2002/08/31 by B. Scott Gaudi, D. Graff, David S. Graff +1
Physics and Astronomy · #Adaptive optics and wavefront sensing #Astronomy and Astrophysical Research #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/367636
published as Astrophys.J. 586 (2003) 451-463 · 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)
arxiv created 2002/11/27 · openalex publication_date 2003/03/20 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We outline a method by which the angular radii of giant and main-sequence stars located in the Galactic bulge can be measured to a few percent accuracy. The method combines comprehensive ground-based photometry of caustic-crossing bulge microlensing events, with a handful of precise (~10 μas) astrometric measurements of the lensed star during the event, to measure the angular radius of the source, θ * . Dense photometric coverage of one caustic crossing yields the crossing timescale Δ t . Less frequent coverage of the entire event yields the Einstein timescale t E and the angle ϕ of source trajectory with respect to the caustic. The photometric light-curve solution predicts the motion of the source centroid up to an orientation on the sky and overall scale. A few precise astrometric measurements therefore yield θ E , the angular Einstein ring radius. Then the angular radius of the source is obtained by θ * = θ E (Δ t / t E ) sin ϕ. We argue that the parameters t E , Δ t , ϕ, and θ E , and therefore θ * , should all be measurable to a few percent accuracy for Galactic bulge giant stars using ground-based photometry from a network of small (1 m class) telescopes, combined with astrometric observations with a precision of ~10 μas to measure θ E . We find that a factor of ~50 times fewer photons are required to measure θ E to a given precision for binary lens events than for single-lens events. Adopting parameters appropriate to the Space Interferometry Mission ( SIM ), we find that ~7 minutes of SIM time is required to measure θ E to ~5% accuracy for giant sources in the bulge. For main-sequence sources, θ E can be measured to ~15% accuracy in ~1.4 hr. Thus, with access to a network of 1 m class telescopes, combined with 10 hr of SIM time, it should be possible to measure θ * to 5% for ~80 giant stars, or to 15% for roughly seven main-sequence stars. We also discuss methods by which the distances and spectral types of the source stars can be measured. A by-product of such a campaign is a significant sample of precise binary lens mass measurements.