Transcript Spatial Structure Evolution of Star Clusters

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
Open Clusters
• What we have
learned/taught in
AST101: irregularly
and sparsely shaped.
• How irregular are
they anyway?
IC 348 by 2MASS
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)
King Model (1962)

1
f k
2
 1  r / rc 


1/ 2

1


1/ 2

1  r / r  
2
t
rc: core radius rt: tidal radius
k  central number density
c
2
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.
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
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
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
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-resolution
spectroscopy
• Kentucky-Yunnan-Taiwan Telescope
(KYTT) 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)