2. W. P. Chen （陳文屏） Graduate Institute of Astronomy |and Department of Physics National Central University Colloquium National Tsing Hua University 2003.03.28 Morphology of Galactic Open Star Clusters

3. NTHU Colloquium2 What we learn/teach in AST101…

4. NTHU Colloquium3 Globular Clusters They are --- large collections of stars --- 10 5 to 10 6 stars spherically shaped and centrally concentrated composed of old stars with little ‘metals’ located in the halo around the Galaxy Shakedown image of M15 with Lulin 1m 2002.09/10

5. NTHU Colloquium4 M104 with LOT 2003.03

6. NTHU Colloquium5 Open Clusters They are irregularly shaped and sparse small groupings of stars --- 10 2 to 10 3 stars composed mostly young stars with abundant ‘metals’ located in the Galactic disk Just how irregular are they anyway?

7. NTHU Colloquium6 Star clusters are good astrophysical laboratories because they contain stars of different masses  of the same age  at the same distance  with the same composition Most stars probably were formed in a clustered environment Cluster Formation = Star Formation Chandra image of the Trapezium

8. NTHU Colloquium7 Globular Clusters Stars concentrate progressively toward the center. The King model (1962) is understood as a combination of an isothermal sphere; i.e., dynamically relaxed in the inner part of the cluster, and tidal truncation by the Milky Way in the outer part. Surface brightness of M3 (Da Costa and Freeman, 1976)

9. NTHU Colloquium8 King Model (1962) r c : core radius r t : tidal radius k  central number density

10. NTHU Colloquium9 Structure of Open Clusters The initial stellar distribution in a star cluster is dictated perhaps by the structure in the parental molecular cloud. (Initial) As the cluster evolves, the distribution is modified by gravitational interaction among member stars. (Internal) Eventually stellar evaporation and external disturbances --- Galactic tidal force, differential rotation, and collision with molecular clouds --- would dissolve the cluster. (External)

11. NTHU Colloquium10 Questions to address … How would an open cluster shape out of the molecular cloud from which it is formed? How would it organize itself by internal gravitational interaction, and by external disturbances? To what extent the Galactic potential influences the morphology of an star cluster?

12. NTHU Colloquium11 Spatial Structure by Star Counting in 2MASS Stellar density within concentric annuli Center too crowded to resolve by 2MASS; outer part follows well a King model Background uniform out to large angular extents K lim (3-  )=15.6 --- not deep enough to reach MS, for distant and old globular clusters Projected radial stellar density of a GC, M55

13. 2MASS has uniformly scanned the entire sky in three near- infrared bands to detect and characterize point sources brighter than about 1 mJy in each band, with signal-to-noise ratio greater than 10, using a pixel size of 2.0". This has achieved an 80,000- fold improvement in sensitivity relative to earlier surveys. 2MASS used two highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ, and one at CTIO, Chile. Each telescope was equipped with a three-channel camera, each channel consisting of a 256×256 array of HgCdTe detectors, capable of observing the sky simultaneously at J (1.25 microns), H (1.65 microns), and K s (2.17 microns), with 10-sigma limit of 15.8 (J), 15.1 (H), 14.3 (K) mag. 2MASS All-Sky Data Release 2003 Mar 25!

14. NTHU Colloquium13 Map of the 2MASS Point Source Catalog (~471 M sources) integrated flux in 5 × 5´ bins in a galactic Aitoff projection

15. NTHU Colloquium14 Optical (left) and near-infrared (right) image of IC348, an embedded star cluster

16. NTHU Colloquium15 2MASS 3-sigma limit

17. NTHU Colloquium16 Open cluster NGC 2506 (1.9 Gyr; 3.3 kpc) mosaiced from 2MASS data IC 348 by 2MASS

18. NTHU Colloquium17 Sources toward NGC2506 and the surface density

19. NTHU Colloquium18

20. NTHU Colloquium19 Locations of sample galactic open clusters. http://www.ipac.caltech.edu/2mass/

21. NTHU Colloquium20 Radial density profile of NGC 2506

22. NTHU Colloquium21 Cumulative stellar density profiles for NGC 2506 (1.9 Gyr; 3.3 kpc) shows apparent evidence of mass segregation …… in contrast to that in M11 found by Mathieu (1984)

23. NTHU Colloquium22 Stars in the young (5 Myr) star cluster IC 348 are centrally concentrated, and seem to segregate  star formation processes more than subsequent dynamic evolution

24. NTHU Colloquium23 The old (9 Gyr) open cluster Berkeley 17 shows no evidence of mass segregation.

25. NTHU Colloquium24 Clusterl,b  (Myr) N*N* M/M ⊙ D (kpc) R s (‘) R (pc)  re (Myr) τ/τ re Segr. Young NGC1893 174, -02 44983094.46.58.92910.01? IC348 160, -18 53222000.3216.51.6140.2Y Intermediate NGC1817 186, -13 8002361462.112.57.91396N? NGC2506 231, +10 1,9001,0386433.317.517.36053Y NGC2420 198, +20 2,2004502792.512.59.422310Y Old NGC6791 070, +11 8,0001,0956794.210.513.254315? Be17 176, -04 9,0003702292.59.57.114263N

