The ionizing cluster of 30 Doradus - III. Star-formation history and initial mass function

A new method is presented and used to determine the IMF of the starburst cl uster NGC2070. A new correction, the magnitude-limit correction is introduc ed, and shown to be crucial when attempting to derive the IMF in the presen ce of variable reddening when the photometry is not several magnitudes deep er than the fainter stars analyzed. Failure to apply this correction is res ponsible for the drop at the low mass end of the IMF found in previous work on this cluster, despite the proper application of incompleteness correcti ons. For masses between 3M(.) less than or similar to M less than or simila r to 120M(.) and outside 15" the IMF of NGC2070 is shown to be consistent w ith being a single power law with a Salpeter exponent. In the central regio n (4.6" < r < 19.2") within 2.8M(.) < M < 120M(.) our data combined with HS T observations yield a slope flatter than Salpeter at the 2-3 sigma level. Furthermore, it is shown that the number of M > 50M(.) stars near the core (Massey & Hunter 1998a, 1998b) is incompatible with the intermediate mass c ounts 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 bi ased towards earlier types. The star-formation history is dominated by thre e bursts of increasing strength occurring 5My, 2.5My, and less than or simi lar to 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, Sigma(R) sim ilar or equal to R-alpha, over 0.4 pc < R < 12 pc, with alpha approximate t o 1.85, significantly steeper than isothermal.