Radio emission of binary stars in the AB Doradus moving group

Autor:  Azulay Romero, Rebecca
Fecha de Lectura:  15/06/2016
Dirección: Guirado Puerta, José Carlos

We present a study of the radio emission and kinematics of a sample of stars belonging to the AB Doradus moving group through VLA and VLBI observations. The main aim of our study is to obtain precise estimates of the dynamical mass of young, low-mass stars, which in combination with photometric measurements provide precise benchmarks for calibrating pre-main-sequence (PMS) stellar evolutionary models. Calibration of PMS models appears essential as they are widely used to predict the masses of low mass objects as brown dwarfs and planets. Previous studies show that model predictions are in disagreement with experimental results for masses below 1.2 solar masses. Among the stars included in our study, we emphasize the results obtained in two of them: AB Dor B and HD 160934.

We observed the binary system AB Dor B in three different epochs between 2007 and 2013 with the Australian Long Baseline Array (LBA) at a frequency of 8.4 GHz. 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 solar masses and 0.25+/-0.05 solar masses, respectively. 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 2-sigma level. Some of the stellar models considered favor an age between 50 and 100 Myr for this system, while others predict older ages. Simultaneously with AB Dor B, we also observed the well-known stellar system AB Dor A, only 9 arcsec apart and composed by the stars AB Dor A itself and AB Dor C. The new data allowed us to revisit the previously published values of both the orbital parameters and dynamical masses. In particular, we found component masses of 0.894+/-0.040 solar masses and 0.090+/-0.005 solar masses for AB Dor A and AB Dor C, respectively, that allowed us for comparison with theoretical stellar models. In the case of AB Dor A, the models predict good estimates within the uncertainties, favouring an early age between 35-50 Myr. In the case of AB Dor C, the models derive a mass similar to the dynamical mass and an age between 40 and 120 Myr.

Regarding HD 160934, we observed this system with the European VLBI Network (EVN) at 5 GHz in three different epochs between 2012 and 2014. We showed that the two components of the binary, HD 160934 A and HD 160934 c, display compact radio emission at VLBI scales, providing precise information on the relative and absolute orbits, which were analyzed in combination with previously reported orbital measurements. Revised orbital elements were estimated and individual masses of 0.70+/-0.07 solar masses and 0.45+/-0.04 solar masses for components A and c, respectively, were determined. Theoretical models predict masses for the component A approximately 10% lower than our dynamical value; likewise the predictions for the component c are 20-40% lower than our dynamical measurement. The age of the system varies between 40 and 65 Myr depending on models.

Other stars in our sample include EK Dra, LO Peg, and PW And, observed with the EVN at 5 GHz. EK Dra and PW And showed compact radio emission at milliarcsecond scales, meanwhile LO Peg, appeared to be «off» at the time of observations. The scarce number of detections prevented us to obtain estimates of their masses, however, they allowed us to set bounds to their kinematics. Complementary, companion infrared observations of EK Dra has led us to a revision of the orbital parameters of this system.

In summary, our project provides new observational data to calibrate stellar evolutionary models of PMS objects, supporting the evidence that these models tend to underestimate the dynamical masses. The arrival of new, more sensitive telescopes with enormous monitoring capacity will allow a comprehensive study of the radio emission present in the moving groups found so far, definitely increasing the number of PMS stars with masses dynamically determined.