26. NTHU Colloquium25 Relaxation Time τ cross = R/V ; τ relax ~τ cross ． N cross N cross = 0.1 N / ln N τ evap ~ 100 τ relax R: radius V: velocity dispersion N: number of member stars

27. NTHU Colloquium26 NGC 1893 --- 4 Myr

28. NTHU Colloquium27 IC 348 --- 5 Myr

29. NTHU Colloquium28 NGC1817 --- 800 Myr

30. NTHU Colloquium29 NGC 6791 --- 8,000 Myr

31. NTHU Colloquium30 Be 17 --- 9,000 Myr

32. NTHU Colloquium31 Summary 2MASS good for study of open clusters Stars in an open cluster, regardless of masses, are concentrated progressively toward the center. The youngest star clusters show evidence of luminosity (mass) segregation, a relic of the parental molecular cloud structure (SMA) By a few Gyr (several relaxation times), clusters become highly relaxed, until dynamical disruptions dominate the structure.

33. NTHU Colloquium32 Our knowledge, or even recognition, of galactic open clusters is highly incomplete, most biased toward the ones that are nearby and with bright stars.

34. NTHU Colloquium33 Open clusters are distributed widely around the galactic disk.

35. NTHU Colloquium34 Studying star clusters, and knowing their origin(s), properties, etc., are fun ….. But They are good tools, too.  They are gravitating systems.  They emit light (easy to trace).  Good test particles in Galactic gravitational potential  Good tracers of Galactic star formation history and chemical evolution

36. NTHU Colloquium35 Dias (2002): a new catalogue of Galactic open clusters which updates the previous catalogues of Lynga (1987) and of Mermilliod (1995). Virtually all the clusters (1607) presently known were included, an increment of 456 objects relative to the Lynga (1987) catalogue. In total, 99% with known apparent diameters 38% with distance, E(B-V) and age determinations 6% with abundance data 9% with proper motion and radial velocity data

37. NTHU Colloquium36 Sample still vastly incomplete; more star clusters away from the Galactic center  Selection effects?  Dynamic effects? e.g., disintegration by encounters with GMCs in the inner part of the Milky Way? e.g., only the most robust survived and lasted? Chen et al. astro-ph0212542

38. NTHU Colloquium37 Chen et al. on Dias catalog; c.f. Ojha et al (1996): thin disk~260 pc; thick disk~760pc A possible connection between the halo and disk populations?

39. NTHU Colloquium38 Chen et al. on Dias catalog c.f. Phelps (1994) z~55 pc for young and 375 pc for old clusters How do old star clusters survive in the disk from tidal disruption and differential rotation?

40. NTHU Colloquium39 NGC1893 NGC2420 NGC1931 Surface star density

41. NTHU Colloquium40 Eccentricity vs. z of 36 open clusters in our sample. Eccentricity vs. Age of our sample clusters.

42. NTHU Colloquium41 Conclusions … for now Most open clusters are elongated. The higher z an open cluster locates, the more circular it is  influence of the Galactic disk on the morphology of a star cluster. Cluster internal dynamics  spherical/circular By 100 Myr, internal relaxation competes with external influence (e.g., Galactic tidal force, differential rotation) in shaping a star cluster. The oldest and most eccentric star clusters  in the process of disintegration?

43. NTHU Colloquium42 NGC2395 l,b: 204.6052, +13.9879 Heliocentric distance: 512 pc Age: 10 9.07 z: 124 pc Going…and going… More than 200 star clusters being analyzed ….

44. NTHU Colloquium43 Density (distribution) of neighbors P star, i = (N obs – N bg )/N obs within an angular range D =  P i Be 17

45. NTHU Colloquium44 Open Cluster Study at NCU Luminosity Function Evolution  age and star formation history (e.g., coeval vs periodic bursts) done Structural Evolution  dynamics  probing galactic mass distribution (e.g., disk vs volume potential) 2/3 done Variability and [rotation vs magnetic activity] doing

46. NTHU Colloquium45 Open Clusters/NCU --- cont. Lulin 1 m telescope (Taiwan) September 2002 Maidanak Observatory (Uzbekistan) 1.5 m and 1 m 1/3 time on 1 m Moletai Obs. 1.65 m (Lithuania) Coravel YALO 1 m (Chile) Kentucky-Yunnan-Taiwan Telescope (KYTT) to lift off by 2004-2005?

47. NTHU Colloquium46 Fast rotating stars P(rot) < 12 d (BLUE) are distinguished from slow rotators (RED) by their X-ray luminosity (normalized to solar – in yellow. The Rossby number gives the rotation period in units of the eddy's lifetime. (http://www.aip.de/groups/turbulence/star_t.html)

48. NTHU Colloquium47