Abstract:
Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low
C
/
He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC.
De
fi
ning atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of
spectroscopic indicators including He
I
and He
II
line pro
fi
les, and the ionization equilibrium of ion pairs such as
C
II
/
C
III
and N
II
/
N
III
. The various indicators provide a consistent set of atmospheric parameters:
T
eff
=
25,000
±
300 K, log
g
=
3.10
±
0.12
(
cgs
)
, and
ξ
=
13
±
2kms
−
1
are provided for V652 Her, and
T
eff
=
22,000
±
600 K, log
g
=
3.45
±
0.15
(
cgs
)
, and
ξ
=
10 km s
−
1
are provided for HD 144941. In contrast to
the non-LTE analyses, the LTE analyses
—
LTE atmospheres and an LTE line analysis
—
with the available
indicators do not provide a consistent set of atmospheric parameters. The principal non-LTE effect on the
elemental abundances is on the neon abundance. It is generally considered that these extreme helium stars with
their very low C
/
He ratio result from the merger of two helium white dwarfs. Indeed, the derived composition of
V652 Her is in excellent agreement with predictions by Zhang & Jeffery, who model the slow merger of helium
white dwarfs; a slow merger results in the merged star having the composition of the accreted white dwarf. In the
case of HD 144941, which appears to have evolved from metal-poor stars, a slow merger is incompatible with the
observed composition but variations of the merger rate may account for the observed composition. More detailed
theoretical studies of the merger of a pair of helium white dwarfs are to be encouraged.
