Microlensing, Brown Dwarfs and GAIA

TL;DR

GAIA's microlensing capabilities enable precise mass measurements of nearby brown dwarfs and stars, especially when combined with ground-based follow-up, offering a new way to inventory their masses regardless of luminosity.

Contribution

This paper proposes strategies for using GAIA data to predict and observe microlensing events caused by high proper motion brown dwarfs, enabling mass measurements of these objects.

Findings

Small sample size of ~100 high proper motion brown dwarfs needed for predictable events.

GAIA can accurately measure lens masses through combined astrometric and photometric data.

Microlensing with GAIA offers a unique method to determine masses of faint objects.

Abstract

The GAIA satellite can precisely measure the masses of nearby brown dwarfs and lower main sequence stars by the microlensing effect. The scientific yield is maximised if the microlensing event is also followed with ground-based telescopes to provide densely sampled photometry. There are two possible strategies. First, ongoing events can be triggered by photometric or astrometric alerts by GAIA. Second, events can be predicted using known high proper motion stars as lenses. This is much easier, as the location and time of an event can be forecast. Using the GAIA source density, we estimate that the sample size of high proper motion ($>300$ mas yr$^{-1}$) brown dwarfs needed to provide predictable events during the 5 year mission lifetime is surprisingly small, only of the order of a hundred. This is comparable to the number of high proper motion brown dwarfs already known from the work of the UKIDSS Large Area Survey and the all-sky WISE satellite. Provided the relative parallax of the lens and the angular Einstein radius can be recovered from astrometric data, then the mass of the lens can be found. Microlensing provides the only way of measuring the masses of individual objects irrespective oftheir luminosity. So, microlensing with GAIA is the best way to carry out an inventory of masses in the brown dwarf regime.