from the Sloan Digital Sky Survey and the Supernova Legacy Survey. Having a cadence of a few days and sufficient non-detections prior to explosion, we constrain rise-times, i.e. the durations from estimated first to maximum light, as a function of effective wavelength. At restframe g-band (4722A), we find a distribution of fast rise-times with median of (7.5+/-0.3) days. Comparing these durations with analytical shock models of Rabinak and Waxman (2013); Nakar and Sari (2010) and hydrodynamical models of Tominaga et al. (2009), which are mostly sensitive to progenitor radius at these epochs, we find a median characteristic radius of less than 400 solar radii. The inferred radii are on average much smaller than the radii obtained for observed red supergiants (RSG). Investigating the post-maximum slopes as a function of effective wavelength in the light of theoretical models, we find that massive hydrogen envelopes are still needed to explain the plateaus of SNe II. We therefore argue that the SN II rise-times we observe are either a) the shock cooling resulting from the core collapse of RSG with small and dense envelopes, or b) the delayed and prolonged shock breakout of the collapse of a RSG with an extended atmosphere or embedded within pre-SN circumstellar material.