Detection of faint secondary companions in spectroscopic binaries

Context. Most known spectroscopic binaries are detected through the variation in the radial velocity of the primary star, while the spectral features of the secondary companion remain hidden in the noise. Aims. We present a novel technique for the spectroscopic detection of low-luminosity secondary companions of binary stars. The main goal is to estimate the mass ratio even when the radial velocity of the secondary cannot be measured in individual spectra. Methods. The method aims to bring together all the spectral information of the secondary component into one single feature. In a first step, a spectral disentangling technique is used in an automatic way for a grid of possible values of the mass ratio. Then, the resulting series of secondary component spectra are compared with a grid of synthetic templates with a technique inspired by spectral cross-correlations. By optimizing a function indicative of the significance of the secondary detection, the mass ratio and an estimate of effective temperature are derived. Results. We apply our method to different types of objects and observational datasets: three single-lined spectroscopic binaries in the open cluster Blanco 1 observed at mid-spectral resolution, an early-type binary in the open cluster NGC 2362, and PX Vir, an F-type binary observed at high resolution for which the secondary companion had been detected in the infrared but not in the optical spectral range. It is shown that from standard-quality spectral datasets it is possible to detect the secondary star in systems in which the secondary contributes less than 0.5–1.0% of the total flux.