Microlensing Constraints on Low-Mass Companions

Microlensing is sensitive to binary, brown dwarf (BD), 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 MJup companions between 1 and 5 AU. Detection of binary and BD 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 BD companions with separations 1−10AU should be detectable with surveyquality 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− 10AU from M-dwarfs in the Galactic bulge.