Session 40 - The Environment of Stars: From Protostars to the Main Sequence.
Display session, Tuesday, June 11
Great Hall,

[40.09] Physical properties of 90 to 250 AU pre-main sequence binaries

W. Brandner (AI Würzburg), H. Zinnecker (AI Potsdam)

We have analyzed photometric and spectroscopic data of a sample of 15 spatially resolved pre-main sequence binaries (separations 0.\!\!^\prime\prime6 to 1.\!\!^\prime\prime7) in the nearby (150 pc) low-mass star--forming regions of Chamaeleon, Lupus, and \rho Ophiuchi. The spectroscopic data have been obtained with the ESO New Technology Telescope (NTT) at La Silla under subarcsec seeing conditions. All binaries (originally unresolved) were identified as pre-main sequence stars based on their strong H\alpha emission --- which classifies them as classical T Tauri stars --- and their association with dark clouds. One of the presumed binaries turned out to be a likely chance projection with the primary'' showing neither H\alpha emission nor Li absorption.

Using the spectral A index (as defined by Kirkpatrick et al.\ 1991 ApJS 77, 417), which measures the strength of the CaH band at 697.5nm relative to the nearby continuum, as a luminosity class indicator, we could show that classical T Tauri stars with spectral types later than M4 are intermediate between luminosity class III and V, whereas earlier spectral types tend to be closer to luminosity class V than III.

Eight out of the 15 pairs could be placed on a 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 ApJ 427, 961) who detected that two third 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, the formation mechanism for a significant fraction of the wider pre-main sequence binaries might be different from that of closer pre-main sequence binaries. All of the latter appear to have formed simultaneously.

The age spread among binaries associated with the individual dark clouds Cha I amp; II and \rho Oph is considerably smaller than the overall scatter of ages among all binaries. Star formation in the individual dark clouds might have been roughly coeval, too.