| Verf.angabe: | L. Rodet, M. Bonnefoy, S. Durkan, H. Beust, A.-M. Lagrange, J.E. Schlieder, M. Janson, A. Grandjean, G. Chauvin, S. Messina, A.-L. Maire, W. Brandner, J. Girard, P. Delorme, B. Biller, C. Bergfors, S. Lacour, M. Feldt, T. Henning, A. Boccaletti, J.-B. Le Bouquin, J.-P. Berger, J.-L. Monin, S. Udry, S. Peretti, D. Segransan, F. Allard, D. Homeier, A. Vigan, M. Langlois, J. Hagelberg, F. Menard, A. Bazzon, J.-L. Beuzit, A. Delboulbe, S. Desidera, R. Gratton, J. Lannier, R. Ligi, D. Maurel, D. Mesa, M. Meyer, A. Pavlov, J. Ramos, R. Rigal, R. Roelfsema, G. Salter, M. Samland, T. Schmidt, E. Stadler, and L. Weber |
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| Abstract: | Evolutionary models are widely used to infer the mass of stars, brown dwarfs, and giant planets. Their predictions are thought to be less reliable at young ages ($<$ 200 Myr) and in the low-mass regime ($\mathrm{<1~M_{\odot}}$). GJ 2060 AB and TWA 22 AB are two rare astrometric M-dwarf binaries respectively members of the AB Doradus and Beta Pictoris moving groups. As their dynamical mass can be measured within a few years, they can be used to calibrate the evolutionary tracks and set new constraints on the age of young moving groups. We find a total mass of $\mathrm{0.18\pm 0.02~M_\odot}$ for TWA 22. That mass is in good agreement with model predictions at the age of the Beta Pic moving group. We obtain a total mass of $\mathrm{1.09 \pm 0.10~M_{\odot}}$ for GJ 2060. We estimate a spectral type of M$1\pm0.5$, $\mathrm{L/L_{\odot}=-1.20\pm0.05}$ dex, and $\mathrm{T_{eff}=3700\pm100}$ K for GJ 2060 A. The B component is a M$3\pm0.5$ dwarf with $\mathrm{L/L_{\odot}=-1.63\pm0.05}$ dex and $\mathrm{T_{eff}=3400\pm100}$ K. The dynamical mass of GJ 2060 AB is inconsistent with the most recent models predictions (BCAH15, PARSEC) for an ABDor age in the range 50-150 Myr. It is 10 to 20\% (1-2 sigma, depending on the assumed age) above the models predictions, corresponding to an underestimation of $0.10$ to $0.20~\mathrm{M_\odot}$. Coevality suggests a young age for the system ($\sim$ 50 Myr) according to most evolutionary models. TWA 22 validates the predictions of recent evolutionary tracks at $\sim$20 Myr. On the other hand, we evidence a 1-2 sigma mismatch between the predicted and observed mass of GJ 2060 AB. This slight departure may indicate that one of the star hosts a tight companion. Alternatively, this would confirm the models tendency to underestimate the mass of young low-mass stars. |
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