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Astron. Astrophys. 347, 532-549 (1999)

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The ionizing cluster of 30 Doradus

III. Star-formation history and initial mass function *

F. Selman 1, J. Melnick 1, G. Bosch 2 and R. Terlevich 2

1 European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

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 [FORMULA] 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 ([FORMULA]) within [FORMULA] our data combined with HST observations yield a slope flatter than Salpeter at the 2-3[FORMULA] level. Furthermore, it is shown that the number of [FORMULA] 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 [FORMULA]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, [FORMULA], over [FORMULA], with [FORMULA], 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

* Based on observations collected with the NTT ESO telescope.

Send offprint requests to: fselman@ESO.org

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999