Carbon Stars and Other Luminous Stellar Populations in M33

The M33 galaxy is a nearby, relatively metal-poor, late-type spiral. Itsproximity and almost face-on inclination means that it projects over a largearea on the sky, making it an ideal candidate for wide-field CCD mosaicimaging. Photometry was obtained for more than 10^6 stars covering a 74' x 56'field centered on M33. Main sequence (MS), supergiant branch (SGB), red giantbranch (RGB) and asymptotic giant branch (AGB) populations are identified andclassified based on broad-band V and I photometry. Narrow-band filters are usedto measure spectral features allowing the AGB population to be further dividedinto C and M-star types. The galactic structure of M33 is examined using starcounts, colour-colour and colour-magnitude selected stellar populations. We usethe C to M-star ratio to investigate the metallicity gradient in the disk ofM33. The C/M-star ratio is found to increase and then flatten with increasinggalactocentric radius in agreement with viscous disk formation models. TheC-star luminosity function is found to be similar to M31 and the SMC,suggesting that C-stars should be useful distance indicators. The ``spectaculararcs of carbon stars'' in M33 postulated recently by Block et al. (2004) arefound in our work to be simply an extension of M33's disk.Comment: 20 pages, 20 figures. Accepted for publication in The Astronomical Journa