Observations indicate that most stars are formed in binary and multiple systems, a quite lucky circumstance, as the study of binary orbits represents the only means to determine the most fundamental parameter of a star, its mass. In spite of this, there are so far only a few direct mass determinations for low-mass stars in the pre-main-sequence (PMS) phase. In particular, eclipsing double-lined spectroscopic binaries (ESB2) represent the most powerful tool for determining fundamental stellar parameters, as they allow to derive, simultaneously, the masses and the radii of the components, and test theoretical models, provided that they make part of a detached system and can thus be assumed to evolve like single stars.
Among PMS stars, only EK Cep and TY CrA have been detected so far as eclipsing systems. However, in both cases, only the low-mass secondary component is still in the PMS phase, whereas the primary is an intermediate-mass star, already on or very close to the ZAMS (Mathieu et al. 1999). This implies that PMS evolutionary tracks for stellar masses smaller than about 1.5 are still very poorly constrained.
We have undertaken a coordinated monitoring campaign to search for and study low-mass PMS eclipsing binaries among a sample of ROSAT-discovered double-lined spectroscopic binaries in Orion (Alcalá et al. 2000) as well as in other star formation regions (Covino et al. 1997, Wichmann et al. 1999), with the aim to determine their orbital and fundamental parameters. Ultimately, these new data will also help to enlarge the statistics on PMS binaries with known orbital elements and allow to address basic questions on the early evolution of binary orbits (Melo et al., in prep.).
In this Letter, we report the discovery that the double-lined low-mass PMS binary RXJ 0529.4+0041 is an eclipsing system. It is thus possible to constrain the orbital inclination angle and to obtain a direct determination of the absolute masses for the binary components, as well as of other orbital and fundamental stellar parameters (namely, orbit size, stellar radii and effective temperatures of the components), through the combined analysis of the radial velocity and photometric curves. This information can then be used to set constraints on current PMS theoretical evolutionary models.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000