成員星的挑選是決定星團特質的關鍵第一步，特別是低質量成員星可以用來回溯星團動力學歷史，如：質量分層、恆星逸散與潮汐拉扯，在其所在的星系環境中。我們已經發展出一個方法，藉由光度與動力學的資料來確認星團成員星。這裡，我們展示對於鄰近的(170 pc)鬼宿星團的分析，從2微米巡天觀測 (2MASS) 的光度資料與PPMXL的動力學資料中，發現有758顆候選成員星，其最低質量到0.2太陽質量；利用泛星計畫 (Pan-STARRS1) 的光度資料，甚至可以找到更低質量(< 0.1太陽質量)的成員星，亮度在g波段可以達到22.8等。以往對於鬼宿星團成員星的研究多仰賴亮星的光度與動力學資料，而大多數的暗星只有光度資料。我們是第一個挑選成員星時，同時包含光度與動力學資料，並將候選星的最低質量推至恆星質量的底限；部分在文獻中發現的候選成員星，其動力學上與鬼宿星團的表現不一致，應該被排除在成員星名單外；質量分層在這個中年齡的星團中清楚地被證實。

Membership identi cation is the critical rst step to determine the properties of a star cluster. Low-mass members in particular could be used to trace the dynamical history | mass segregation, stellar evaporation, tidal stripping of a star cluster in its Galactic environment. We have initiated a program to identify members in star clusters by photometric and kinematic data sets. Here we show our analysis of the nearby (distance of 170 pc) Praesepe cluster (M 44), by using the 2MASS photometry and the PPMXL proper motions, from which a total of 758 member candidates have been identi ed down to a mass limit of 0.2 M . Even lower mass members were found by using Pan-STARRS1 photometry reaching as faint as gP1 = 22.8 mag, corresponding to 0.1 M . Previous studies of Praesepe member candidates relied on photometry and proper motions of bright stars, and mostly photometry only of faint stars. Our work is the first to select membership down to almost the stellar mass end by both photometric and kinematic criteria. Some member candidates found in previous studies have proper motions inconsistent with that of M44, so should be excluded from membership. Mass segregation is clearly evidenced in this mid-aged (900 Myr) cluster.

1. Abazajian, K. N., et al. 2009, ApJS, 182, 543
2. Adams, J., et al. 2002, AJ, 124, 1570
3. Bara e, I., Chabrier, G., Allard, F., and Hauschildt, P.H. 1998, A&A, 337, 403 (BCAH)
4. Boudreault, S., et al. 2009, A&A, 510, 27
5. Chappelle, R. J., et al. 2005, MNRAS, 361, 1323
6. de La Fuente Marcos, R., & de La Fuente Marcos, C. 2000, Ap&SS, 271, 127
7. Eggen, O. J. 1960, MNRAS, 160, 540
8. Gonz alez-Garc a, B. M., et al. 2006, A&A, 460, 799
9. Hambly, N., Steele, I., Hawkins, M., & Jameson, R. 1995, AJ, 109, 29
10. Hodapp, K. W., Walker,C. H., Reipurth, B., et al. 2004, ApJ, 601, 79
11. Ho g, E., Fabricius, C., Makarov, V. V., et al. 2000, A&A, 355, 27
12. Jones, B.,and Cudworth, K. 1983, AJ, 88, 215
13. Jones, B.,and Stau er, J. 1991, AJ, 102, 1080
14. Klein-Wassink W. 1927, Publ. Kapteyn Astron. Lab. Groningen, 41, 1
15. Kenyon, S. J., and Hartmann L. 1995, ApJ, 101, 117
16. Kraus, A. L., and Hillenbrand, L. A. 2007, AJ, 134, 2340
17. Marigo, P., et al. 2008, A&A, 482, 883
18. Mermilliod, J.-C., et al. 1990, A&A, 235, 114
19. Pin eld, D. J., et al. 1997, MNRAS, 287, 180
20. Reglero, V., and Fabregat, J. 1991, A &AS, 90, 25
21. Roeser, S., Demleitner,M., & Schilbach, E. 2010, AJ, 139, 2440
22. Siess L., Dufour E., Forestini M. 2000, A&A, 358, 593
23. Skrutskie, M. F., Cutri, R. M., Stiening, R., et al., 2006, AJ, 131, 1163
24. Spitzer, L., Jr., & Shull, J. M. 1975, ApJ, 201, 773
25. Taylor, B. J. 2006, AJ, 132, 2453
26. Tonry, J. L., Stubbs, C. W., Lykke, K. R., et al. 2012, ApJ, 750, 99
27. Vandenberg, D. A., and Bridges, T. J. 1984, ApJ, 278, 679