M ORPHOLOGY OF G ALACTIC O PEN C LUSTERS

1. Fig3: NGC1893 Fig.4: NGC1931 Fig.5 NGC2420 Fig6: Eccentricity vs. z of the 36 open clusters in our sample. Fig.7: Eccentricity vs. age of our sample clusters. Fig.1. Left: z vs. Galactic longitude for open clusters younger than 100 million years. Right: The same for open clusters older than 100 million years. Young O.C. are more concentrated toward the plane. Fig.2. Optical (left) and near-infrared (right) image of IC348. 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 latestopen 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. MORPHOLOGYOF GALACTIC OPEN CLUSTERS 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 A&As