Probing the M33 Halo Using RR Lyrae Stars

Details Probing the M33 Halo Using RR Lyrae Stars Probing the M33 Halo Using RR Lyrae Stars

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aj....142..198p&link_type=abstract

The Astronomical Journal, Volume 142, Issue 6, article id. 198 (2011).

Astronomy and Astrophysics

Astronomy

Galaxies: Individual: M33, Local Group, Stars: Variables: General, Stars: Variables: Rr Lyrae

Scientific paper

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 10.8 kpc deprojected distance of about 19 kpc 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 ± 0.04 mag. In the other field (25' to the southeast of the M33 center; projected distance of 5.6 kpc deprojected distance of about 10 kpc 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 ± 0.14 mag. We found a distance modulus of lang(m - M)0rang = 24.69 ± 0.17 mag from the RRab stars in the field 25' from M33. The mean periods of the RRab stars in both fields (0.630 ± 0.002 days, 45' southeast; 0.628 ± 0.055 days, 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 ± 0.30 dex). 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.
Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).

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