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Astron. Astrophys. 364, 665-673 (2000)

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1. Introduction

Accurate masses for binary stars provide a crucial test of our understanding of stellar physics (e.g. Andersen 1991). Mass, the basic input of evolutionary models, is directly measured, and the models must reproduce the effective temperatures and luminosities (or radii) of both components, for a single age and a single chemical composition. Given the strong mass dependency of all stellar parameters however, this discriminating diagnostic only shows its power for relative mass errors [FORMULA]1-3%.

Very accurate mass measurements have long been the exclusive province of double-lined detached eclipsing binaries (Andersen 1991, 1998), for which as a bonus the stellar radii are simultaneously determined. Such systems are however unfortunately rare, and only 44 pairs had yielded masses accurate enough to be included in Andersen 1991's critical compilation, mostly for intermediate mass stars. Relatively few eclipsing binaries have had their masses measured since then. In the mass range of interest here, the litterature still contains no more than three well detached eclipsing binaries with substantially subsolar component masses: YY Gem (M0Ve, 0.6+0.6[FORMULA]; Bopp 1974; Leung & Schneider 1978), the recently identified GJ 2069A (M3.5Ve, 0.4+0.4[FORMULA]; Delfosse et al. 1999a), and CM Dra (M4Ve, 0.2+0.2[FORMULA]; Lacy 1977; Metcalfe et al. 1996).

Angularly resolved spectroscopic binaries provide stellar masses in parts of the HR diagram where eclipsing systems are rare or missing, and in particular for very low mass stars. Until recently however, these measurements did not match the [FORMULA]1% accuracy which can be obtained in detached eclipsing systems. As a consequence, the best representation to date of the empirical M-L relation for M dwarfs had to mostly rely on masses determined with 5-20% accuracy (Henry & McCarthy 1993; Henry et al. 1999). The last two years have seen a dramatic evolution in this respect, with two groups breaking through the former [FORMULA]5% accuracy barrier. The first group to do so used the 1 mas per measurement astrometric accuracy of the Fine Guidance Sensors (Benedict et al. 1999) on HST to determine a few masses of angularly resolved binaries with 2 to 10% accuracy (Franz et al. 1998; Torres et al. 1999; Henry et al. 1999; Benedict et al. 2000). Slightly more recently, we have demonstrated that the combination of very accurate radial velocities with angular separations from adaptive optics imaging can yield masses for VLMS with even better accuracy, of only 1-3% (Forveille et al. 1999; Delfosse et al. 1999b). Here we present 16 new or improved masses determined with the same method, with accuracies that now range between 0.2 and 5%. In a companion paper (Delfosse et al. 2000a) we rediscuss the VLMS mass-luminosity relation in the light of these new data. We first discuss the observing program and its sample in Sect. 2 and then present the observations and data processing in Sect. 3. Sect. 4 describes the orbit adjustment and the mass determination.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001