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

¹ Dr. Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: veronika.schaffenroth@sternwarte.uni-erlangen.de
² Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
³ Department of Physics, High Point University, 833 Montlieu Ave, High Point, NC 27268, USA
⁴ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA

Received: 20 January 2015
Accepted: 14 February 2015

Abstract

Hot subdwarf B stars (sdBs) are evolved, core helium-burning objects located on the extreme horizontal branch. Their formation history is still puzzling because 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, which implies that 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, since 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 curves indicates a mass ratio close to q = 0.146, an radial velocity semi-amplitude of K = 54.6 km s^-1, and an inclination of i = 86.8°. Combining these results with our spectroscopic determination of the surface gravity, log  g = 5.83, the best-fitting model yields an sdB mass of 0.47 M_⊙ and a companion mass of 69 M_Jup. Because the latter mass is below the hydrogen-burning limit, V2008-1753 represents the first HW Vir system that is known to consist of a pulsating sdB and a brown dwarf companion. Consequently, it holds strong potential for better constraining models of sdB binary evolution and asteroseismology.