96 research outputs found
Recommended from our members
Galactic R Coronae Borealis Stars: The C-2 Swan Bands, The Carbon Problem, And The C-12/C-13 Ratio
Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars are analyzed by synthesizing the C-2 Swan bands (1, 0), (0, 0), and (0, 1) using our detailed line list and the Uppsala model atmospheres. The (0, 1) and (0, 0) C-2 bands are used to derive the C-12 abundance, and the (1, 0) (CC)-C-12-C-13 band to determine the C-12/C-13 ratios. The carbon abundance derived from the C-2 Swan bands is about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). Carbon abundances derived from C I lines are about a factor of four lower than the carbon abundance of the adopted model atmosphere over the same C/He interval, as reported by Asplund et al., who dubbed the mismatch between adopted and derived C abundance as the "carbon problem." In principle, the carbon abundances obtained from C-2 Swan bands and that assumed for the model atmosphere can be equated for a particular choice of C/He that varies from star to star. Then, the carbon problem for C-2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. A more likely solution to the C-2 carbon problem may lie in a modification of the model atmosphere's temperature structure. The derived carbon abundances and the C-12/C-13 ratios are discussed in light of the double degenerate and the final flash scenarios.Robert A. Welch Foundation of Houston, TX F-634McDonald Observator
Abundance analysis of cool extreme helium star: LSS 3378
Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is
presented. The abundance analysis is done using LTE line formation and LTE
model atmospheres constructed for EHe stars.
The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and
s-process nucleosynthesis. The derived abundances of iron-peak and
alpha-elements indicate absence of selective fractionation or any other
processes that can distort chemical composition of these elements. Hence, the
Fe abundance (log epsilon(Fe) = 6.1) is adopted as an initial metallicity
indicator. The measured abundances of LSS 3378 are compared with those of R
Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.Comment: 7 pages, 4 figures, 3 tables, MNRAS format, accepted for publication
in MNRA
On the binary helium star DY Centauri: Chemical composition and evolutionary state
DY Cen has shown a steady fading of its visual light by about 1 magnitude in
the last 40 years suggesting a secular increase in its effective temperature.
We have conducted non-LTE and LTE abundance analyses to determine the star's
effective temperature, surface gravity, and chemical composition using
high-resolution spectra obtained over two decades. The derived stellar
parameters for three epochs suggest that DY Cen has evolved at a constant
luminosity and has become hotter by about 5000 K in 23 years. We show that the
derived abundances remain unchanged for the three epochs. The derived
abundances of the key elements, including F and Ne, are as observed for the
extreme helium stars resulting from a merger of an He white dwarf with a C-O
white dwarf. Thus, DY Cen by chemical composition appears to be also a product
of a merger of two white dwarfs. This appearance seems to be at odds with the
recent suggestion that DY Cen is a single-lined spectroscopic binary.Comment: Accepted to ApJ with 8 Figures and 5 Table
- …