Eclipsing binaries in open clusters - I. V615 Per and V618 Per in h Persei

Abstract

We derive absolute dimensions for two early-type main-sequence detached eclipsing binaries in the young open cluster h Persei (NGC 869). V615 Persei has a spectral type of B7 V and a period of 13.7 d. V618 Persei is A2 V and has a period of 6.4 d. New ephemerides are calculated for both systems. The masses of the component stars have been derived using high-resolution spectroscopy and are 4.08 ± 0.06 and 3.18 ± 0.05 M ○. for V615 Per and 2.33 ± 0.03 and 1.56 ± 0.02 M ○. for V618 Per. The radii have been measured by fitting the available light curves using EBOP and are 2.29 ± 0.14 and 1.90 ± 0.09 R ○. for V615 Per and 1.64 + 0.07 and 1.32 ± 0.07 R ○. for V618 Per. By comparing the observed spectra of V615 Per with synthetic spectra from model atmospheres we find that the effective temperatures of the stars are 15 000 ± 500 K for the primary and 11000 ± 500 K for the secondary. The equatorial rotational velocities of the primary and secondary components of V615 Per are 28 ± 5 and 8 ± 5 km s -1 , respectively. Both components of V618 Per rotate at 10 ± 5 km s -1 . The equatorial rotational velocities for synchronous rotation are about 10 km s -1 for all four stars. The time-scales for orbital circularization for both systems, and the time-scale for rotational synchronization of V615 Per, are much greater than the age of h Per. Their negligible eccentricities and equatorial rotational velocities therefore support the hypothesis that they were formed by delayed breakup. We have compared the radii of these stars with models by the Granada and the Padova groups for stars of the same masses but different compositions. We conclude that the metallicity of the stars is Z 0.01. This appears to be the first estimate of the bulk metallicity of h Per. Recent photometric studies have assumed a solar metallicity so their results should be reviewed.