Carbon Stars and Other Luminous Stellar Populations in M33

The M33 galaxy is a nearby, relatively metal-poor, late-type spiral. Its proximity and almost face-on inclination means that it projects over a large area on the sky, making it an ideal candidate for wide-field CCD mosaic imaging. Photometry was obtained for more than 106 stars covering a 74′ × 56′ field centered on M33. Main-sequence, supergiant branch, red giant branch, and asymptotic giant branch (AGB) populations are identified and classified based on broadband V and I photometry. Narrowband filters are used to measure spectral features allowing, the AGB population to be further divided into C and M star types. The galactic structure of M33 is examined using star counts, color-color–, and color-magnitude–selected stellar populations. We use the C to M star ratio to investigate the metallicity gradient in the disk of M33. The C/M star ratio is found to increase and then flatten with increasing galactocentric radius, in agreement with viscous disk formation models. The C star luminosity function is found to be similar to M31 and the SMC, suggesting that C stars should be useful distance indicators. The "spectacular arcs of carbon stars" in M33 postulated recently by Block et al. are found in our work to be simply an extension of M33's disk.