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

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Physical properties of 90 AU to 250 AU pre-main-sequence binaries *

Wolfgang Brandner 1 and Hans Zinnecker 2

1 Astronomisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany (brandner@astro.uni-wuerzburg.de)
2 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany (hzinnecker@aip.de)

Received 12 September 1996 / Accepted 9 October 1996

Abstract

We have analyzed photometric and spectroscopic data of a sample of 14 spatially resolved pre-main-sequence binaries (separations [FORMULA] 6 to [FORMULA] 7) in the nearby (150 pc) low-mass star-forming regions of Chamaeleon, Lupus, and [FORMULA] 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 [FORMULA] 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 [FORMULA] emission nor Li absorption.

Using the spectral A index (as defined by Kirkpatrick et al. 1991), 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 the classical T Tauri stars in our sample tend to be close to luminosity class V.

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 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.

Key words: stars: binaries: visual – stars: pre-main-sequence – T association: Chamaeleon, Lupus, ae Ophiuchi

* Based on observations obtained at the European Southern Observatory, La Silla; ESO Proposal No. 52.7-0058, 53.7-0107, 54.D-0911

Send offprint requests to: Wolfgang Brandner

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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