A Search for the Progenitors of Two Type-Ia Supernovae in NGC 1316

TL;DR

This study searches for progenitor stars of two Type-Ia supernovae in NGC 1316 using pre-explosion HST images, finding no candidates and setting limits on possible progenitor brightness, thus constraining progenitor models.

Contribution

It provides the first direct observational constraints on the progenitors of two SNe-Ia in NGC 1316, ruling out certain stellar types as progenitors prior to explosion.

Findings

No progenitor candidates detected at SN sites.

Limits exclude normal stars > 6 M_sun at AGB tip as progenitors.

Limits exclude post-AGB stars > 4 M_sun and post-RG stars > 9 M_sun.

Abstract

Recent evidence of a young progenitor population for many Type-Ia SNe (SNe-Ia) raises the possibility that evolved intermediate-mass progenitor stars may be detected in pre-explosion images. NGC 1316, a radio galaxy in the Fornax cluster, is a prolific producer of SNe-Ia, with four detected since 1980. We analyze Hubble Space Telescope (HST) pre-explosion images of the sites of two of the SNe-Ia that exploded in this galaxy, SN2006dd (a normal Type-Ia) and SN2006mr (likely a subluminous, 1991bg-like, SN-Ia). Astrometric positions are obtained from optical and near-IR ground-based images of the events. We find no candidate point sources at either location, and set upper limits on the flux in B, V, and I from any such progenitors. We also estimate the amount of extinction that could be present, based on analysis of the surface-brightness inhomogeneities in the HST images themselves. At the distance of NGC 1316, the limits correspond to absolute magnitudes of about -5.5, -5.4, and -6.0 mag in M_B, M_V, and M_I, respectively. Comparison to stellar evolution models argues against the presence at the SN sites, 3 years prior to the explosion, of normal stars with initial masses > 6 M_sun at the tip of their asymptotic-giant branch (AGB) evolution, young post-AGB stars that had initial masses > 4 M_sun, and post-red-giant stars of initial masses > 9 M_sun.