Annu. Rev. Astron. Astrophys. 1997. 35: 309-355 Copyright © 1997 by Annual Reviews. All rights reserved

2.3. Environments and Progenitors

The locations at which SNe occur provide important clues to their nature and to the mass of their progenitor stars. SNe II, Ib, and Ic have never been seen in elliptical galaxies and rarely if ever in S0 galaxies. They are generally in or near spiral arms and H II regions (Huang 1987, Porter & Filippenko 1987, Van Dyk 1992, Van Dyk et al 1996a), implying that their progenitors must have started their lives as massive stars ( 8-10 M). SNe Ia, on the other hand, occur in all types of galaxies, including ellipticals, and in spirals there is no strong preference for spiral arms (Maza & van den Bergh 1976, Van Dyk 1992, McMillan & Ciardullo 1996; but see Bartunov et al 1994b). Because SNe Ia occur most frequently in spiral galaxies, with the rate per unit K-band (2.2 µm) luminosity increasing from early to late Hubble types (Della Valle & Livio 1994), the majority of SNe Ia probably come from intermediate-age (~ 0.1-0.5 billion years), moderately massive stars (4-7 M); see Oemler & Tinsley (1979) for an interesting early discussion.

The progenitors of SNe Ia are carbon-oxygen white dwarfs that accrete matter from a companion star and undergo thermonuclear runaway (Nomoto et al 1984, Woosley & Weaver 1986, and references therein). Although the white dwarfs probably reach the Chandrasekhar limit prior to exploding, this is not yet certain (Woosley et al 1994, Livne & Arnett 1995, but see Höflich et al 1996a). SNe II are thought to arise from evolved, massive progenitors (initial mass 8-10 M) that suffer core collapse (generally iron) and subsequently rebound (e.g. Arnett et al 1989), leaving a neutron star or perhaps in some cases a black hole (Brown & Bethe 1994). Most workers now believe that SNe Ib/Ic are produced by the same mechanism as SNe II, except that the progenitors were stripped of their hydrogen (SN Ib) and possibly helium (SN Ic) envelopes prior to exploding, either via mass transfer to companion stars (Nomoto et al 1994, Woosley et al 1995) or through winds (Woosley et al 1993, Swartz et al 1993b, and references therein). White dwarf models have been discussed (e.g. Branch & Nomoto 1986) but are implausible.