Continuing Observations from the Columbia-VATT Microlensing Survey of M31

Details Continuing Observations from the Columbia-VATT Microlensing Survey of M31 Continuing Observations from the Columbia-VATT Microlensing Survey of M31

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...190.4301u&link_type=abstract

American Astronomical Society, 190th AAS Meeting, #43.01; Bulletin of the American Astronomical Society, Vol. 29, p.837

Astronomy and Astrophysics

Astrophysics

1

Scientific paper

A microlensing survey of M31 has the advantages of sensitivity to lensing by Machos in the halos of both the Galaxy and M31, with a high optical depth, tau_ {M31} up to 10(-5) (c.f. tau_ {Gal} of 10(-6) assuming all Macho spherical halos [Crotts 1992, ApJ, 399, L43]). In contrast to single sight-line surveys through the Galactic halo towards the LMC (MACHO, Alcock et al. 1997a ApJ in press [astroph/9606165]; EROS, Ansari et al., 1996, A&A, 314, 94) and the Galactic Bulge (OGLE, Udalski et al. 1994, Act. Ast., 44, 165; MACHO, Alcock et al. 1997b ApJ in press), M31 offers many sight lines in a small area of sky. Furthermore, due to the high inclination of M31 tau_ {M31} 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. Given these motivations we have been engaged in a survey since 1994 to search for microlensing in M31 on various timescales, primarily with the KPNO 4-m and the VATT 1.8-m telescopes. We use an image subtraction technique to isolate and photometer individual variables among M31's unresolved stars (Tomaney & Crotts 1996, AJ, 112, 2872). The survey has been successful at detecting many variables: in particular a 11'x11' far-side field in the bulge/inner disk has yielded over 2200 variables. From these we have reported six candidates that are consistent with microlensing from objects with minimum masses around 0.5 M_sun within the constraints of our data (Crotts & Tomaney 1996, ApJ, 473, 87). We present data taken in 1996 which extends these light curves and eliminates one candidate. We also discuss how Miras with periods around 240(d) could mimick these light curves given our present sampling.

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