HS 2333+3927: A new sdB+dM binary with a large reflection effect

Abstract

Abstract. We have discovered periodic light variations (P = 0.1718023 d) in the sdB star HS 2333+3927 in the BVR bands with amplitudes of 0.21, 0.28 and 0.33 mag, respectively. Sinusoidal radial velocity variations at the same period were detected with a semi-amplitude of K 1 = 89.6 km s −1 , indicating that it is binary system and that the light variations are caused by the reflection effect with no eclipses. A mass function of f (m) = 0.0128 M has been determined. The analysis of the light curve did not yield a unique solution, mainly because the albedo of the secondary is poorly constrained. Two solutions of equal quality with a high (A 2 = 1.0) and a low (A 2 = 0.39) albedo were considered further. Variability of the Balmer line profiles, most notably for Hα, was discovered, probably also caused by the reflection effect. A spectroscopic analysis results in T eff = 36 500 K, log g = 5.70, and log (n He /n H ) = −2.15. These characteristics are typical for sdB stars. Mass-radius relations are derived from the results of the analysis of light and radial-velocity curves. Comparison with the observed mass-radius relation of the sdB star and with that of lower main sequence stars for the companion allows us to discard the high albedo solution, because the resulting mass of the primary and the radius of the secondary would be unreasonably low. From a discussion of evolutionary models we constrain the plausible mass of the sdB to the range between 0.29 M and 0.47 M . Accordingly, the mass of the secondary is between 0.24 M and 0.32 M , indicating a spectral type of M3 to M4. HS 2333+3927 is only the sixth sdB+dM system discovered so far. An improved measurement of the gravity and the projected rotational velocity of the sdB star is required to further constrain the masses and to identify the evolutionary state of the sdB star uniquely.

A survey for pulsating subdwarf B stars with the Nordic Optical Telescope

“…The discovery of an eclipsing sdB+dM system, HS 0705+6700, first detected during run N4 was presented by Drechsel et al (2001), and the non-eclipsing sdB+dM system HS 2333+3927 discovered during run N2 was presented by Heber et al (2004). Two more sdBs with dM companions were found during run S2: the eclipsing HS 2231+2441 (Østensen et al 2007), and HS 2043+0615 (Østensen et al 2010, A&A, in prep.…”

Astrophysics in 2004

“…6 binary sdBϩdM pairs (Heber et al 2004). The sdB surfaces are on their way toward the very large H/He ratios of DA white dwarfs.…”

The MUCHFUSS photometric campaign

“…Each spectrum was fitted separately, because in several other HW Vir systems an apparent change of the atmospheric parameters with the orbital phase has been detected Heber et al 2004), which is caused by the time-variable contribution of the irradiated companion. This effect is only visible when the atmospheric parameters can be derived with sufficient accuracy.…”