Constrained fitting of disentangled binary star spectra: application to V615Persei in the open cluster hPersei (Research Note)

Context. Using the technique of spectral disentangling, it is possible to determine the individual spectra of the components of a multiple star system from composite spectra observed at a range of orbital phases. This method has several advantages: it is unaffected by line blending, does not use template spectra, and returns individual component spectra with very high signal-to-noise ratios. Aims. The disentangled spectra of a binary star system are very well suited to spectroscopic analysis but for one problem: the absolute spectral line depths are unknown because this information is not contained in the original spectra (unless there is one taken in eclipse) without making assumptions about the spectral characteristics of the component stars. Here we present a method for obtaining the atmospheric parameters of the component stars by the constrained fitting of synthetic spectra to observed and disentangled spectra. Methods. Disentangled spectra are fitted using synthetic spectra and a genetic algorithm in order to determine the effective temperatures, surface gravities and relative light contributions of the two stars in a binary system. The method is demonstrated on synthetic spectra and then applied to the eclipsing binary V615 Per, a member of the young open cluster NGC 869 (h Persei). Results. The method works well for disentangled spectra with signal-to-noise ratios of 100 or more. For V615 Per we find a normal He abundance but an Mg abundance, which indicates bulk metallicity, a factor of two lower than typical for nearby OB stars.