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

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.