AAS Meeting #193 - Austin, Texas, January 1999
Session 46. Photometric Observations of Variable Stars
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1

## [46.11] Observations of RR Lyrae in M31 using Difference Imaging Photometry

B. E. Sugerman, R. R. Uglesich, A. P. S. Crotts (Columbia U.)

The incidence and population statistics of RR Lyrae in the inner parts of M31 are extremely poorly-determined. At a distance modulus 24.3, RR Lyrae have an R~25 superposed on a surface brightness between 18 and 20 mag arcsec-2 at the inner disk, making them extremely enigmatic to detect. Pritchet & van den Bergh (1987, ApJ, 316, 517) found only 30 RR Lyrae candidates in the halo [40\prime (9 kpc) from the nucleus] using the CFHT 3.6-m. With \langle B \rangle = 25.68, this implies a specific incidence of ~100 for each B=15.9 (or R ~ 14.5).

Using the Difference Image Photometry'' (Tomaney & Crotts, 1996, AJ, 112, 2872) technique to isolate variable sources within unresolved fields, we seek to better determine the population of RR Lyrae within M31. Observations globally-distributed in longitude (including the VATT 1.8-m, INT 2.5-m and Wise 1-m) were made between Oct. 15 to Oct. 21, 1996 on a 10\prime \times 10\prime far-side field 1.\prime5 (0.3 to 2.5 kpc) from the nucleus in the bulge/inner disk. Our study incorporates INT data, together with concurrent data from the ongoing Columbia-VATT M31 microlensing survey (Crotts & Tomaney 1996, ApJ, 473, L87). Identification of RR Lyrae is nontrivial since they lie deeply embedded within the unresolved bulge/disk. We median-combine differenced images using a temporal-filter in phase and period to identify variables and their tentative periods.

Adopting the above halo specific-incidence and a halo metallicity, KPNO 4-m observations of M31 indicate that we expect to find \lesssim 0.9 RR Lyrae per square arcsec. This corresponds to \lesssim 3 \times 105 RR Lyrae in our field. Since lower incidence indicates higher metallicity, this study is also a good diagnostic for metallicity, which is otherwise difficult to determine due to crowding. We also search for additional short-period populations of variables, such as eclipsing binaries and \beta Cepheids.