Synthesis of the elements in stars: forty years of progress

1008 Wallerstein et al.: Synthesis of the elements
sult in a dramatic increase in the mass-loss rate with
increasing luminosity. Stothers and Chin (1993a, 1993b,
1996) suggest another mechanism of enhanced mass loss
involving a classical ionization instability in the stellar
envelope. In both cases, enhanced mass loss inhibits
evolution into the Humphreys-Davidson forbidden
zone. Whatever the source of the instability, the observations
show that single stars initially more massive than
50M do not become giants before experiencing a supernova
explosion, and this has important ramifications
for massive close binary star evolution.
It is clear that massive stars interact with the interstellar
medium both as energy sources and sources of isotope
enrichment prior to the supernova explosion that
terminates their lives. Some aspects of this interaction
are explored in a recent volume by Kunth et al. (1997).
Models of Galactic chemical evolution (e.g., Pagel, 1989;
Matteuchi, 1989; and Taylor, 1990) rely heavily on theoretical
estimates of the nucleosynthetic yields of massive
stars (e.g., Maeder, 1992; Timmes et al., 1996;
Thielemann et al., 1986; Arnett 1996), but uncertainties
still remain (Arnett, 1995), particularly with regard to
the extent of mass loss prior to the supernova explosion
and with regard to the dividing line in initial mass between
those stars which form BHs and therefore do not
return iron-peak elements to the interstellar medium
and those which form NSs and do (Maeder, 1992;
Timmes et al., 1996).
After the core carbon-burning phase, the chemically
evolved interior of massive stars follows one of two evolutionary
paths, depending on the initial stellar mass and
composition. For solarlike initial composition, models of
initial mass in the 11 – 13 M range experience core collapse
initiated by electron capture before they form an
‘‘Fe-Ni’’ core (see below); in more massive models, core
collapse is initiated after the formation of the Fe-Ni
core. A model star of mass 11M is just at the borderline
between stars that become TPAGB stars with stable
electron-degenerate cores of ONe and stars that immediately
form electron-degenerate ONe cores massive
enough that electron captures on 20 Ne, 23 Na, and 24 Mg
trigger a rapid contraction that cannot be halted even by
explosive O and Ne burning (Miyaji et al., 1980; Nomoto,
1984, 1987; Miyaji and Nomoto, 1987; Gutierrez
et al., 1996). Nuclear reactions convert the composition
of the core into iron-peak isotopes, and the Fe-Ni core
collapses dynamically to nuclear matter densities. The
real analog of the 11M model and real analogs of models
slightly less massive (say 10.75M ) probably live
long enough as AGB stars that the ONe core becomes
massive enough and therefore dense enough to evolve
into a NS (Nomoto, 1987).
Model stars more massive than 13M burn carbon,
neon, oxygen, and silicon quiescently and then form a
core of iron-peak isotopes in statistical equilibrium (see
Clayton, 1968; Arnett 1996; and Meyer, this review, Sec.
XV). Beyond the core carbon-burning phase (e.g., the
third heavy portion along the 25M track in Fig. 1),
there is essentially no motion in the H-R diagram; the
rapidly evolving core and the hydrogen-rich envelope
are essentially decoupled. Contraction and heating of
the Fe-Ni core leads to partial photodisintegration of
iron-peak isotopes into alpha particles and neutrons (B
2 FH; Hoyle and Fowler, 1960; Fowler and Hoyle, 1964).
Details of the subsequent dynamical collapse of the
core, neutronization, trapping of neutrinos, core bounce,
and expulsion of the envelope in a type II supernova
(SNII) explosion are described by many authors (e.g.,
Woosley and Weaver 1986; Arnett, 1996). The supernova
ejecta contains 56 Ni, which beta decays into 56 Co,
which in turn beta decays into 56 Fe. Associated gamma
emission from the decay of excited nuclear levels helps
power the light curve. Analysis of the light curve of
SN1987a in the Large Magellanic Cloud (Arnett et al.,
1989 and references therein) suggests that 0.1M of
56 Fe is ejected into the interstellar medium when a star
of initial mass 20M explodes, showing that SNeII are
potent sources of iron in the Universe. That they must
also be major sources of other heavy elements follows
from detailed models of explosions set off in the envelopes
of evolutionary models that have been carried to
the stage of Fe-Ni core collapse (e.g., Thielemann et al.,
1996, and references therein).
Despite much effort expended over the past 40 years,
an unambiguous theoretical picture of the detachment
of the neutronized, lepton-degenerate core has not yet
emerged, although the evolution of the final detached
core is reasonably well understood (Burrows and Lattimer,
1986, 1987). The details of how energy is transferred
from the core in such a way as to cause expulsion
of the stellar envelope and a quantitative estimate of
how much matter falls back onto the core are not yet
known theoretically. Thus, a secure theoretical mapping
between initial main-sequence mass and final NS or BH
mass is not presently available (although, see Timmes
et al., 1996 for an encouraging effort), and the critical
initial mass M crit which separates stars that form NS
remnants from those that form BH remnants is not
known. However, an understanding of massive close binary
evolution requires this information. In order to
make progress, a concrete choice must be made, and, in
the following, it will be assumed that M crit 40M ,
M NS 1.4M , and M BH 10M . The choice of 1.4M
for the gravitational mass of a typical neutron star is not
inconsistent with the average value of 1.350.27 estimated
by Thorsett et al. (1993) for 17 NSs in binary systems.
Historically, supernovae have been classified primarily
on the basis of their spectral features, with type I
SNe (SNeI) being hydrogen deficient and SNeII exhibiting
hydrogen lines. The SN light curve has in recent
years become important in identifying the nature of the
explosion. Comparison between spectral features and
light curves of theoretical models of explosions (see,
e.g., Wheeler et al., 1995) suggest that SNeII are the end
result of the evolution of massive stars that retain their
hydrogen-rich envelopes and explode as red or blue supergiants
(e.g., SN1987A), and that SNeIb,c are the result
of the evolution of stars that become WR stars before
exploding. In close binaries, the primary may
Rev. Mod. Phys., Vol. 69, No. 4, October 1997