Resolved astrometric orbits of ten O-type binaries^⋆,⋆⋆

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: jean-baptiste.lebouquin@univ-grenoble-alpes.fr
² Institute of Astrophysics, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
³ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du Six Août, 4000 Liège, Belgium
⁴ European Southern Observatory, Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁵ Unidad Mixta Internacional Franco-Chilena de Astronomía (UMI 3386), CNRS/INSU France; Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁶ LESIA, UMR 8109, Observatoire de Paris, CNRS, UPMC, Univ. Paris-Diderot, PSL, 5 place Jules Janssen, 92195 Meudon, France
⁷ European Southern Observatory, Alonso de Cordova 3107, Vitacura, 19001 Casilla, Santiago 19, Chile
⁸ I. physikalisches Institut, Universität zu Köln, Zülpicherstr. 77, 50937 Köln, Germany
⁹ Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Católica de Chile, Casilla 306, 22 Santiago, Chile
¹⁰ University of Exeter, Astrophysics Group, School of Physics, Stocker Road, Exeter EX4 4QL, UK
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 7 July 2016
Accepted: 23 July 2016

Abstract

Aims. Our long-term aim is to derive model-independent stellar masses and distances for long period massive binaries by combining apparent astrometric orbit with double-lined radial velocity amplitudes (SB2).

Methods. We followed-up ten O+O binaries with AMBER, PIONIER and GRAVITY at the VLTI. Here, we report on 130 astrometric observations over the last seven years. We combined this dataset with distance estimates to compute the total mass of the systems. We also computed preliminary individual component masses for the five systems with available SB2 radial velocities.

Results. Nine of the ten binaries have their three-dimensional orbit well constrained. Four of them are known to be colliding wind, non-thermal radio emitters, and thus constitute valuable targets for future high angular resolution radio imaging. Two binaries break the correlation between period and eccentricity tentatively observed in previous studies. This suggests either that massive star formation produces a wide range of systems, or that several binary formation mechanisms are at play. Finally, we found that the use of existing SB2 radial velocity amplitudes can lead to unrealistic masses and distances.

Conclusions. If not understood, the biases in radial velocity amplitudes will represent an intrinsic limitation for estimating dynamical masses from SB2+interferometry or SB2+Gaia. Nevertheless, our results can be combined with future Gaia astrometry to measure the dynamical masses and distances of the individual components with an accuracy of 5 to 15%, completely independently of the radial velocities.