MORPHOLOGY OF GALACTIC OPEN CLUSTERS

1
MORPHOLOGY OF GALACTIC OPEN CLUSTERS
Chin-Wei Chen Wen-Ping ChenGraduate Institute
of AstronomyNational Central University, Taiwan
Thirty six open clusters were selected from the
2MASS database on the basis of the latest open
cluster catalogue (Dias et al 2002). The
morphological parameters such as eccentricity,
orientation were obtained via isodensity elliptic
fitting. Most star clusters are elongated, and
the eccentricity is correlated with z as an
indication of the influence of the Galactic disk.
The morphology shows clear evidence of
competing internal dynamics and external Galactic
disturbances when a cluster becomes 10 Myr old.
I. Introduction Galactic open clusters are
distributed throughout and around the Galactic
disk, so are useful tools to probe the Galactic
structure and star formation history. Study of
the structure of open clusters shows that stars
in an open cluster are concentrated
progressively toward the center (Chen Chen
2002). 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?

• III. Morphology of open clusters
• Dividing each 2MASS field (typically 42) around
  a star cluster into spatial grids whose size
  depends on the richness of the cluster.
• Computing the surface number density of stars in
  each grid.
• Subtracting the Galactic background number
  density by fitting a surface to the outer part of
  each field.
• Fitting the star cluster with an ellipse, from
  which the eccentricity and orientation are
  obtained.
• Figures 3-5 are examples of clusters with
  eccentricity 0.9, 0.7, and 0.5, respectively.
  Figure 6 and 7 show the eccentricities versus
  distances and ages, respectively.

IIa. The Latest Open Cluster Catalogue Dias et al
(2002) compiled a new catalogue of open clusters
which updates those of Lynga and of Mermilliod.
New objects and new data on kinematics such as
radial velocity, proper motion were included.
Four hundred and fifty-six objects were added
into the new catalogue to make it become 1607
clusters, of which 37 have distances, ages and
color-excesses. Figure 1 plots the Galactic z
distribution for young (less than 100 Myr) and
relatively older star clusters. It can be seen
that young clusters are located closer toward the
Galactic plane.
IIb. Open clusters in 2MASS We used the
Two-Micron All-Sky Survey (2MASS) star catalogue
to analyze the morphology of open clusters. The
2MASS data, being taken at near-infrared
wavelengths, are free from much of interstellar
extinction, and would thus reveal the true shape
of a star cluster better than in optical bands
(Fig. 2). We selected 36 open clusters from the
Dias et al catalogue that have distance and age
determinations, and also have complete 2MASS
coverage.

• Conclusion
• Most open clusters are elongated.
• The higher z an open cluster locates, the more
  circular it is, showing the general influence of
  the Galactic disk on the morphology of a star
  cluster.
• As a cluster evolves, its internal dynamics
  would cause it to appear circular. However as a
  cluster becomes some 100 Myr, the internal
  relaxation and external influence (e.g., Galactic
  tidal force, differential rotation) become
  competing factors in shaping a star cluster.
• The oldest and most eccentric star clusters may
  be in the process of disintegration.

• References
• Chen, W. P. Chen, C. W. 2002, IAU-APRM
  Proceedings
• Dias, 2002 AAs