Extreme Horizontal Branch Stars: Observational Evidence on Binary Nature and Evolutionary History

Details Extreme Horizontal Branch Stars: Observational Evidence on Binary Nature and Evolutionary History Extreme Horizontal Branch Stars: Observational Evidence on Binary Nature and Evolutionary History

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007jena.confr..16p&link_type=abstract

"JENAM-2007, "Our non-stable Universe", held 20-25 August 2007 in Yerevan, Armenia. Abstract book, p. 16-16"

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Scientific paper

Underluminous sdB stars are thought to be He burning stars with very low mass H envelopes. Effective temperatures (>25,000 K) and surface gravities (log g>5) place them on EHB (extreme horizontal branch), i.e. they appear in the same region of Teff - log g plane as evolutionary tracks for core He burning stars with core masses of about 0.5 Msun and extremely thin l = 0.02 Msun inert H envelopes. It has been shown quite recently that dB stars, EHBs in high probability all belong to binary systems. We study in detail the mass and angular momentum loss from the giant progenitors of sdB stars in an attempt to clarify why binarity must be a crucial factor in producing EHBs. Assuming that the progenitors of EHBs belong to the binaries with initial separations of roughly a hundred solar radii and fill in their critical Roche lobes when being close to the tip of red giant branch we have found that considerable shrinkage of the orbit can be achieved due to a combined effect of angular momentum loss from the red giant and accretion on its low mass companion on the hydrodynamical timescale of the donor resulting in formation of He WD with masses roughly equal 0.5 Msun.

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