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

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1. Introduction

In a study entitled "Are wide pre-main-sequence binaries coeval? " Hartigan, Strom & Strom (1994) investigated young T Tauri binary stars with separations [FORMULA] 400 AU in order to find clues on how binaries form. A comparison with theoretical pre-main-sequence evolutionary tracks showed that 2/3 of the 26 binaries in their sample are coeval. Thus these binaries very likely formed through fragmentation shortly before or during the collapse phase of a molecular cloud, a process discussed, e.g., by Larson (1978), Boss (1988), Bodenheimer et al. (1988), and Pringle (1989). In the other 1/3 of the cases, however, the secondary appears to be considerably younger than the primary.

Our study aimed at expanding the investigation towards closer pre-main-sequence (PMS) binaries. Close binaries sample a different regime, because for smaller separations circumstellar disks in binaries might be strongly disturbed by the presence of the companion (Papaloizou & Pringle 1977; Artymowicz & Lubow 1994). Thus, the PMS components of a close binary evolve not independently from each other due to disk mediated interaction.

We have analyzed photometric and spectroscopic data of a sample of 14 spatially resolved PMS binaries (separations [FORMULA] to [FORMULA], which corresponds to 90 AU to 250 AU at an average distance of 150 pc) in the nearby low-mass star-forming regions of Chamaeleon, Lupus, and [FORMULA] Ophiuchi. The stars had been originally identified as T Tauri stars based on their strong H [FORMULA] emission and their association with dark clouds by Schwartz (1977, desig. "Sz") and Wilking et al. (1987, desig. "WSB"), and were later on resolved as binaries by Brandner (1992) and Reipurth & Zinnecker (1993).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998