Copyright © 1998 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 1998. 36:
17-55 Copyright © 1998 by Annual Reviews. All rights reserved |
4.1. Progenitors
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 |
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 |