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Thursday 9 November at 3pm
Ken Mighell - NOAO/KPNO
"HST Observations of the Stellar Populations Near
the Nucleus of M33"
ABSTRACT: "I will show how HST WFPC2 observations of the stellar populations
near the nucleus of M33 have improved our understanding of the complex
star formation history of the center of this nearby spiral galaxy. The
WFPC2 color-magnitude diagrams 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 about I = 24 mag and a signal-to-noise ratio SNR >= 10 in the F555W
(~V) and F814W (~I) 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 WFPC2 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. Many of our brightest V-band dropouts are clearly
seen on Davidge's 1200-s K-band image of the nuclear region of M33. 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)."
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