Constrained fitting of disentangled binary star spectra: application to V615 Persei in the open cluster h Persei

Abstract

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.

Absolute Properties of the Pre-Main-Sequence Eclipsing Binary Star Np Persei

“…That these uncertainties are very similar could be either a coincidence, or the result of our careful selection of lines with good atomic data, resulting in a similar scatter. The renormalized disentangled spectrum of the secondary has too small a S/N to allow accurate abundance determinations, though we were able to derive its effective temperature from an optimal fit (Tamajo et al 2011) to the renormalized spectrum over the 5050-5700 Å region, albeit with much larger uncertainty than the primary. A search for the signatures of a third star in the systemʼs spectra by using SPD was negative.…”

Asiago eclipsing binaries program IV. SZ Camelopardalis, aβ Cephei pulsator in a quadruple, eclipsing system

“…A genetic algorithm (GENFITT, Tamajo 2009) is used for the optimization (Tamajo et al 2011) in order to ensure that the best solution is found in a parameter space that suffers from strong degeneracies, in particular between effective temperature (T eff ) and surface gravity (log g). Table 4 compares the results of the NLTE analysis when it is run un-constrained and when the gravities of the two stars member of the eclipsing pair are fixed to the values derived by the orbital solution of Table 5.…”

“…In OB stars, numerous oxygen, nitrogen, and silicon lines, are superimposed on the Balmer-lines are superimposed, so we carefully selected parts of their profiles that are free of such blendings as suitable for matching to theoretical line profiles. The code GENFITT has been developed to fit theoretical line profile to hydrogen lines by χ 2 minimization in T eff (Tamajo et al 2011). …”

CoRoT 102918586: aγDoradus pulsator in a short-period eccentric eclipsing binary

“…In the present analysis we used a constrained optimal fitting method (Tamajo et al 2011). In the new release of the computer code for constrained optimal fitting of disentangled component spectra the following parameters can be adjusted for both stars: effective temperature, surface gravity, light dilution factor, Doppler shift, projected rotational velocity, and the vertical shift to adjust for the continuum.…”