Total: 41 pulsars
8.2 Outcomes of Stellar Evolution
Chapter 8
Neutron Stars in Supernova Remnants
D. L. Kaplan
8.2.1 White Dwarfs
Below some mass limit, the star will end its life without a supernova and will leave a degenerate carbon- oxygen white dwarf (Weidemann 1987). This limit is likely in the range 7–8M⊙(Weidemann 2000; Koester 2002). In the range 8 to≈10M⊙, carbon burning and significant mass-loss occur in the proto-white dwarf and the result will be a O-Ne-Mg white dwarf. It is possible, though, that in a narrow range of masses below the upper threshold stars form O-Ne-Mg white dwarfs where the mass-loss is insufficient for them to maintain stability. Therefore they then explode off-center due to accelerated electron capture (e.g., Miyaji et al. 1980; Nomoto 1984, 1987). These supernovae probably leave neutron stars, but they may entirely destroy the stellar cores and leave nothing behind.
8.2.2 Neutron Stars
Above the threshold for white dwarf formation and below an unknown boundary (MBH ≈ 20−25M⊙; Ergma & van den Heuvel 1998; Fryer 1999 and references therein), stars are expected to undergo traditional Type II supernovae and end as neutron stars. As mentioned above, the detailed states (mass, rotation, velocity) of these neutron stars are currently unknown so precise prediction of the post-collapse properties is not possible, but the NSs should have masses of roughly 1.2–1.4M⊙ (after radiating away some fraction of their binding energies as neutrinos).
Neutron stars may also emerge from considerably more massive stars. Some authors (Woosley et al.
2002) suggest that there may be a range above 50M⊙ where neutron stars form due to extreme mass loss from the progenitor that strips away much of the original material. The supernovae in these cases would be Type Ib/c, since the hydrogen envelopes would have been ejected. On the other hand, Ergma & van den Heuvel (1998) cite evidence that the objects with progenitors > 25M⊙ can have a mix of outcomes (neutron star or black hole), perhaps depending on one of the many complex parameters that are currently ignored or simplified (rotation, magnetic fields, asymmetries, etc.). In either case it is likely that some neutron stars can be formed by stars with initial masses> MBH, but most do not.
8.2.3 Black Holes
The conventional expectation is that above MBH black holes will result from core collapse. These black holes can either form directly from collapse of the iron core, or can occur due to fallback of supernova ejecta onto a nascent neutron star. If the progenitor is near the neutron star boundary fallback or other delayed formation scenarios are more likely, but if the progenitor is more massive and has less than solar metallicity then the direct and immediate formation of the black hole could prevent any supernova. Overall, direct collapse is more likely for low metallicities, while fallback should occur for solar metallicities and above.
8.2.4 No Remnant
For the most massive stars (∼>100M⊙) and low metallicities, pair-instability supernovae will likely occur.
Here electron-positron pair creation absorbs energy that could have raised the temperature (and stabilized the collapse), leading to runaway collapse and then explosion. The explosion occurs with extreme violence and can entirely disrupt the core, leaving nothing behind.
8.2.5 Population Rates
With some prescription for outcomes, including a mapping of progenitor mass and metallicity to compact remnant type (e.g., Heger et al. 2003), one can then determine the rates for the formations of different compact objects by multiplying this mapping with an initial mass function (IMF). While this has many
8.2 Outcomes of Stellar Evolution 105
Table 8.1. Summary of Outcomes for Massive Stars of Solar Metallicity Initial Mass Compact Object SN Type Fractiona
(M⊙)
<8 . . . C/O WD · · · 94%
8–10 . . . O/Ne/Mg WD · · · 1.5%
≈10 . . . NS e− capture <0.5%
10–25 . . . NS IIp 3.2%
25–34 . . . BH weak(?) IIL/b 0.5%
34–50 . . . BH weak Ib/c 0.4%
>50 . . . NS Ib/c 0.4%
aFraction of stars with initial masses ≥ 1M⊙, assuming a Salpeter (1955) initial mass function.
Note. — All mass boundaries are approximate. From Woosley et al. (2002) and Heger et al. (2003).
uncertainties, Heger et al. (2003) do it and find that for solar metallicity one expects roughly 15% of massive stars (∼> 10M⊙) to leave black holes and 85% to leave neutron stars. For lower metallicities the fractions of black holes and remnant-less pair-instability SNe increase, while for higher metallicities the neutron star fraction increases up to unity (as extreme mass-loss prevents black-hole formation). We have summarized this in Table 8.1. We note that the values in Table 8.1 are very rough.
The supernova remnants (SNRs) that form around the compact objects to some degree track the compact object type. Neutron stars typically form from traditional Type II SNe, while black holes result from either weak Type II SNe or Type Ib/c SNe (either strong or weak). Weak Type II SNe occur primarily at low metallicities (below solar) and at masses just above the black hole transition where nickel falls back onto the compact remnant, lessening the optical emission, reducing the explosion energy, and creating a black hole through fallback. Weak Type Ib/c SNe can occur at higher metallicities and also produce black holes through fallback. The Type II vs. Type Ib/c distinction (based on the presence of hydrogen in the stellar envelope) is not too important for supernova remnants (except for the youngest remnants where detailed compositional studies are possible), but the difference between the strong and weak SNe may significantly affect the SNRs. The weak SNe are almost always associated with black hole formation, and so the observed population of SNRs may be slightly deficient in those containing black holes.
[The above discussion applies only to single stars, or to stars with distant companions that do not alter their evolution. For binary systems the situation is more complicated, especially with regards to mass-loss, and no detailed population estimates are available.]
8.2.6 Type Ia SNe
Type Ia SNe are believed to result from carbon detonation/deflagration of a white dwarf that has been pushed beyond its mass limit through accretion from a companion star or merger with another white dwarf. The resulting explosion completely disrupts the star, synthesizing nearly a solar mass of56Ni which
ultimately decays to Fe. No compact core is left behind. The SNRs from such events thus form a subsample in which we do not expect to find an associated neutron star. The mean rate for Type Ia SNe is about 20–25% of that for core-collapse supernovae (Cappellaro et al. 1999) based on observations of extragalactic samples.
The fraction of detected supernova remnants in this Galaxy that are the results of Ia SNe is unknown.
Simplistically it would be the same 20–25% of core-collapse remnants. However, there are a number of effects that could alter this number in either direction. First, since Type Ia SNe come from evolved low- mass stars, they have much wider spatial distributions than do core-collapse SNe, especially with respect to the height above the Galactic plane. The ambient density would then be lower on average, and the SNR could fade more rapidly. However, the sample of Galactic SNRs is by no means complete, and since Type Ia SNRs would occur at higher Galactic latitude where there is less confusion, these SNRs may be over-represented in the Galactic sample.