Long Baseline Interferometry: a Promising Tool for Multiplicity Investigations of Massive Stars

The issue of the multiplicity of massive stars can be addressed through the investigation of radial velocity variations in spectral lines, or by the detection of blended asymmetric line profiles. However, the absence of such signatures should by no mean be interpreted as an evidence that the star is single. The non-detection of the spectral line of the secondary can indeed be due to a high luminosity ratio of the two components of the binary. The lack of significant radial velocity variation can also be due to a very low inclination angle of the orbit, a poor sampling of an eccentric orbit, or even a very long orbital period. On the other hand, interferometry can also be used to investigate stellar multiplicity, but most of the Galactic massive stars studied so far are located at distances of a few kpc. Considering the angular resolution of present facilities such as the VLTI (∼mas), typical orbital separations of a few AUs are accessible. Applying Keplers laws, this translates into periods of at least a few years or so. A large inclination angle and a large eccentricity may compromise further the resolution of binaries, at least during a significant fraction of their orbit. Interferometry is therefore mostly efficient at investigating long period binaries. Both approaches present therefore different limitations, and should be considered as complementary. Long baseline interferometry is therefore expected to open a new part of the parameter space of massive binaries (see also Sana & Le Bouquin in this volume).