Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

with only three known to date. Most mass determinations for Very Low Mass Stars (VLMS) are therefore instead
obtained from visual and interferometric pairs, which until recently, have not yielded comparable precisions on
mass estimates. By combining very accurate radial velocities with precise angular separations from adaptive
optics, our team reached accuracies of only 1-3% for VLMS (Forveille et al. 1999; Delfosse et al. 1999b;
Ségransan et al. 2000; 2003). To date, masses were determined for ∼30 M dwarfs with accuracies ranging
between 0.5 and 5% by us, with additional measurements becoming available as the time span of our surveys
catches up with longer period systems.
As predicted by stellar structure models, the metallicity dispersion of the field population induces a large
scatter around the mean V-band relation, while the infrared relations are much tighter.
Figure 6.10: V-band Mass-Metalllicity-Luminosity relation. The red filled circles represent our metallicity
determinations and the blue open circles those from WW05. The symbol size is proportional to the metallicity,
and the dashed contours represent iso-metallicity for our calibration, spaced by 0.25 dex from +0.25 (left) to
-1.75 dex (right). The solid lines are the V-band empirical M/L relation of Delfosse et al. 2000.
In Delfosse et al. (2000) it was suggested that metallicity might explain most of this intrinsic dispersion
on the visible mass-luminosity relation, but for lack of quantitative metallicity estimates we could not pursue
this suggestion further. Recently (Bonfils et al. 2005, in press) determined the metallicities for 20 M-dwarfs in
wide-binary systems that also contain an F-, G- or K-star, under the simple assumption that the two stars have
the same composition. We used this data to derive a photometric calibration of the metallicities of very lowmass
stars. The calibration is valid between 0.8 and 0.1 M⊙, needs V- and K-band photometry and an accurate
parallax, and provides metallicity estimates with ∼0.2 dex uncertainties. We use these new metallicity estimates
to take a fresh look at the V-band mass-luminosity relation, and demonstrate that its intrinsic dispersion is
indeed due to metallicity. The first mass-metallicity-luminosity relation for M dwarfs is hence determined (see
Fig. 6.10).
6.5 Selected topics on Extra-Solar Planets
About 150 planetary system have to date been unveiled around stars other than our Sun. They were initially
searched for, and found, around solar-type stars (e.g., Mayor & Queloz 1995, Nature 378, 355; Marcy G.W. &
Butler R.P., ARAA, 36, 57). As a result of this bias, only a few planets around stars with very different masses
are known.
One major motivation to look for planets around “non solar-like stars” is to constrain the planet formation
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