A search for the progenitors of two type Ia Supernovae in NGC 1316

Abstract

Recent evidence of a young progenitor population for many Type Ia supernovae (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 analyse 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-infrared 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 ~-5.5, -5.4 and -6.0 mag in MB, MV and MI, respectively. Comparison to stellar evolution models argues against the presence at the supernova sites, 3 yr prior to the explosion, of normal stars with initial masses ≳6MȮ at the tip of their asymptotic-giant branch (AGB) evolution, young post-AGB stars that had initial masses ≳4MȮ and post-red giant stars of initial masses ≳9MȮ. © 2008 The Authors. Journal compilation © 2008 RAS.