疏散星团的质量分层

Mass Segregation of Open Clusters

对疏散星团质量分层的有关问题做了简要的评述,包括空间质量分层和速度质量分层的表现形式和探测途径,质量分层形成机制的研究现状.最后概要介绍了2MASS测光资料对探讨疏散星团质量分层效应的作用.

The mass segregation effects of open clusters, including spatial mass segregation and velocity mass segregation, are of great importance for studies of formation and evolution of clusters. The mass segregation properties are tightly related not only to the evolution of member stars within clusters and its influence on cluster＇s global evolution, but also to the gravitational field of the Galaxy and the interstellar clouds where clusters form. Observations show that, in an open cluster, the spatial distribution and radial number density profiles for high-mass member stars and low-mass members are different. High-mass stars are more concentrated in the core of the cluster while low-mass ones being transferred to the halo. In order to examine spatial mass segregation effect of a cluster, a number of different approaches have been developed, such as detecting the variation of the mass function slopes in different spatial regions of the cluster, and analyzing the surface number density profiles or the half-mass radii of member stars with different masses. Current data show that the spatial mass segregations have been found in many open clusters, both old and young. On the other side, detecting velocity mass segregation of clusters is not as easy as spatial mass segregation, for which the only observational data can be used are proper motions with high accuracies of the cluster members. The observational facts to reflect velocity mass segregation of an open cluster are the intrinsic dispersions in proper motion components of high-mass members should be smaller than those of low-mass ones. Indeed, velocity mass segregation effects have been detected for some open clusters, while the dependence of the intrinsic dispersions in proper motion components on the masses of stars expected from energy equipartition is not found for these clusters. Concerning the explanations of mass segregation of open clusters, two possible mechanisms have been suggested. Due to dynamical evolution of clusters, high-mass stars will lose part of their kinematical energy and tend to move toward the center of the clusters, and low-mass stars will get some kinematical energy from high-mass ones because of two-body relaxation and move to the periphery of the clusters. The other point of view is the so-called primordial mass segregation, which means initial formation processes of clusters can also leads to radial profiles of stars with different masses reflecting mass segregation effect. Therefore, the present observational effects of the mass segregation of open clusters could be from both dynamical evolution and initial formation processes of clusters. Finally, the importance of the 2MASS photometry on analyzing the mass segregation of open clusters is also described briefly.