Evidence for Circumstellar Material in Type Ia Supernovae via Sodium Absorption Features

Type Ia supernovae are very good tools for measuring distances on a cosmic scale. The consensus view is that mass transfer onto a white dwarf in a close binary system leads to a thermonuclear explosion, though the nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star. In the double-degenerate model it is another white dwarf. We study the velocity structure of absorbing material along the line of sight to 35 Type Ia supernovae and find a statistical preference for blueshifted structures, likely arising in gas outflows from the supernova progenitor systems, consistent with a single-degenerate progenitor for a substantial fraction of Type Ia supernovae in nearby spiral galaxies.