Dynamical masses of the low-mass stellar binary AB Doradus B

¹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
e-mail: Rebecca.Azulay@uv.es
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
⁴ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁵ CSIRO Astronomy and Space Science, Canberra, 2122 Marsfield, Australia
⁶ Research School of Astronomy & Astrophysics, Australian National University, 0200 Canberra, Australia
⁷ Observatoire de Paris/LERMA, 61 rue de l’Observatoire, 75014 Paris, France
⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA
⁹ University of Roma Tor Vergata, Department of Physics, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁰ INFN, Section of Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
¹¹ INAF–Observatory of Rome, via Frascati 33, 00040 Monteporzio Catone (RM), Italy

Received: 21 January 2015
Accepted: 31 March 2015

Abstract

Context. AB Doradus is the main system of the AB Doradus moving group. It is a quadruple system formed by two widely separated binaries of pre-main-sequence (PMS) stars: AB Dor A/C and AB Dor Ba/Bb. The pair AB Dor A/C has been extensively studied and its dynamical masses have been determined with high precision, thus making AB Dor C a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair AB Dor Ba/Bb could be determined, they could play a similar role to that of AB Dor C in calibrating PMS models, and would also help to better understand the dynamics of the whole AB Doradus system.

Aims. We aim to determine the individual masses of the pair AB Dor Ba/Bb using VLBI observations and archive infrared data as part of a larger program that monitors binary systems in the AB Doradus moving group.

Methods. We observed the system AB Dor B between 2007 and 2013 with the Australian Long Baseline Array (LBA) at a frequency of 8.4 GHz in phase-reference mode.

Results. We detected, for the first time, compact radio emission from both stars in the binary, AB Dor Ba and AB Dor Bb. This result allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28 ± 0.05 M_⊙ and 0.25 ± 0.05 M_⊙, respectively.

Conclusions. Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by 10–30% and 10–40%, respectively, although they still agree at the 2σ level. Some of the stellar models considered favor an age between 50 and 100 Myr for this system, while others predict older ages. We also discuss the evolutionary status of AB Dor Ba/Bb in terms of an earlier double-double star scenario that might explain the strong radio emission detected in both components.