Dynamical masses of M-dwarf binaries in young moving groups

I. The case of TWA 22 and GJ 2060

¹ Univ. Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: laetitia.rodet@univ-grenoble-alpes.fr
² Astrophysics Research Centre, Queens University Belfast, Belfast, UK
³ Department of Astronomy, Stockholm University, Stockholm, Sweden
⁴ Exoplanet and Stellar Astrophysics Laboratory, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 2286 and Departemento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo della Osservatorio 5, 35122 Padova, Italy
⁸ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, 19001 Casilla, Santiago, Chile
⁹ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ University College London, London, UK
¹¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC, Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
¹² Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
¹³ CRAL, UMR 5574, CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France
¹⁴ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
¹⁵ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁶ NOVA Optical Infrared Instrumentation Group, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁷ Anton Pannekoek Institute for Astronomy, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁸ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland

Received: 28 February 2018
Accepted: 13 June 2018

Abstract

Context. 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 (< 1 M_⊙). GJ 2060 AB and TWA 22 AB are two rare astrometric M-dwarf binaries, respectively members of the AB Doradus (AB Dor) and Beta Pictoris (β Pic) moving groups. As their dynamical mass can be measured to within a few years, they can be used to calibrate the evolutionary tracks and set new constraints on the age of young moving groups.

Aims. We provide the first dynamical mass measurement of GJ 2060 and a refined measurement of the total mass of TWA 22. We also characterize the atmospheric properties of the individual components of GJ 2060 that can be used as inputs to the evolutionary models.

Methods. We used NaCo and SPHERE observations at VLT and archival Keck/NIRC2 data to complement the astrometric monitoring of the binaries. We combined the astrometry with new HARPS radial velocities (RVs) and FEROS RVs of GJ 2060. We used a Markov chain Monte-Carlo (MCMC) module to estimate posteriors on the orbital parameters and dynamical masses of GJ 2060 AB and TWA 22 AB from the astrometry and RVs. Complementary data obtained with the integral field spectrograph VLT/SINFONI were gathered to extract the individual near-infrared (1.1–2.5 μm) medium-resolution (R ∼ 1500 − 2000) spectra of GJ 2060 A and B. We compared the spectra to those of known objects and to grids of BT-SETTL model spectra to infer the spectral type, bolometric luminosities, and temperatures of those objects.

Results. We find a total mass of 0.18 ± 0.02 M_⊙ for TWA 22, which is in good agreement with model predictions at the age of the β Pic moving group. We obtain a total mass of 1.09 ± 0.10 M_⊙ for GJ 2060. We estimate a spectral type of M1 ± 0.5, L/L_⊙ = −1.20 ± 0.05 dex, and T_eff = 3700 ± 100 K for GJ 2060 A. The B component is a M3 ± 0.5 dwarf with L/L_⊙ = −1.63 ± 0.05 dex and T_eff = 3400 ± 100 K. The dynamical mass of GJ 2060 AB is inconsistent with the most recent models predictions (BCAH15, PARSEC) for an AB Dor age in the range 50–150 Myr. It is 10%–20% (1–2σ, depending on the assumed age) above the model’s predictions, corresponding to an underestimation of 0.10–0.20 M_⊙. Coevality suggests a young age for the system (∼50 Myr) according to most evolutionary models.

Conclusions. TWA 22 validates the predictions of recent evolutionary tracks at ∼20 Myr. On the other hand, we evidence a 1–2σ mismatch between the predicted and observed mass of GJ 2060 AB. This slight departure may indicate that one of the stars hosts a tight companion. Alternatively, this would confirm the model’s tendency to underestimate the mass of young low-mass stars.