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

Abstract

Context. The study of pre-main-sequence (PMS) stars with model-independent measurements of their masses is essential to check the validity of theoretical models of stellar evolution. The well-known PMS binary AB Dor A/C is an important benchmark for this task, since it displays intense and compact radio emission, which makes possible the application of high-precision astrometric techniques to this system. Aims. We aim to revisit the dynamical masses of the components of AB Dor A/C to refine earlier comparisons between the measurements of stellar parameters and the predictions of stellar models. Methods. We observed in phase-reference mode the binary AB Dor A/C, 0.2 separation, with the Australian Long Baseline Array at 8.4 GHz. The astrometric information resulting from our observations was analyzed along with previously reported VLBI, optical (Hipparcos), and infrared measurements. Results. The main star AB Dor A is clearly detected in all the VLBI observations, which allowed us to analyze the orbital motion of the system and to obtain model-independent dynamical masses of 0.90 ± 0.08 M and 0.090 ± 0.008 M , for AB Dor A and AB Dor C, respectively. Comparisons with PMS stellar evolution models favor and age of 40−50 Myr for AB Dor A and of 25−120 Myr for AB Dor C. Conclusions. We show that the orbital motion of the AB Dor A/C system is remarkably well determined, leading to precise estimates of the dynamical masses. Comparison of our results with the prediction of evolutionary models support the observational evidence that theoretical models tend to slightly underestimate the mass of the low-mass stars.

Evidence of a Substellar Companion to AB Dor C

“…This implies masses of 0.072 ± 0.013 M and 0.013 ± 0.001 M for each one of the components of the binary, which interestingly lie near the hydrogen-burning limit for the case of AB Dor Ca, and near the deuterium-burning limit, straddling the boundary between brown dwarfs and giant planets, for the case of AB Dor Cb. Given the relatively large mass ratio between AB Dor Ca/Cb, we notice that the tentative binarity of AB Dor C would not result in a substantial change in the age range when compared with previous estimates based on the same evolutionary models (Azulay et al 2017). Likewise, the presence of AB Dor Cb does not affect the age determinations based on model isochrones fitting to the members of the AB Dor moving group (Bell et al 2015).…”

“…The median values given in their Table 6 for very low-mass binaries range from 0.85 to 1.16, which, considering our Ca/Cb separation (38.1 mas) and the dynamical mass of AB Dor C (0.090 M ), translate to a semimajor axis for the Ca/Cb orbit of 32.4-44.2 mas with a period of 418-666 days. The Ca/Cb binary is, in turn, orbiting the 0.89 M AB Dor A with a period of 11.78 yr (Azulay et al 2017). It is obvious that the complete system is dynamically dominated by the presence of AB Dor A, and The first row represents the bolometric luminosity derived from the photometric measurements and the bolometric corrections found in Filippazzo et al (2015).…”

“…The critical semimajor axis at which the orbit of the system Ca/Cb is stable depends on the eccentricity of the binary A/C, the mass ratio A/C, and the separation between the host object (Ca) and the perturber (A). Assuming the estimated mass of AB Dor Ca, and adopting the orbital parameters of AB Dor C around AB Dor A given in Azulay et al (2017), the formulae provided by Holman & Wiegert (1999) yield stable orbits for AB Dor Ca/Cb for separations <50 mas. This implies that our measured separation for the binary in AB Dor C (∼38 mas) would correspond to a stable binary system.…”

“…Absolute orbits of AB Dor A and AB Dor C, adapted from(Azulay et al 2017). The map is centered at the center of mass of the system.…”

“…Mass versus age at constant K s magnitude: 8.43 ± 0.16 (blue; Ca) and 11.7 ± 0.3 (red; Cb) and the corresponding binary system (green; Ca+Cb). A dashed-dotted line represents the measured dynamical mass of 0.090 M(Azulay et al 2017), while the shadowed gray area represents the age range (44-66 Myr) resulting from the intersection of this measured dynamical mass with the measured Ca+Cb magnitude, according to the models byTognelli et al (2018). The irregularities in the model are more likely a product of the interpolation than a real physical effect.…”