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Дата изменения: Fri Jan 25 15:37:50 2002 Дата индексирования: Tue Oct 2 09:35:22 2012 Кодировка: Поисковые слова: neutron |
The characteristic surface properties, effective temperature ,
surface gravity
and composition
, of EHes have been summarized
by Jeffery (1996). The distribution of EHes in the
plane is
shown in Fig. 1 together with the location of other
H-deficient stars, notably the putatively related RCrB stars. It may
be seen that they lie roughly along a locus given by
,
which also corresponds to the evolution track of post-AGB stars,
contracting towards the white dwarf track. This has guided most of
discussion of their evolutionary status within the context of post-AGB
evolution.
The surface compositions of EHes vary considerably from star to star,
but may be briefly summarized as follows.
Hydrogen: (
) The fact that some
hydrogen is found in the atmospheres of nearly all EHes indicates that
some remnant of the outermost hydrogen-rich layers of the progenitor
has been retained.
Helium: (
) By definition, helium
dominates the atmospheric abundances.
Nitrogen: (
) Nitrogen is enriched in
most EHes, implying that helium has been produced by CNO cycling. If
all nitrogen comes from such a source, it should reflect the total
C+N(+O) abundances in the progenitor. This is supported by a
correlation between N and Fe abundances in EHes.
Carbon: (
) Carbon is substantially
enriched in nearly all EHes, indicating the presence of material
processed through
helium burning.
Oxygen: (
) Since
indicates the
destruction of oxygen, nearly all observed oxygen has probably been
produced by
. The O/C ratio
should therefore provide a diagnostic of conditions in the C-rich
layers of the progenitor. Observed O/C ratios do not approach the
value expected in CO cores, except in two cases (DYCen and LSE78).
Iron: The iron abundances fall approximately into two
groups (with the exceptions of DYCen and HD144941) clustered about
and
. It remains to be seen
whether the bimodal distribution of iron abundances persists to the
complete EHe sample.
Consequently, any model for the origin of EHes must result in a
surface mixture which includes a remnant of the hydrogen envelope,
predominantly CNO-processed helium and a significant quantity of
and
products, as well as reproducing
the overall dimensions such as luminosity, mass and effective
temperature.