Mass segregation in young compact star clusters in the Large Magellanic Cloud - II. Mass functions

Abstract

We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young Large Magellanic Cloud (LMC) star clusters NGC 1805 and 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii of r≲20–30 arcsec, for masses in excess of ∼1.6–2.5 M⊙. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius, are not simple power laws. The global and the annular MFs near the core radii appear to be characterized by similar slopes in the mass range (-0.15log m/M⊙0.85), and the MFs beyond r≳30 arcsec have significantly steeper slopes. We estimate that while the NGC 1818 cluster core is between ∼5 and ∼30 crossing times old, the core of NGC 1805 is likely to be ≲3–4 crossing times old. However, since strong mass segregation is observed out to ∼6Rcore and ∼3Rcore in NGC 1805 and 1818, respectively, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805.