疏散星团M67中的红团簇巨星
摘要
2MASS (the two micron all sky survey) near-infrared data and UCAC4 (the fourth U.S. naval observatory CCD astrograph catalog) data have been used to analyze the old open cluster M67. Seven red clump giants are found in the color-magnitude diagram, and all can be identified as membership by their accurate radial velocities and metallcities. Using the seven red clump giants, the absolute distance modulus of M67 is determined to be 9.48±0.04 mag (787±15 pc), which is significantly lower than the values derived based on main-sequence fitting. Reasons for this discrepancy are discussed. In addition, a systematic analysis of positions and proper motions of the red clump giants reveals that one is escaping from M67 at a speed of 18±10 km/s. One possible explanation is that internal dynamical relaxation of M67 led to mass segregation; energy equipartition has brought to a redistribution of the stars within the cluster; massive stars would preferentially sink towards the center of the cluster; while low mass stars populate the cluster halo. Low mass stars with higher velocities would evaporate from the cluster halo.