Binary systems with post-T Tauri secondaries

The identification of post-T Tauri (pTT) stars selected through spectroscop ic criteria by Pallavicini et al. (1992) and by Martin et al. (1992) among the candidates belonging to visual binary systems is revisited in the prese nt paper by studying their position in the HR diagram. These stars belong t o the so-called Lindroos binary sample (Lindroos 1985), i.e. to systems wit h early-type primaries and late-type companions. If these binaries are phys ical 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 th e uvby beta indices. In this paper, we revisit these ages through the posit ion 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 eac h system component, as well as their masses, using parallaxes of the early- type component derived from Hipparcos data and by forcing the late-type com panion to be at the same distance. T-eff and log g of the early-type compon ents have been computed using the calibrations of two independent photometr ic systems: the uvby beta photometry and the Geneva system. The Teff of the late-type stars have been determined by using various calibrations of seve ral 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 sou rces of recent evolutionary models are considered and discussed. The consis tency of the age of the late type component with that of its early type pri mary is examined; the validity of this criterion for a selection of physica l pairs is discussed. The accuracy of the observational and theoretical dat a are not sufficient to assign stringent values to the age for several of t he examined systems. Nevertheless, in spite of the large error bars, we hav e established that we could select a number of systems which, according to their position in the HR diagram, may be physically associated. The selecti on of possible physically bounded systems obtained with the present approac h and that made by Pallavicini et al. (1992) or Martin et al. (1992) on the basis of spectroscopic criteria are not always coincident. Spectroscopic c riteria, for example the presence of a strong Li feature, are more stringen t conditions than that of coherent ages of primaries and secondaries; howev er the Li I 6708 doublet is expected to fade in the latest stages of the pr e-main sequence life of a star, so that the "oldest" pTTs may not be detect ed by spectroscopy only. The different results so obtained are discussed fo r each system and we conclude that the present approach may be used as a po werful criterion to select new pTT candidates in visual binaries to be obse rved and analyzed with high resolution spectrographs and to select candidat es that have almost reached the main sequence.