The initial mass function of the rich young cluster NGC 1818 in the Large Magellanic Cloud

Abstract

We use deep Hubble Space Telescope photometry of the rich, young (~20- to 45-Myr old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its stellar mass function (MF) down to ~0.15M_solar. This represents the deepest robust MF thus far obtained for a stellar system in an extragalactic, low-metallicity ([Fe/H] ~= -0.4dex) environment. Combining our results with the published MF for masses above 1.0M_solar, we obtain a complete present-day MF. This is a good representation of the cluster's initial MF (IMF), particularly at low masses, because our observations are centred on the cluster's uncrowded half-mass radius. Therefore, stellar and dynamical evolution of the cluster will not have affected the low-mass stars significantly. The NGC 1818 IMF is well described by both a lognormal and a broken power-law distribution with slopes of Γ = 0.46 +/- 0.10 and Γ ~= -1.35 (Salpeter-like) for masses in the range from 0.15 to 0.8M_solar and greater than 0.8M_solar, respectively. Within the uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa solar neighbourhood and the Chabrier lognormal mass distributions.