Type-la supernovae in dense circumstellar gas

We propose a model for the bolometric light curve of a type-la supernova (SN Ia) that explodes in a dense circumstellar (CS) envelope. Our modeling of the light curves for SN 2002ic and SN 1997cy shows that the densities of the CS envelopes around both supernovae at a radius of similar to7 x10(15) cm are similar, while the characteristic ejection time for this envelope around SN 1997cy does not exceed 600 yr. We analyze two possible evolutionary scenarios that could lead to the explosion of a SN la inside a dense CS hydrogen envelope: accretion onto a CO white dwarf in a symbiotic binary and the evolution of a single star with an initial mass of about 8M(circle dot). If the hypothesis of a SN la explosion in a dense CS envelope is correct for SN 2002ic and SN 1997cy, then we must assume that the the rapid loss of the red-supergiant envelope in several hundred years and the subsequent explosion of the CO white dwarf are synchronized by some physical mechanism. This mechanism may be related to the contraction of the white dwarf as it approaches the Chandrasekhar limit. We show that the formation of a (super-)Chandrasekhar mass due to the merger of a CO white dwarf and the CO core of a red supergiant followed by a supernovae explosion is unlikely, since this mechanism does not provide the required synchronization of the rapid mass loss and the explosion. (C) 2004 MAIK"Nauka/Interperiodica".