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Several scenarios have been proposed to account for the depletion of
surface hydrogen in extreme helium stars. Those which may be
successful in producing extremely hydrogen-poor surfaces are the
following:
Case BB mass transfer in a binary. Following
main-sequence evolution, a red giant star in a close binary system may
expand to fill its Roche lobe. Transferring mass to the less massive
companion (Case B) will reduce but not remove the H-rich envelope. If
core-helium burning is completed before the secondary completes its
main-sequence evolution and the primary expands to the giant region
for a second time, then a further phase of mass transfer (case BB) can
completely remove the H-rich envelope, exposing CNO-processed helium
(Plavec 1973, Schönberner & Drilling 1983). Although Iben &
Tutukov (1984) used this model to account for the EHes, the latter are
not binaries (Jeffery et al. 1987). Case BB mass transfer does
account successfully for the observations of hydrogen-deficient
binaries such as
Sgr and KSPer.
Final helium-shell flash in a post-AGB star. The model
proposed by Iben et al. (1983) derives from evolutionary calculations
of post-AGB stars. At some point during contraction from the AGB to
the white dwarf (WD) track, some models were found to experience a
late thermal pulse - or helium-shell flash. The energy output of this
last shell flash causes large-scale mixing and a brief expansion of
the envelope to giant dimensions. Strong evidence that such late shell
flashes do occur comes from three objects, V652 Aql, FG Sge and V4334
Sge, all of which have been observed to evolve from faint blue star to
a red supergiant on timescales of 3 - 50 years. In the case of V652
Aql, contraction after the shell flash to the WD track was also
rapid. Recently, more detailed evolutionary calculations have been
carried out for WDs which experience a late shell flash (Herwig et al
1999). The post-expansion tracks have been compared favourably with
observations of other H-deficient objects including some central stars
of PN ([WC] stars) and very hot pre-WDs (PG1159 stars). All of these
objects have a surface carbon abundance of
or greater. If
the final shell-flash model successfully explains such stars, the
question is whether it can also explain EHes, with
carbon
abundances and apparently slower evolutionary timescales.
Merger of CO and He white dwarf. While most proposed
models for EHes invoke post-AGB evolution, the model introduced by
Webbink (1984) is completely different. A binary system with
appropriate initial masses and orbital separation can evolve to the
point where both stars are WDs, one being a carbon-oxygen WD of
, the other a helium WD of 0.3-0.4
, with an
orbital period in the range 1-10 hours. Over a long interval, the
orbital angular momentum can be reduced by a combination of
gravitational-wave radiation and magnetic-wind braking to the point at
which the less massive WD fills its Roche lobe. Tidal disruption will
follow on a dynamical timescale, the WD being transformed into a thick
disk around the more massive companion. Accretion from the disk onto
the surviving WD creates a star with a degenerate CO core and a helium
envelope. Depending on the accretion rate, helium may be ignited
either explosively (slow accretion) or quiescently (fast accretion),
resulting in either a type Ib supernova or a helium giant (Iben &
Tutukov 1985). Numerical models for the mergers of two CO WDs and two
He WDs have been computed (Saio & Nomoto 1998, Saio & Jeffery 2000),
but the CO+He case has yet to be treated successfully.
Other models. Schönberner (1986) discusses a variety
of unsuccessful models which have been proposed at one time or
another. Only the final-flash and WD-merger models currently seem
capable of reproducing most of the observed properties of EHes.