AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 90. Cosmology and Dark Matter
Display, Thursday, June 3, 1999, 9:20am-4:00pm, Southwest Exhibit Hall

## [90.04] The Columbia/VATT Microlensing Survey of M31: Preliminary results from 5 seasons of observations

R. Uglesich, A.P.S. Crotts (Columbia University), A.B. Tomaney (University of Washington), G. Gyuk (University of California, San Diego)

The Columbia/VATT Microlensing Survey has been monitoring fields in the outer bulge and inner disk of M31 in a search for microlensing activity. The survey has the advantages of sensitivity to lensing by Machos in the halos of both the Galaxy and M31, with a high optical depth, \tauM31 up to 10-5. In contrast to single sight-line surveys through the Galactic halo towards the LMC (MACHO, EROS) and the Galactic Bulge (OGLE), M31 offers many sight lines in a small area of sky. Furthermore, due to the high inclination of M31, \tauM31 is strongly modulated from the far to near side of the galaxy, implying that a measure of \tau across the face of the galaxy could constrain the shape of its halo.

We present the results from 5 seasons (1994-1998) of data obtained with the 1.8-m VATT, MDM 1.3-m McGraw-Hill and KPNO 4-m telescopes. Variability is detected and monitored in two broad bandpasses using the technique of difference image photometry'' (Tomaney & Crotts 1996, AJ, 112, 2872). Previously, we reported 6 events from the 1995 season consistent with microlensing due to masses of about 1 M\sun (Crotts & Tomaney 1996, ApJ, 473, L87). Additional baseline coverage from data obtained in subsequent seasons, as well as HST snapshot images presented here, confirm that three of these candidates are inconsistent with being long-period variable stars. New candidate microlensing events discovered during the 1996-1998 seasons are discussed.

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