| Abstract | Recent evidence of a young progenitor population for many TypeIa supernovae (SNeIa) 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 SNeIa, with four detected since 1980. We analyse Hubble Space Telescope (HST) pre-explosion images of the sites of two of the SNeIa that exploded in this galaxy, SN2006dd (a normal Type Ia) and SN2006mr (likely a subluminous, 1991bg-like, SNIa). 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 >~6Msolar at the tip of their asymptotic-giant branch (AGB) evolution, young post-AGB stars that had initial masses >~4Msolar and post-red giant stars of initial masses >~9Msolar. |
