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Astron. Astrophys. 321, 220-228 (1997)

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5. Summary

We have surveyed 14 binaries with separations between [FORMULA] and [FORMULA]. In 27 of the 28 individual stars we did find Lithium absorption, which (together with their H [FORMULA] emission and association to dark clouds) classifies them as T Tauri stars. One of the presumed binaries turned out to be a likely chance projection with the "primary" showing neither H [FORMULA] emission nor Lithium absorption. This object (ESO H [FORMULA] 281 A) is very likely a background giant. Similarly, Aspin et al. (1994) found the 'companion' to the PMS star ESO H [FORMULA] 279 A to be a background giant.

A comparison of the equivalent width of the H [FORMULA] emission line of the primaries and secondaries showed that they are not correlated with each other. Some of the originally unresolved T Tauri stars were only picked up in H [FORMULA] surveys because they are binaries and have a secondary with strong H [FORMULA] emission whereas the primary shows only weak H [FORMULA] emission. Therefore, samples of H [FORMULA] selected T Tauri stars might be biased towards binaries.

Eight out of the 14 pairs could be placed on an H-R diagram. A comparison with theoretical pre-main-sequence evolutionary tracks yields that for all pairs the individual components appear to be coeval within the observational errors. This finding is similar to Hartigan et al. (1994) who found that 2/3 of the wider pairs with separations from 400 AU to 6000 AU are coeval. However, unlike Hartigan et al. for the wider pairs, we find no non-coeval pairs among our sample. Thus, young binaries with separations less than 400 AU might indeed represent a different regime as compared to binaries with separations between 400 AU and 6000 AU. Otherwise, it might very well be that the wide non-coeval binaries in the sample from Hartigan et al. are just chance projections and not physically related to each other.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998