The young populous clusters in the Magellanic Clouds are attractive targets for studies of the initial mass function (IMF) of stars; they have a large range of main-sequence stellar masses, they are dynamically unevolved and have large numbers of stars.
After having found mass segregation in NGC 2157 (Fischer et al. 1998), we now present results for two other young LMC clusters, NGC 2004 and NGC 2031 (Richtler et al. 1998). We find clear evidence for mass segregation in NGC 2004, which we interpret as a signature of the star formation process. NGC 2031 also shows signs for mass segregation, but the evidence is only marginal.
The young cluster NGC 2004 (age 1.5·107 years) clearly shows mass segregation in the sense that the mass function slope becomes steeper at large radius (see Fig. 1). For the mass interval 4.27 Msun>m>1.3 Msun the mass function slope decreases from α = -1.5±0.1 at small radius to α = -2.2±0.15 at large radius. The mass function appears to peak at 1.1 Msun and strongly declines for lower masses. This feature has not been seen in any other cluster mass spectrum that we are aware of (except perhaps in NGC 2031).
The older cluster NGC 2031 (age 1.4·108 years) shows less pronounced mass segregation, which is hardly significant given our uncertainties (see Fig. 2). A similar peak, but again less pronounced, can also be seen at 1.1 Msun. It's mass spectrum in the outer region (α = -2.4±0.15 in the mass interval 3.7 Msun>m>1.2 Msun) agrees well with that of NGC 2004 and of other young clusters (e.g. Sagar & Richtler 1991; Hunter et al. 1997; Phelps & Janes 1993), supporting the idea of a universal mass spectrum at least for the mass interval under consideration. The mass spectrum in the inner region is only marginally flatter (α = -2.1±0.17).
This type of mass segregation does not appear to be just a peculiar feature of rich and dense star clusters like NGC 2004. Earlier photographic work on galactic open clusters already indicated a similar behaviour of the mass spectrum (see the references in Scalo 1986).
For instance, a scenario in which the stellar mass is determined by the duration of matter infall onto a protostellar core becomes improbable, since interactions between fragments are expected to terminate infall, and accordingly, massive stars are preferentially formed in the outer cluster regions.
On the other hand, a steepening of the mass spectrum with radius in the sense which we observed is predicted by theories in which encounters between protostars enhance the stellar mass. In the picture of Murray & Lin (1991, 1996), the protocluster consists of pressure-supported cloudlets, which undergo dissipative mergers to build up protostars. The formation of massive stars then preferably takes place in high-density environments, i.e. the center of the cluster.
Seeking the reason for mass segregation in such a general mechanism leads to the question, why mass segregation in NGC 2031 is less pronounced than in NGC 2004. One is tempted to speculate that the density profile and elliptical shape of this cluster make it a candidate for a merged binary cluster. In that case one would expect any mass segregation to be wiped out.
Regarding the universality of the IMF, Scalo (1998) demonstrates that a universal mass function is not well justified empirically for stars with masses less than 1 Msun. Guided by the existence of mass segregation in combination with peculiar features like that in NGC 2004, one may think of interaction processes to be dominantly responsible for the shape of the IMF at low masses. However, the high mass end of an IMF formed in low density environment might still be universal. In support of this, at large radii the mass spectrum in the clusters studied here are consistent and also are consistent with the average slope of galactic open clusters.
| First version: | 09th | September, | 1998 |
| Last update: | 04th | October, | 1998 |