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On the Peculiarities of Evolutionary History of EHB Objects in Binary Systems with Hot Subdwarf Companions

Pustynski V.-V., Pustylnik I.

Acta Astronomica

2006

Vol. 56, No. 1

83--95

It has been shown quite recently (Maxted et al. 2001, Morales-Rueda et al. 2003) that dB stars, extreme horizontal branch (EHB) objects, likely all belong to binary systems. We study in detail the mass and angular momentum loss in the giant progenitors of sdB stars in an attempt to clarify why binarity must be a crucial factor in producing EHB objects. Assuming that the progenitors of EHB objects belong to the binaries with initial separations of 100-150 Rsolar and fill in their critical Roche lobes while being close to the RGB tip we have found that considerable shrinkage of the orbit can be achieved due to the combined effect of angular momentum loss from the red giant and appreciable accretion on its low mass companion on the hydrodynamical time scale of the donor resulting in formation of helium white dwarfs with masses about 0.5 Msolar and thus evading the common envelope stage.