|Annu. Rev. Astron. Astrophys. 1998. 36:
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Although numerous articles and several recent reviews (Branch et al 1995, Renzini 1996, Iben 1997, Ruiz-Lapuente et al 1997a) have been written about SN Ia progenitors, we still do not know even whether (or how often) the progenitor binary system contains one white dwarf or two.
In the standard "single-degenerate" scenario, the white dwarf accretes from the Roche lobe or wind of a nondegenerate companion until it approaches the Chandrasekhar mass and ignites carbon deep in its interior. There has been much recent interest in the possibility that single-degenerate pre-SN Ia systems are being observed as supersoft X-ray sources (van den Heuvel et al 1992, Rappaport et al 1994, Yungelson et al 1996), especially since Hachisu et al (1996) found a new strong-wind solution for mass transfer from a lobe-filling companion. According to Hachisu et al, the formation and expulsion of a common envelope can be avoided more easily than previously believed. This may open up two promising channels (Figure 10) for the accretor to reach the Chandrasekhar mass, and both could be observed as supersofts: close systems in which the donor is a main sequence or subgiant star in the range 2-3.5 M and wide systems in which the donor is a red giant of 1 M (Hachisu et al 1996, Nomoto et al 1997, Li & van den Heuvel 1997), but see Yungelson & Livio 1998). In the single-degenerate scenario, a significant amount of circumstellar matter is expected to be in the vicinity of the explosion. So far, no convincing evidence for narrow circumstellar hydrogen or helium lines in SN Ia spectra has been found (Ho & Filippenko 1995, Cumming et al 1996), nor has X-ray (Schlegel & Petre 1993) or radio (Eck et al 1996) emission from circumstellar interaction been seen. These nondetections are not yet quite stringent enough, however, to rule out single-degenerate progenitor systems (Lundqvist & Cumming 1997). So far, only one SN Ia has been found to be polarized (Wang et al 1997b); the general lack of polarization may lead to constraints on the presence of circumstellar matter and the nature of the progenitor systems (Wang et al 1996).
Figure 10. Donor masses are plotted against orbital period for candidate single-degenerate SN Ia progenitor binary systems. Filled circles are for an initial white-dwarf accretor of 1.2 M and open circles are for 1.0 M. The dotted lines represent the boundaries of mass transfer in case A (left) and case B (right). From Li & van den Heuvel (1997).
In the standard "double-degenerate" scenario, two white dwarfs spiral together as a consequence of the emission of gravitational radiation to form a super-Chandrasekhar merger product. According to a population-synthesis study by Tutukov & Yungelson (1994), mergers that form within 3 × 108 years of star formation have a mean mass that is greater than 2 M (Figure 11). Those researchers who have made recent attempts to model the merging process using SPH calculations (Mochkovitch & Livio 1990, Benz et al 1990, Rasio & Shapiro 1995, Mochkovitch et al 1997) are not uniformly optimistic about producing SNe Ia in this way. On the other hand there are arguments (Section 4.4) that peculiar events like SN 1991T, at least, may be super-Chandrasekhar merger products. It is not clear that there should be a significant amount of circumstellar matter in the vicinity of a merger SN Ia, but if so, it would be carbon and oxygen rather than hydrogen and helium. In this regard, the detection of narrow [O I] lines in late-time spectra of SN 1937C by Minkowski (1939) and the possible detection of narrow [O I] 8446 emission in a very early spectrum of SN 1991T by Ruiz-Lapuente et al (1992) are intriguing.
Figure 11. The distributions of the masses of double-degenerate mergers for three age intervals. The peaks near 2.0, 0.8, and 0.5 M correspond to CO-CO, CO-He, and He-He white-dwarf mergers. Explosions are not expected for total masses below 1.4 M. From Tutukov & Yungelson (1994).