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The ionizing cluster of 30 Doradus
III. Star-formation history and initial mass function *
Received 5 February 1999 / Accepted 6 May 1999
A new method is presented and used to determine the IMF of the starburst cluster NGC2070. A new correction, the magnitude-limit correction is introduced, and shown to be crucial when attempting to derive the IMF in the presence of variable reddening when the photometry is not several magnitudes deeper than the fainter stars analyzed. Failure to apply this correction is responsible for the drop at the low mass end of the IMF found in previous work on this cluster, despite the proper application of incompleteness corrections. For masses between and outside 15" the IMF of NGC2070 is shown to be consistent with being a single power law with a Salpeter exponent. In the central region () within our data combined with HST observations yield a slope flatter than Salpeter at the 2-3 level. Furthermore, it is shown that the number of stars near the core (Massey & Hunter 1998a, 1998b) is incompatible with the intermediate mass counts of Hunter et al. (1995, 1996) extrapolated with a Salpeter slope, so either the slope is flatter than Salpeter, or the HST spectral types are biased towards earlier types. The star-formation history is dominated by three bursts of increasing strength occurring 5My, 2.5My, and 1.5My ago, the latest one responsible for most of the star-formation within 6 pc from the cluster center. A spherically symmetric structure is detected at about 6 pc from the cluster center which contains predominantly massive stars and has a flatter IMF. The surface number density profile of the cluster is shown to be well modeled by a single power law, , over , with , significantly steeper than isothermal.
Key words: stars: early-type stars: Hertzsprung Russel (HR) and C-M diagrams stars: luminosity function, mass function ISM: dust, extinction Galaxy: open clusters and associations: general galaxies: Magellanic Clouds
Send offprint requests to: fselman@ESO.org
Online publication: June 30, 1999