Astron. Astrophys. 322, L9-L12 (1997)

1. Introduction

Supersoft X-ray sources (hereafter SSS), originally discovered with the Einstein satellite in the Large Magellanic Cloud (Long, Helfand & Grabelsky 1981), and more recently with ROSAT (Trümper et al. 1991), are a class of luminous (bolometric luminosity erg s^-1) objects, with a characteristic radiation temperature of 30 to 60 eV. The number of known SSS exceeds 30, and their main properties have been reviewed by Hasinger (1994), Kahabka & Trümper (1996) and in the recent book edited by Greiner (1996).

The presently accepted interpretation of SSS is that they are binary systems containing massive white dwarfs () which steadily burn nuclear fuel on their surface accreted from a to main-sequence or subgiant companion star at a rate near or above the Eddington limit (van den Heuvel et al. 1992). Because of the large white dwarf masses and the high mass accretion rates in SSS, a fraction of these objects could actually grow beyond the Chandrasekhar limit and undergo explosion or collapse. This opens the evolutionary perspective that SSS may be the long-sought progenitors of SNe Ia.

Detailed population studies for SSS have been made by Rappaport, Di Stefano & Smith (1994) and by Yungelson et al. (1996). Rappaport et al. (1994), assuming a constant mass transfer rate and a efficient conversion of the accreted hydrogen into helium, found that close-binary supersoft sources (CBSS) may grow to the Chandrasekhar mass with a rate as high as yr^-1 in the Galaxy. A lower estimate ( yr^-1) of the SNe Ia rate was derived by Yungelson et al. (1996). Recently, a new model for progenitors of SNe Ia was proposed by Hachisu, Kato & Nomoto (1996). The model consists of a white dwarf and a lobe-filling red giant. Hachisu et al. found a new strong wind solution when the mass accretion rate exceeds a critical value. The strong winds stabilize the mass transfer, even though the mass ratio of the donor star to the white dwarf is larger than 0.79, in which case the mass transfer was conventionally thought to be unstable to form a common envelope (cf. Iben & Livio 1993 for a review). Thus the strong winds from the mass-accreting white dwarf change the stability condition and are able to open a new channel to SN Ia explosions.

Motivated by the work of Hachisu et al. (1996), we have performed evolutionary calculations of white dwarf binaries with a lobe-filling companion star. The aim of our work is to investigate the distribution of the possible SN Ia progenitors, and to test whether such systems can produce SNe Ia with a rate compatible with the observations. In the next section we describe our main assumptions. The results and discussion are presented in section 3.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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