The Yellow Supergiant Progenitor of the Type II Supernova 2011dh in M51

TL;DR

This paper identifies the progenitor of supernova 2011dh as a yellow supergiant star, using archival Hubble data and adaptive optics imaging, and concludes it was a single star with a specific initial mass, not a cluster or binary.

Contribution

First direct identification and characterization of the supernova 2011dh progenitor as a yellow supergiant star, refining its properties and ruling out alternative progenitor scenarios.

Findings

Progenitor is consistent with a F8 supergiant star.

Progenitor's initial mass estimated at 13+/-3 solar masses.

Progenitor likely not part of a cluster or significantly affected by a binary companion.

Abstract

We present the detection of the progenitor of the Type II SN 2011dh in archival pre-explosion Hubble Space Telescope images. Using post-explosion Adaptive Optics imaging with Gemini NIRI+ALTAIR, the position of the SN in the pre-explosion images was determined to within 23mas. The progenitor object was found to be consistent with a F8 supergiant star (log L/L_{\odot}=4.92+/-0.20 and T_{eff}=6000+/-280K). Through comparison with stellar evolution tracks, this corresponds to a single star at the end of core C-burning with an initial mass of M_{ZAMS}=13+/-3M_{\odot}. The possibility of the progenitor source being a cluster is rejected, on the basis of: 1) the source is not spatially extended; 2) the absence of excess H\alpha\, emission; and 3) the poor fit to synthetic cluster SEDs. It is unclear if a binary companion is contributing to the observed SED, although given the excellent correspondence of the observed photometry to a single star SED we suggest the companion does not contribute significantly. Early photometric and spectroscopic observations show fast evolution similar to the transitional Type IIb SN 2008ax, and suggest that a large amount of the progenitor's hydrogen envelope was removed before explosion.