UBV Stellar Photometry of the 30 Doradus Region of the Large Magellanic Cloud With the Hubble Space Telescope

We report on Planetary Camera observations of the central region of 30 Doradus in the Large Magellanic Cloud. These images of 30 Doradus are the first "deep" HST exposures that have appropriate photometric calibration. The B band (F439W) image, which shows RI 36a at the center of the PC6 CCD chip, reveals over 200 stars within 3" of the center of R136a, and over 800 stars in a 35" X 35" area. We used Malumuth eta/.'s (The First Year of HST Observations, edited by A. L. Kinney and J. C. Blades (ST ScI, Baltimore) (1991)] PSF-fitting method to measure the magnitudes of all stars on the PC6 chip. These new B magnitudes, along with U and V magnitudes from archival PC images, yield a luminosity function, mass density profile, and initial mass function of the 30 Doradus ionizing cluster. The mass distribution is well fit by a King model with a core radius, R_c_ = 0.96" (0.24 pc), a tidal radius, R_t_ = 110" (28 pc), and a total mass, M = 16 800 M_sun_. Both the luminosity function and initial mass function show evidence for mass segregation, in the sense that the central region has a higher fraction of massive stars than the outer regions. This is the first observational evidence for mass segregation in a very young cluster (age 3 million years). The observations admit the hypothesis that the mass segregation occurred in the process of star formation and/or that the mass segregation is the result of dynamical evolution.