Observations of RR Lyrae in M31 using Difference Imaging Photometry

Details Observations of RR Lyrae in M31 using Difference Imaging Photometry Observations of RR Lyrae in M31 using Difference Imaging Photometry

May 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...194.8201s&link_type=abstract

American Astronomical Society, 194th AAS Meeting, #82.01; Bulletin of the American Astronomical Society, Vol. 31, p.967

Statistics

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Scientific paper

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.

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