Triage of astrometric binaries – how to find triple systems and dormant black hole secondaries in the Gaia orbits

ABSTRACT Preparing for the expected wealth of Gaia detections, we consider here a simple algorithm for classifying unresolved astrometric binaries with main-sequence (MS) primary into three classes: binaries with a probable MS secondary, with two possible values for the mass ratio; probable hierarchical triple MS systems with an astrometric secondary as a close binary, with a limited range of mass-ratio values; and binaries with a compact-object secondary, with a minimal value of the mass ratio. This is done by defining a unitless observational parameter ‘Astrometric Mass-Ratio Function’ (AMRF), $\mathcal {A}$, of a binary, based on primary-mass estimation, in addition to the astrometric parameters – the angular semimajor axis, the period, and the parallax. We derive the $\mathcal {A}$ value that differentiates the three classes by forward modelling representative binaries of each class, assuming some mass–luminosity relation. To demonstrate the potential of the algorithm, we consider the orbits of 98 Hipparcos astrometric binaries with MS primaries, using the Hipparcos parallaxes and the primary-mass estimates. For systems with known spectroscopic orbital solution, our results are consistent with the spectroscopic elements, validating the suggested approach. The algorithm will be able to identify hierarchical triple systems and dormant neutron star and black hole companions in the Gaia astrometric binaries.

Spectroscopic orbits of nearby stars

Aims. We observed stars with variable radial velocities to determine their spectroscopic orbits. Methods. Velocities are presented of 132 targets taken over a time span reaching 30 years. These were measured with the correlation radial velocity spectrometers (1917 velocities) and the new VUES echelle spectrograph (627 velocities), with a typical accuracy of 0.5 and 0.2 km s−1, respectively. Results. We derived spectroscopic orbits of 57 stars (including 53 first-time orbits), mostly nearby dwarfs of spectral types K and M, with some being HIPPARCOS astrometric binaries. Their periods range from 2.2 days to 14 years. Comments on individual objects are provided. Many stars belong to hierarchical systems containing three or more components, including 20 new hierarchies resulting from this project. The preliminary orbit of the young star HIP 47110B has a large eccentricity e = 0.47 despite having a short period of 4.4 d; it could still be circularizing. Conclusions. Our results enrich the data on nearby stars and contribute to a better definition of the multiplicity statistics.