W. P. Chen (???)

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Morphology of Galactic Open Star Clusters

• W. P. Chen (???)
• Graduate Institute of Astronomy and Department
  of PhysicsNational Central University
• Colloquium National Tsing Hua University2003.03.2
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What we learn/teach in AST101
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Globular Clusters

• They are ---
• large collections of stars --- 105 to 106 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
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M104 with LOT 2003.03
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Open Clusters

• They are
• irregularly shaped and sparse
• small groupings of stars --- 102 to 103 stars
• composed mostly young stars with abundant
  metals
• located in the Galactic disk

Just how irregular are they anyway?
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• 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
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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 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)

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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?

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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 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 256256
array of HgCdTe detectors, capable of observing
the sky simultaneously at J (1.25 microns), H
(1.65 microns), and Ks (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!
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Map of the 2MASS Point Source Catalog (471 M
sources) integrated flux in 5 5 bins in a
galactic Aitoff projection
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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
IC 348 by 2MASS
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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
• 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.

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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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• 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

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• 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

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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
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A possible connection between the halo and disk
populations?
Chen et al. on Dias catalog c.f. Ojha et al
(1996) thin disk260 pc thick disk760pc
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How do old star clusters survive in the disk from
tidal disruption and differential rotation?
Chen et al. on Dias catalogc.f. Phelps (1994)
z55 pc for young and 375 pc for old clusters
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Surface star density
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Eccentricity vs. z of 36 open clusters in our
sample.
Eccentricity vs. Age of our sample clusters.
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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?

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NGC2395 l,b 204.6052, 13.9879 Heliocentric
distance 512 pc Age 109.07 z 124 pc

Goingand going More than 200 star clusters
being analyzed .
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Be 17
Density (distribution) of neighbors Pstar, i
(Nobs Nbg)/Nobs within an angular range D ?
Pi
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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) 2/3 done
• Variability and rotation vs magnetic activity
  doing

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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?

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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)
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