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 possibl e progenitor systems for Type Ia super-novae. We have therefore looked for narrow H alpha in a high-resolution spectrum of the normal Type I a supernova 1994D taken 10 d before maximum and only 6.5 d after explo sion. 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 estima te the limit that this puts on wind density, we have made time-depende nt 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 symbiot ic 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 fin d that X-ray observations can probably provide the most useful limits on the progenitor mass loss, while high-resolution optical spectroscop y offers our only chance of actually identifying circumstellar hydroge n.