Angular Radii of Stars via Microlensing

We outline a method by which the angular radii of giant and main sequencestars in the Galactic bulge can be measured to a few percent accuracy. Themethod combines ground-based photometry of caustic-crossing bulge microlensingevents, with a handful of precise astrometric measurements of the lensed starduring the event, to measure the angular radius of the source, theta_*. Densephotometric coverage of one caustic crossing yields the crossing timescale dt.Less frequent coverage of the entire event yields the Einstein timescale t_Eand the angle phi of source trajectory with respect to the caustic. Thephotometric light curve solution predicts the motion of the source centroid upto an orientation on the sky and overall scale. A few precise astrometricmeasurements therefore yield theta_E, the angular Einstein ring radius. Thenthe angular radius of the source is obtained by theta_*=theta_E(dt/t_E)sin(phi). We argue that theta_* should be measurable to a few percent accuracyfor Galactic bulge giant stars using ground-based photometry from a network ofsmall (1m-class) telescopes, combined with astrometric observations with aprecision of ~10 microarcsec to measure theta_E. We find that a factor of ~50times fewer photons are required to measure theta_E to a given precision forbinary-lens events than single-lens events. Adopting parameters appropriate tothe Space Interferometry Mission (SIM), ~7 min of SIM time is required tomeasure theta_E to ~5% accuracy for giant sources in the bulge. Formain-sequence sources, theta_E can be measured to ~15% accuracy in ~1.4 hours.With 10 hrs of SIM time, it should be possible to measure theta_* to ~5% for\~80 giant stars, or to 15% for ~7 main sequence stars. A byproduct of such acampaign is a significant sample of precise binary-lens mass measurements.Comment: 13 pages, 3 figures. Revised version, minor changes, required SIM integration times revised upward by ~60%. Accepted to ApJ, to appear in the March 20, 2003 issue (v586