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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.
The author(s) of this abstract have provided an email address for comments about the abstract: ben@astro.columbia.edu