Dynamical Masses of Young Stellar Multiple Systems with the VLBA (DYNAMO-VLBA)

Abstract

Very Long Baseline Interferometry (VLBI) provides high angular resolution images and has been used for stellar astrometry for decades. The DYNAMO-VLBA project utilizes the Very Long Baseline Array (VLBA) to study tight binary and multiple pre-main sequence stars, whose components have detectable radio emission and typical separations on the order of milli-arcseconds. Such systems cannot be resolved by Gaia, making VLBI an essential tool for the study of their orbital parameters and, eventually, the determination of their mass. Here, we report VLBA dynamical mass measurements of the individual stars in the S1 system in Ophiuchus and EC\,95 in Serpens. S1 is the most luminous and massive stellar member of the nearby Ophiuchus star-forming region. We find that the primary component, S1A, has a mass of $4.11 \pm 0.10\,M_{\odot}$. This is significantly less than the value of $\sim6\,M_{\odot}$ expected from theoretical models given the location of S1A on the HR diagram. The secondary, S1B, has a mass of $0.831 \pm 0.014\,M_{\odot}$ and is most likely a T Tauri star. In the Serpens triple system EC\,95, we measure the masses of EC\,95A and EC\,95B, finding $2.15\pm0.10$ M$_\odot$ and $2.00\pm0.12$ M$_\odot$, respectively. In this case, the measured masses agree with the location of the stars in the HR diagram for very young 2 $M_\odot$ stars. For the first time, we also estimated the mass of tertiary, EC\,95C, to be 0.26 $^{+0.53}_{-0.46}$ M$_\odot$.

Figures (5)

• Figure 1: Measured positions of S1A (red dots) and S1B (blue dots) shown as offsets from the position of S1A in the first detected epoch (2005 June 24).
• Figure 2: Stellar relative positions and orbital fit model of S1. The blue dots indicate the relative positions of S1B with respect to S1A, and the errorbars consider the position errors of both components which are added in quadrature. The dashed black line traces the line of nodes from the model, and the black cross indicates the position of the primary.
• Figure 3: Measured positions of EC 95A (red dots) and EC 95B (blue dots) shown as offsets from the position of EC 95A in the first detected epoch (2007 December 22).
• Figure 4: Stellar relative positions and orbital fit model of EC 95. The blue dots indicate the relative positions of EC 95B with respect to EC 95A. The remaining description is similar to that provided for the S1 Figure \ref{['s1_2']}.
• Figure 5: Orbits of EC 95A (red line), EC 95B (blue line) and EC 95C (green line) around the center of mass of the system (black cross). The colored squares indicate the measured positions, while the arrows show the direction of the orbits.