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TOPIC: Subdwarf B (sdB) stars

Blobrana

L

Posts: 131433

Date: Nov 13 11:10 2012

Subdwarf B stars	Permalink

Title: Hot subdwarf stars in close-up view - III. Metal abundances of subdwarf B stars Authors: S. Geier Context: Hot subdwarf B stars (sdBs) are considered to be core helium-burning stars with very thin hydrogen envelopes situated on or near the extreme horizontal branch (EHB). The formation of sdBs is still unclear as well as the chemical composition of their atmospheres. The observed helium depletion is attributed to atmospheric diffusion. Metal abundances have been determined for about a dozen sdBs only resulting in puzzling patterns with enrichment of heavy metals and depletion of lighter ones. Aims: In this paper we present a detailed metal abundance analysis of 106 sdBs. Methods: From high resolution spectra we measured elemental abundances of up to 24 different ions per star. A semi-automatic analysis pipeline was developed to calculate and fit LTE models to a standard set of spectral lines. Results: A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from star to star, the general abundance patterns in most sdBs are similar. State-of-the-art diffusion models predict such patterns and are in qualitative agreement with our results. However, the highest enrichments measured cannot not be explained with these models. Peculiar line shapes of the strongest metal lines in some stars indicate vertical stratification to be present in the atmospheres. Such effects are not accounted for in current diffusion models and may be responsible for some of the yet unexplained abundance anomalies. Read more (116kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Aug 4 01:44 2010

Subdwarf B (sdB) stars	Permalink

Title: First Kepler results on compact pulsators V: Slowly pulsating subdwarf B stars in short-period binaries Authors: S. D. Kawaler, M. D. Reed, R. H. Østensen, S. Bloemen, D. W. Kurtz, A. C. Quint, R. Silvotti, A. S. Baran, E. M. Green, S. Charpinet, J. Telting, C. Aerts, G. Handler, H. Kjeldsen, J. Christensen-Dalsgaard, W. J. Borucki, D. G. Koch, J. Robinson The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for nonradial pulsations. We present our analysis of two sdB stars that are found to be g-mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M-dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer-duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal-to-noise, perhaps revealing lower amplitude periodicities that could confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high-overtone g-mode pulsations. Read more (155kb, PDF) __________________

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