Spatial Structure Evolution of Open Star Clusters

W. P. Chen and J. W. ChenGraduate Institute of Astronomy

National Central UniversityIAU-APRM2002.7.03 Tokyo

Open Clusters

• What we have learned/taught in AST101: irregularly and sparsely shaped.

• How irregular are they anyway?

IC 348 by 2MASS

Globular ClustersStars 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)

King Model (1962)

2

2/122/12 /1

1

/1

1

ctc rrrrkf

rc: core radius rt: tidal radius k central number density

Structure of an Open Cluster• 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)

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

• Klim (3-)=15.6 --- not deep enough to reach MS, for distant and old globular clusters Projected radial stellar

density of a GC, M55

2MASS 3-sigma limit

Open cluster NGC 2506 (1.9 Gyr; 3.3 kpc) mosaiced from 2MASS data

Sources toward NGC2506 and the surface density

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

Radial density profile of NGC 2506

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)

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

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

Cluster l,b (Myr)

N* M/M⊙ D (kpc)

Rs

(‘)

R (pc)

re

(Myr)

τ/τ re Segr.

YoungNGC1893 174, -02 4 498 309 4.4 6.5 8.9 291 0.01 ?

IC348 160, -18 5 322 200 0.32 16.5 1.6 14 0.2 Y

IntermediateNGC1817 186, -13 800 236 146 2.1 12.5 7.9 139 6 N?

NGC2506 231, +10 1,900 1,038 643 3.3 17.5 17.3 605 3 Y

NGC2420 198, +20 2,200 450 279 2.5 12.5 9.4 223 10 Y

OldNGC6791 070, +11 8,000 1,095 679 4.2 10.5 13.2 543 15 ?

Be17 176, -04 9,000 370 229 2.5 9.5 7.1 142 63 N

Relaxation Time

τcross = R/V ; τrelax ~τcross．Ncross Ncross = 0.1 N / ln N

τevap ~ 100 τrelax

R: radius V: velocity dispersion

N: number of member stars

NGC 1893 --- 4 Myr

IC 348 --- 5 Myr

NGC1817 --- 800 Myr

NGC 6791 --- 8,000 Myr

Be 17 --- 9,000 Myr

Summary

• 2MASS good for study of open clusters - Full data release expected end of 2002 - Deep IR images to differentiate the MS (IR camera with Nagoya U & PMO)

• Stars in an open cluster, regardless of masses, are concentrated progressively toward the center.

• The youngest star clusters show evidence of luminosity (mass) segregation - cf. molecular cloud structure (SMA)

Summary --- II

• By a few Gyr (several relaxation times), clusters become highly relaxed, until dynamical disruptions dominate.

• Tidal distortions (age, location, massive vs low-mass stars) Open clusters (scattering around the galactic disk) as probes of galactic mass distribution e.g., disk vs volume potential - galactic disk and (dark) halo models

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

Open clusters are distributed widely around the galactic disk.

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) ½ done

• Variability and [rotation vs magnetic activity] doing

Open Clusters/NCU --- cont.

• Lulin 1 m telescope (Taiwan) August 2002

• Maidanak Observatory (Uzbekistan) 1.5 m and 1 m

• Moletai Obs. 1.65 m (Lithuania)• YALO 1 m (Chile)• Imaging plus CORAVEL high-resolutio

n spectroscopy • Kentucky-Yunnan-Taiwan Telescope (K

YTT) to lift off by 2004-2005?

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)