Newly discovered active binaries in the RasTyc sample of stellar X-ray sources

I. Orbital and physical parameters of six new binaries

¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy e-mail: afrasca@oact.inaf.it
² Observatoire Astronomique, UMR 7550, 11 rue de l'Université, 67000 Strasbourg, France
³ Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia, 78, 95123 Catania, Italy

Received: 23 November 2005
Accepted: 17 March 2006

Abstract

We present the first results from follow-up optical observations, both photometric and spectroscopic, of stellar X-ray sources, selected from the RasTyc sample, resulting from the cross-correlation of ROSAT All-Sky Survey (RASS) and TYCHO catalogues. In particular, we report on the discovery of six late-type binaries, for which we obtained good radial velocity curves and solved their orbits. We performed an automatic spectral classification of both single-lined and double-lined binaries with codes developed by us and found two binaries composed of two main-sequence stars and four binaries with an evolved (giant or subgiant) component. Filled-in or pure emission Hα profiles indicative of a moderate or high level of chromospheric activity were observed. In nearly all the systems, we also detected a photometric modulation ascribable to surface inhomogeneities that is correlated with the orbital period, suggesting a synchronization between rotational and orbital periods. The position on the HR diagram of the components of the five sources with a known parallax indicates three binaries containing only main-sequence stars and two single-lined systems with a giant component. The kinematical properties of two, or possibly four, of the observed systems are consistent with a young disk population.