Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer

TL;DR

This paper reports the discovery of the progenitor of SN 2008S in NGC 6946 using archival Spitzer mid-infrared data, revealing a dust-enshrouded star with modest mass undetectable in optical images.

Contribution

First identification of a supernova progenitor solely through mid-infrared observations, challenging previous assumptions about massive star progenitors.

Findings

Progenitor detected in 4.5, 5.8, 8.0 micron IRAC bands.

Progenitor not detected in optical or 24 micron data.

Progenitor likely a ~10 solar mass star enshrouded in dust.

Abstract

We report the discovery of the progenitor of the recent type IIn SN 2008S in the nearby galaxy NGC 6946. Surprisingly, it was not found in deep, pre-explosion optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the SN 2008S position is clearly detected in the 4.5, 5.8, and 8.0 micron IRAC bands, showing no evident variability in the three years prior to the explosion, yet is undetected at 3.6 and 24 micron. The distinct presence of ~440 K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S was engulfed in a shroud of its own dust. The inferred luminosity of 3.5x10^4 Lsun implies a modest mass of ~10 Msun. We conclude that objects like SN 2008S are not exclusively associated with the deaths or outbursts of very massive eta Carinae-like objects. This conclusion holds based solely on the optical flux limits even if our identification of the progenitor with the mid-IR source is incorrect.