Circumstellar Hα from SN 1994D and future Type Ia supernovae: an observational test of progenitor models

Searching for the presence of circumstellar material is currently the only direct way to discriminate between the different types of possible progenitor systems for Type Ia supernovae. We have therefore looked for narrow Hin a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 days before maximum and only 6.5 days after explosion. We derive an upper limit of 2.0×10 −16 erg s −1 cm −2 for an unresolved emission line at the local H ii region velocity. To estimate the limit this puts on wind density, we have made time-dependent photoionization calculations. Assuming spherical symmetry we find an upper limit of u M � 1.5 × 10 −5 M⊙ yr −1 for a wind speed of 10 km s −1 . This limit can exclude only the highest-mass-loss-rate symbiotic systems as progenitors. We discuss the effect of asymmetry and assess the relative merits of early optical, radio and X-ray limits in constraining mass loss from Type Ia progenitors. We find that X-ray observations can probably provide the most useful limits on the progenitor mass loss, while high-resolution optical spectroscopy offers our only chance of actually identifying circumstellar hydrogen.