Substellar Companion Masses from Minimal Radial Velocity or Astrometric Information: a Monte Carlo Approach

Abstract

Slow drifts detected in the radial velocities of a number of nearby stars indicate the presence of companions in orbit around the primaries, even though a complete spectroscopic orbital solution is not possible because of the long periods. Some of these companions may have substellar masses and are thus of considerable interest for the study of the characteristics of extrasolar planets and brown dwarfs. Similar indications can be obtained from the measurement of small astrometric accelerations of the primary star, either from the ground or more likely from future highly precise space astrometry missions. We show here that a single measurement of the angular separation of such companions, if they can be imaged directly, provides valuable statistical information on their mass when combined with the accelerations. We derive the probability distribution of the companion mass separately for the radial velocity case and for the astrometric case by Monte Carlo simulations, and illustrate the application of the technique with several examples.The information on the masses of such wide substellar companions is complementary to that from closer spectroscopic companions, and a full analysis of the mass distribution of these objects should include both to avoid possible biases.