OGLE-2003-BLG-238: Microlensing Mass Estimate of an Isolated Star

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 = (c 2/4G)r̃ Eθ E and so requires the measurement of both the angular Einstein radius θ E and the projected Einstein radius r̃ 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 = 650 μas. Although the timescale of the event is only t E = 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < r̃ E < 18 AU at the 1 σ level. Together these two parameter measurements yield a range for the lens mass of 0.36 M ⊙ < M < 1.48 M ⊙. 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.