The Low-Mass Initial Mass Function in the 30 Doradus Starburst Cluster
Abstract
We present deep Hubble Space Telescope NICMOS 2 F160W band observations of the central 56'' × 57'' (14 pc × 14.25 pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar initial mass function (IMF) down to ~1 M sun in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayear. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 M sun and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 M sun for 3 Myr old objects. We find an IMF of {dN}/{d log M }∝ M^{-1.20± 0.2} over the mass range 1.1-20 M sun only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 M sun at a distance of 5 pc from the center, in contrast to a flattening at 2 M sun at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results including the possible presence of mass segregation and the effects of differential extinction, particularly for the pre-main sequence sources. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2009
- DOI:
- 10.1088/0004-637X/707/2/1347
- arXiv:
- arXiv:0911.2755
- Bibcode:
- 2009ApJ...707.1347A
- Keywords:
-
- globular clusters: individual: 30 Doradus;
- stars: formation;
- stars: luminosity function;
- mass function;
- stars: pre-main sequence;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted in ApJ. Abstract abridged