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

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 of AB Dor C a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair AB Dor Ba/Bb can be determined, they could not only play a similar role to that of AB Dor C in calibrating PMS models, but 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 directed to monitor 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 all still agree at the 2-σ level. Some of stellar models considered favour an age between 50 and 100 Myr for this system, meanwhile 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.

The AB Doradus system revisited: The dynamical mass of AB Dor A/C

“…A vivid discussion about the nature, age, and characterization of this low-mass object followed from these results, including new observations made using the VLT chronograph (Boccaletti et al 2008), VLT SINFONI , Spitzer (Plavchan et al 2009), and VLTI (Guirado et al 2011). In this work we present a deeper analysis of the VLBI observations of the AB Doradus system reported in Azulay et al (2015), based on a new correlation pass centered on AB Dor A/C. Our analysis improves the determination of the orbital parameters of the system and obtains precise values of the dynamical masses of both components.…”

“…The sequence targetcalibrator-target lasted about four minutes. Results from these observations were reported in Azulay et al (2015) for AB Dor B, the companion to AB Dor A, that is 10 away from it. Since both sources lie within the primary beam of all telescopes, the AB Dor A data were obtained from a new correlator pass using the nominal AB Dor A position as phase center.…”

“…We followed the process already described in Azulay et al (2015), and outlined briefly here: (i) calibration of the visibility amplitudes using the gain curves of each antenna and the system temperatures of each station; (ii) correction of both the parallactic angle and the ionospheric contribution; and (iii) fringe-search on the calibrator PKS 0516−621 to remove residual contributions to the phases, whose corrections were applied to the target source AB Dor A. The phase-referenced naturally-weighted images of AB Dor A were obtained using the Caltech imaging program DIFMAP (Shepherd et al 1994) and are shown in Fig.…”

“…First, we followed the least-squares fit method detailed in Azulay et al (2015). We combined the absolute positions of AB Dor A and all the relative positions available to determine the astrometric and orbital parameters which are shown in Table 4.…”

“…We combined the absolute positions of AB Dor A and all the relative positions available to determine the astrometric and orbital parameters which are shown in Table 4. As stated in Azulay et al (2015), a priori values of the orbital elements are essential to find convergence of the least-squares method; we used the orbital elements given in Guirado et al (2006) as a priori values for the least-squares analysis, which greatly facilitated the convergence of our procedure. Diagrams of the absolute orbits are shown in Fig.…”