Spatial%20Structure%20Evolution%20of%20Open%20Star%20Clusters

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Spatial Structure Evolution of Open Star Clusters

• W. P. Chen and J. W. Chen
• Graduate Institute of Astronomy National Central
  University
• IAU-APRM2002.7.03 Tokyo

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Open Clusters

• What we have learned/taught in AST101
  irregularly and sparsely shaped.
• How irregular are they anyway?

IC 348 by 2MASS
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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)
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King Model (1962)
rc core radius rt tidal radius k ? central
number density
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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)

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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
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2MASS 3-sigma limit
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Open cluster NGC 2506 (1.9 Gyr 3.3 kpc) mosaiced
from 2MASS data
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Sources toward NGC2506 and the surface density
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Locations of sample galactic open clusters.
http//www.ipac.caltech.edu/2mass/
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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)
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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
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The old (9 Gyr) open cluster Berkeley 17 shows no
evidence of mass segregation.
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Cluster l,b ?(Myr) N M/M? D (kpc) Rs() R (pc) ? re(Myr) t/t re Segr.
Young Young Young Young Young Young Young Young Young Young Young
NGC1893 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
Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate Intermediate
NGC1817 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
Old Old Old Old Old Old Old Old Old Old Old
NGC6791 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
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Relaxation Time

• tcross R/V trelax tcross.Ncross
• Ncross 0.1 N / ln N
• tevap 100 trelax
• R radius V velocity dispersion
• N number of member stars

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NGC 1893 --- 4 Myr
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IC 348 --- 5 Myr
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NGC1817 --- 800 Myr
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NGC 6791 --- 8,000 Myr
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Be 17 --- 9,000 Myr
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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)

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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

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

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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-resolution spectroscopy
  
• Kentucky-Yunnan-Taiwan Telescope (KYTT) to lift
  off by 2004-2005?

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Fast rotating stars P(rot) lt 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)