Binary systems with post-T Tauri secondaries

The identification of post-T Tauri (pTT) stars selected through spectroscopic criteria by Pallavicini et al. (1992) and by Martin et al. (1992) among the candidates belonging to visual binary systems is revisited in the present paper by studying their position in the HR diagram. These stars belong to the so-called Lindroos binary sample (Lindroos 1985), i.e. to systems with early-type primaries and late-type companions. If these binaries are physical and not simply optical pairs, similar ages must be found for the early-type primary and the late-type component of each system. The ages of these systems have been derived by Lindroos in 1986, by using calibrations of the uvby beta indices. In this paper, we revisit these ages through the position of these stars among new evolutionary tracks in the HR diagram for pre- and post-main sequence stars. We derive new estimations of the ages of each system component, as well as their masses, using parallaxes of the early-type component derived from Hipparcos data and by forcing the late-type companion to be at the same distance. T-eff and log g of the early-type components have been computed using the calibrations of two independent photometric systems: the uvby beta photometry and the Geneva system. The Teff of the late-type stars have been determined by using various calibrations of several photometric systems: uvby beta, UBV and VRI, in order to determine the uncertainties and systematic errors on these parameters and consequently on the ages. Differences in the ages and masses obtained by using various sources of recent evolutionary models are considered and discussed. The consistency of the age of the late type component with that of its early type primary is examined; the validity of this criterion for a selection of physical pairs is discussed. The accuracy of the observational and theoretical data are not sufficient to assign stringent values to the age for several of the examined systems. Nevertheless, in spite of the large error bars, we have established that we could select a number of systems which, according to their position in the HR diagram, may be physically associated. The selection of possible physically bounded systems obtained with the present approach and that made by Pallavicini et al. (1992) or Martin et al. (1992) on the basis of spectroscopic criteria are not always coincident. Spectroscopic criteria, for example the presence of a strong Li feature, are more stringent conditions than that of coherent ages of primaries and secondaries; however the Li I 6708 doublet is expected to fade in the latest stages of the pre-main sequence life of a star, so that the "oldest" pTTs may not be detected by spectroscopy only. The different results so obtained are discussed for each system and we conclude that the present approach may be used as a powerful criterion to select new pTT candidates in visual binaries to be observed and analyzed with high resolution spectrographs and to select candidates that have almost reached the main sequence.