星团质量分层的探索途径

Study on Description Methods of Mass Segregation of Star Clusters

星团质量分层是指质量较大的恒星趋于向团中心集聚,而质量较小的恒星更多地分布在星团的外围部分。研究质量分层对理解星团的动力学演化规律以及约束大质量恒星的形成条件有重要意义。质量分层的探索无统一方法,往往因作者而异,但大体上可分为两条途径:即按不同天区探究成员星的质量分布,或者按不同质量范围探究成员星的空间分布。对常用的具体方法做了简单介绍和比较式讨论,并探讨了它们的适用范围。

Mass segregation means that the higher-mass stars are preferably found in the center of a cluster, whereas the lower-mass stars in the outer region. Studying mass segregation can help us to understand the dynamical evolution of clusters and constrain initial conditions of massive stars. There are a few different methods of probing mass segregation （or luminosity segregation when the masses of member stars are not available）. These methods can be classified into two categories. One is to investigate the difference between stellar mass/luminosity functions （MF/LF） of two or more subregions of a cluster. In order to determine whether these functions are different or not, besides direct comparison, one may make use of some characteristic quantities, e.g. the ratio of mean stellar masses of two different subregions （（M）＇）. The other is to investigate the difference between the stellar number density profiles （no-r） of two or more sets of stars within different mass ranges. In order to determine whether these profiles are different or not, besides direct comparison, one may make use of some characteristic quantities, e.g. the ratio of half-number radii, lengths of the minimum spanning tree （A）, and mean stellar mutual distances （R）. Some tips of these methods are summarized as follows： 1） Since massive stars are usually scarce and MF is a piecewise defined function, describing mass segregation by the slope of MF should be careful. 2） （M）＇ may be biased by the presence of a few massive stars. 3） The no-r of one cluster may be not universal. 4） The ratio of half-number radii depends on the cluster center, while it is not the case for A and R. There is no predominant method to describe mass segregation. In simulation, authors often study the evolution of mass segregation as a function of time, so （M）＇ and R are preferred. In observation, authors often analyze only one cluster, so LF and no-r seem to be more appropriate. In addition, if the velocities of member stars are available, one may consider the correlation between velocities and stellar masses as well.