The POINT-AGAPE Survey - I. The variable stars in M31

For the purposes of identifying microlensing events, the POINT-AGAPE collaboration has been monitoring the Andromeda galaxy (M31) for three seasons (1999-2001) with the Wide Field Camera on the Isaac Newton Telescope. In each season, data are taken for one hour per night for roughly sixty nights during the six months that M31 is visible. The two 33 ′ ×33 ′ fields of view straddle the central bulge, northwards and sou thwards. We have calculated the locations, periods and brightness of 35414 variable stars i n M31 as a by-product of the microlensing search. The variables are classified according t o their period and brightness. Rough correspondences with classical types of variable star (suc h as population I and II Cepheids, Miras and semi-regular long-period variables) are established. The spatial distribution of population I Cepheids is clearly associated with the spiral arm s, while the central concentration of the Miras and long-period variables varies noticeably, t he brighter and the shorter period Miras being much more centrally concentrated. A crucial role in the microlensing experiment is played by the asymmetry signal ‐ the excess of events expected in the southern or more distant fiel ds as measured against those in the northern or nearer fields. It was initially assumed that t he variable star populations in M31 would be symmetric with respect to the major axis, and thus variable stars would not be a serious contaminant for measuring the microlensing asymmetry signal. We demonstrate that this assumption is not correct. All the variable star distributi ons are asymmetric primarily because of the effects of differential extinction associated with the dust lanes. The siz e and direction of the asymmetry of the variable stars is measured as a function of period and brightness. The implications of this discovery for the successful completi on of the microlensing experiments towards M31 are discussed.