Circumstellar H alpha 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 super-novae. We have therefore looked for narrow H alpha in a high-resolution spectrum of the normal Type Ia supernova 1994D taken 10 d before maximum and only 6.5 d after explosion. We derive an upper limit of 2.0 x 10(-16) erg s(-1)cm(-2) for an unresolved emission line at the local H II region velocity. To estimate the limit that this puts on wind density, we have made time-dependent photoionization calculations. Assuming spherical symmetry, we find an upper limit of M similar to 1.5 x 10(-5) M. yr(-1) for a wind speed of 10 km s(-1). This limit excludes as progenitors only those symbiotic systems with the highest mass-loss rates. 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.