A spectroscopic study of the close eclipsing binary HS Herculis

Abstract

Aims. New high-resolution spectroscopic observations of the eclipsing binary HS Herculis are presented. The main aims were to determine the stellar parameters of the two components, their evolutionary stage, and to search for spectroscopic evidence for a possible third body. Methods. Using the cross-correlation technique, we detect spectroscopically the faint secondary component of HS Herculis, and for the firts time measure its radial velocity. The primary and secondary radial velocity curves are analyzed simultaneously and the results of the orbital solution are combined with those derived from multiband light curve analysis to derive orbital and stellar parameters. Results. We find the masses to be M 1 = 4.49±0.16 and M 2 = 1.75±0.09 M ⊙ , the radii to be R 1 = 2.83±0.04 and R 2 = 1.61±0.02 R ⊙ , and the effective temperatures to be T 1 = 15 200 ± 750 K and T 2 = 7600 ± 400 K for the primary and secondary stars, respectively. We also derive projected rotational velocities of the components as υ 1 sin i = 81 ± 3 and υ 2 sin i = 24 ± 6. Conclusions. While a synchronous rotation for the primary star is indicated by the broadening of the spectral lines, the secondary component appears to rotate more slowly, nearly one half the synchronous rotation velocity. This discrepancy indicates that the less massive secondary component could have not yet attained tidal synchronization. Although the presence of a third body physically bound to the eclipsing pair has been suggested by many investigators, we find no sign of its presence in our CCD spectra. The evolutionary stage of the system's components is briefly discussed by comparing their physical parameters with those of theoretical models. We find that the two components are located near the zero-age main sequence, with an age of about 32 Myr.