Mass segregation of compact star clusters and its origins.

The phenomenon of mass segregation of a star cluster means a preferential concentra- tion of high mass stars towards the cluster center and a preferential allocation of lower mass stars towards the periphery of the cluster,which seems to occur in star systems with widely distinct physical properties.As a kind of compact star systems,globular clusters are very old ones with ages of older than 10~(10)years,which are different from another kind of young compact clusters, super star clusters(SSC)or massive young clusters(MYC),with ages ranging from a few 10~6 to 10~8 years.It is interesting that mass segregation of member stars has been observed for these two kinds of compact clusters. An important observational effect of mass segregation of a star cluster is expected that the slopeαof the PDMF(present day mass function)of the cluster is dependent on radial distance r from the cluster center,in the senseαbecoming steeper with r increasing.This radial variation of the mass function reveals mass segregation for both old globular clusters and young SSC or MYC.On the other hand,it is found that stars of different masses in a cluster show the pro- nounced,systematic variations of the core radius,which is another observational effect of mass segregation.Mass segregation mainly affects the innermost region of clusters,while dynamical interaction with the Galaxy acts predominantly their outermost regions inducing stellar evapo- ration and stripping. Further evidence of mass segregation in a globular cluster comes from the detection of color gradients whereby the color indices such as(U-B)or(B-V)get bluer towards the cluster center.A possible explanation for such color gradients is mass segregation of main sequence (MS)stars,since stars near the MS turnoff,with higher and bluer color,are expected to be more centrally concentrated than the fainter,redder,and lower mass stars.A core concentration of blue stars is likely due to binary-single star interaction or the central lack of low mass MS stars. Mass segregation cannot be observed directly via star counts for young compact clusters in which individual stars are unresolved.However,a mass-segregated cluster should appear smaller in the UV and IR,where the radiation is dominated by red supergiants,than in the optical, where the radiation is dominated by intermediate mass MS stars.The negative correlation be- tween cluster size and observed wavelength suggests that the massive red evolved stars are more centrally concentrated than the intermediate mass MS stars.The other effect of mass segregation for SSC is that core radii of stars with high mass stars and with lower mass ones are different in the sense that core radii decrease with star's mass becoming higher. There are two possible and distinct origins for the effect of mass segregation:dynamical and primordial.The dynamical mass segregation is caused by the dynamical evolution of the cluster led by two-body relaxation,where the cluster member stars trend to reach the kinetic energy equipartition due to stellar encounters,in the course of which the high mass stars decrease their velocities,sinking towards the cluster center,and meanwhile the low mass ones speed up and spread out to the periphery of the cluster. It is indicated from numerical stimulation that dynamical evolution from initially unsegre- gated systems cannot account for the degree of mass segregation observed in some young clusters. Therefore,the obvious explanation is that these clusters were born mass segregated.Recent ob- servational and theoretical studies do indeed suggest that high mass stars form preferentially in the centers of star-forming regions,and the mechanism invoked to explain this primordial mass segregation relies mainly on the higher accretion rate for stars in the centers of young clusters.