WFPC2 Observations of the Stellar Populations Near the Nucleus of M33

We have studied the stellar populations near the nucleus of the nearby spiral galaxy M33 using {Hubble Space Telescope} Wide Field Planetary Camera 2 (WFPC2) observations in the F555W ( ~V) and F814W ( ~I) filters. The color-magnitude diagrams (CMDs) reveal a young stellar population indicated by a bright blue main-sequence (M_{V≳ -6.2} mag), a red supergiant plume (M_{I≳ -6.3} mag), very red [ (V-I)>3.5 mag] asymptotic giant branch (AGB) stars, and a wide red giant branch (RGB) with a limiting magnitude of I≈24 mag and a signal-to-noise ratio SNR >= 10 in both filters. Davidge (2000, AJ, 119, 748) recently reported on near-infrared adaptive-optics observations of the nuclear region of M33 in the J, H, and K bands obtained with the KIR imager and the Canada-France-Hawaii Telescope (CFHT) AO system. Comparing our optical space-based photometry with Davidge's near-infrared ground-based photometry, we have identified 51 of Davidge's stars on our {HST} images. While most of the infrared-bright stars seen near the nucleus of M33 are old AGB stars, some of these stars are young red supergiants (RSGs); we find that the upper ~0.9 mag tip of Davidge's near-infrared CMDs of the circumnuclear region of M33 samples a mixed stellar population of old AGB stars and young RSGs. Many of our brightest V-band dropouts are clearly seen on Davidge's 1200-s K-band image of the nuclear region of M33. We demonstrate that the intermediate-age infrared-bright AGB stars are more centrally concentrated than the ancient low-mass RGB stars; we associate this stellar population with the M33 bulge identified in the H band by Minniti, Olszewski, & Rieke (1993, ApJ, 410, L79). This work is carried out through the NOAO/KPNO REU Program, supported by the National Science Foundation. Support was also provided by a grant from the National Aeronautics and Space Administration (NASA), Order No. S-67046-F, which was awarded by the Long-Term Space Astrophysics Program (NRA 95-OSS-16).