Weighing the Darkness: Astrometric Mass Measurement of Hidden Stellar Companions using Gaia

TL;DR

This paper assesses Gaia's capability to measure the orbits and masses of binary systems with hidden stellar companions, such as brown dwarfs, white dwarfs, neutron stars, and black holes, using astrometric data and simulations.

Contribution

It provides a detailed analysis of Gaia's potential to determine the orbits and component masses of binaries with dark companions across various distances and orbital periods.

Findings

Gaia can measure orbits of binaries with hidden brown dwarfs up to tens of parsecs.

Gaia can detect white dwarf and neutron star companions at hundreds of parsecs.

Black hole companions may be measured out to 1 kiloparsec or more.

Abstract

In astrometric binaries, the presence of a dark, unseen star can be inferred from the gravitational pull it induces on its luminous binary companion. While the orbit of such binaries can be characterized with precise astrometric measurements, constraints made from astrometry alone are not enough to measure the component masses. In this work, we determine the precision with which Gaia can astrometrically measure the orbits and -- with additional observations -- the component masses, for luminous stars hosting hidden companions. Using realistic mock Gaia observations, we find that Gaia can precisely measure the orbits of binaries hosting hidden brown-dwarfs out to tens of pc and hidden white dwarf and neutron star companions at distances as far as several hundred pc. Heavier black hole companions may be measured out to 1 kpc or farther. We further determine how orbital period affects this precision, finding that Gaia can characterize orbits with periods as short as 10 days and as long as a few 10$^3$ days, with the best measured orbits having periods just short of Gaia's mission lifetime. Extending Gaia's nominal five-year mission lifetime by an additional five years not only allows for the measurement of longer period orbits, but those longer period binaries can be seen at even greater distances.