Microlensing Constraints on Low-Mass Companions

Microlensing is sensitive to binary, brown dwarf, and planetary companions to normal stars in the Galactic bulge with separations between about 1-10 AU. The accurate, densely-sampled photometry of microlensing events needed to detect planetary companions has been achieved by several follow-up collaborations. Detailed analysis of microlensing events toward the bulge demonstrates that less than 45% of M-dwarfs in the bulge have Jupiter-mass companions between 1 and 5 AU. Detection of binary and brown-dwarf companions using microlensing is considerably easier; however, the interpretation is hampered by their non-perturbative influence on the parent lightcurve. I demonstrate that ~25% of brown-dwarf companions with separations 1-10 AU should be detectable with survey-quality data (~1 day sampling and ~5% photometry). Survey data is more amenable to generic, brute-force analysis methods and less prone to selection biases. An analysis of the ~1500 microlensing events detected by OGLE-III in the next three years should test whether the BD desert exists at separations 1-10 AU from M-dwarfs in the Galactic bulge.