Wednesday, February 25, 2015

A Pulsating Hot Subdwarf B Star With a Brown Dwarf Companion

An eclipsing post common-envelope system consisting of a pulsating hot subdwarf B star and a brown dwarf companion


Schaffenroth et al


Hot subdwarf B stars (sdBs) are evolved, core helium-burning objects located on the extreme horizontal branch. Their formation history is still puzzling as the sdB progenitors must lose nearly all of their hydrogen envelope during the red-giant phase. About half of the known sdBs are in close binaries with periods from 1.2 h to a few days, a fact that implies they experienced a common-envelope phase. Eclipsing hot subdwarf binaries (also called HW Virginis systems) are rare but important objects for determining fundamental stellar parameters. Even more significant and uncommon are those binaries containing a pulsating sdB, as the mass can be determined independently by asteroseismology.

Here we present a first analysis of the eclipsing hot subdwarf binary V2008-1753. The light curve shows a total eclipse, a prominent reflection effect, and low--amplitude pulsations with periods from 150 to 180 s. An analysis of the light-- and radial velocity (RV) curves indicates a mass ratio close to q=0.146, an RV semi-amplitude of K=54.6kms−1, and an inclination of i=86.8∘. Combining these results with our spectroscopic determination of the surface gravity, logg=5.83, the best--fitting model yields an sdB mass of 0.47M⊙ and a companion mass of 69 MJup. As the latter mass is below the hydrogen-burning limit, V2008-1753 represents the first HW Vir system known consisting of a pulsating sdB and a brown dwarf companion. Consequently, it holds great potential for better constraining models of sdB binary evolution and asteroseismology.

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