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| Paper: |
The Chemical Composition of Subdwarf B Stars: Spectroscopic
Diagnostics and Clues to Evolution |
| Volume: |
452, Fifth Meeting on Hot Subdwarf Stars and Related Objects |
| Page: |
41 |
| Authors: |
Jeffery, C. S.; Pereira, C.; Naslim; N.; Behara, N. |
| Abstract: |
The principal signature in the optical spectrum of a normal subdwarf B star
is the presence of strong gravity-broadened hydrogen lines, and the
weakness or absence of neutral helium lines. The latter represents
a very low photospheric abundance of helium, and has been attributed
to the gravitational settling of helium beneath the photosphere by
diffusion. In a few cases, helium lines dominate the spectrum: these helium-rich
subdwarfs raise important questions about the formation of all hot subdwarfs.
With access to ultraviolet spectra, and also to high signal-to-noise
high-resolution optical spectra, it has become possible to explore the
abundances of elements other than hydrogen and helium systematically.
Questions arise
regarding the choice of model atmosphere to use for such chemically
peculiar stars. We report on
i) a comparison between model grids with
radically different chemical distributions and physical assumptions
and the consequences for measurement of effective temperature and surface gravity,
ii) a multi-wavelength analysis of normal sdB stars, comparing and
combining abundances for several stars from FUSE, HST/STIS and
optical spectroscopy, and
iii) recent work on helium-rich subdwarfs which explores
relationships between normal, moderately
helium-rich and extremely helium-rich sdB stars. |
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