Probing the M33 halo using rr lyrae stars

We present the discovery of RR Lyrae (RRL) variable stars in two fields of M33 to the southeast of its center using Gemini North observations. In the outermost field (45′ to the southeast of the M33 center; projected distance of 10.8kpc; deprojected distance of about 19kpc; about 5 V-band disk scale lengths) we detected two RRLs, consisting of one RRab star and one RRc star. An additional variable was found in this field that is a possible Cepheid. The mean g′-band magnitude for the two RRL stars is 25.60 0.04mag. In the other field (25′ to the southeast of the M33 center; projected distance of 5.6kpc; deprojected distance of about 10kpc; about 3 V-band disk scale lengths) we found 12 RRL stars, all of which are RRab stars. We also detected two candidate Cepheid variables in this field. The mean magnitude for the RRL stars in this field is 25.64 ± 0.14mag. We found a distance modulus of 〈(m - M)₀〉 = 24.69 ± 0.17mag from the RRab stars in the field 25′ from M33. The mean periods of the RRab stars in both fields (0.630 ± 0.002days, 45′ southeast; 0.628 ± 0.055days, 25′ southeast) are much longer than found for previously detected RRL stars in the inner regions of M33. This finding argues for a low metallicity for the RRab stars in the field 25′ from M33 ([Fe/H] =-1.84 0.30dex). Given these properties, we conclude that the RRLs we have detected belong to the halo of M33 and thus that there is an old component in the outer regions of M33.