Due to distance and crowding, the incidence and population statistics of short-period variables in the inner parts of M31 are extremely poorly-determined. Specifically, 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(') (9 kpc) from the nucleus] using the CFHT 3.6-m. 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 in M31. Observations (including the VATT 1.8-m and INT 2.5-m) were made between Oct. 15 and Oct. 21, 1996 on a 10(') x 10(') far-side field 1.(') 5 (0.3 to 2.5 kpc) from the nucleus in the bulge/inner disk. Our current study is restricted to INT data. Identification of RR Lyrae is nontrivial since they lie deeply embedded within the unresolved bulge/disk. To achieve the required high S/N to identify candidate varirables, we median-combine differenced data by folding frames along a given test period at evenly-spaced phases. Since the constant background has been removed, any high-sigma residual is a potential variable-source. We compare the resulting light-curves against custom templates for a variety of short-period variables (including RRab, RRc and eclipsing binaries) to identify true variables and ignore sporadic signal. Adopting the metallicity and specific-incidence of RR Lyrae in M31 from Pritchet & van den Bergh, KPNO 4-m observations of M31 indicate that we expect to find ≲ 0.9 RR Lyrae per square arcsec. This corresponds to ≲ 3 x 10(5) 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. Preliminary estimates from findings on a small subsample of the full dataset indicate that there are gtrsim 2 x 10(5) variables with periods between 3 and 24 hours. Statistics for the full dataset, as well as completeness estimates will be presented.
American Astronomical Society Meeting Abstracts #194
- Pub Date:
- May 1999