2004ApJ...617.1307J

Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M=(c²/4G)r{d5}_Eθ_Eand so requires the measurement of both the angular Einstein radius θ_Eand the projected Einstein radius r{d5}_E. Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I_min=10.3), high-magnification (A_max=170) microlensing event. Pronounced finite-source effects permit a measurement of θ_E=650µas. Although the timescale of the event is only t_E=38days, one can still obtain weak constraints on the microlens parallax: 4.4AU<r{d5}_E<18AU at the 1 σ level. Together these two parameter measurements yield a range for the lens mass of 0.36M_☉<M<1.48M_☉. As was the case for MACHO-LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements.