The Study, Based On The Structure Of The Milky Way Open Clusters

In addiction to the field stars, there are many star clusters in the Milky Way. They play an important role in understanding the formation, structure and evolution of the Galaxy. However, their observational properties differ in many aspects, such as luminosities, compactness, and metallicities. Star clusters with compact structures and high luminosities are named as"globular clusters"; while the others with lose structures and low luminosities are named as"open clusters".Observations have pointed out that stars form in molecular clouds and most of them are clustered. Since the stars in a cluster have the same origins, their observations present a good laboratory to test the theory of star formation; Secondly, because of the wide distributions of open clusters in age (overlap the whole history of the Galactic disk) and mass (from hundreds to more than 10 thousand solar masses), they have been taken effectively to probe the formation and evolution of the Galactic disk. For example, on one hand young open clusters are adopted to determine the structures of spiral arms of the Galaxy, to study the rotation curve of the Galaxy, to investigate the star formation mechanisms and the dynamic evolution of star clusters, and to constrain the stellar initial mass function and the mass function of clouds where clusters were born. On the other hand, because ages of old open clusters are relatively easy to obtain, they have become one of the best tools to investigate the early epoch of the Galactic disk formation. Furthermore, the brightest member stars in an old open cluster are red giants always with strong emission lines, so that they can be observed spectroscopily to get their chemical composition and radial velocities. As a result, open clusters are effective tools to investigate the structure and evolution, dynamics and chemical evolution of the Galaxy. There have been several good papers as the overview of open clusters.The WEBDA open cluster catalogue (version 2007), developed and maintained by Ernst Paunzen working in the Institute of Astronomy of the University of Vienna, is adopted in the present study. The total number of open clusters in the catalogue is 970, within which 911 clusters have age determinations, 920 clusters have distances, and 911 clusters have color excesses. The goal of the present thesis is to study statistically the structure of the Galactic disk by open clusters, such as the scale height and length of the disk, and the position of the Sun above the Galactic plane as well.It implies that an active star forming period might occur 3.4Gyr ago when the age distribution of open clusters is taken into account. Most of old open clusters were destroyed in the inner solar circle due to the interactions with giant molecular clouds. The Sun locates about 20pc north to the Galactic plane. The scale height perpendicular to the Galactic plane for open clusters increases with their ages.Although WEBDA catalogue provides a large amount of information on open clusters, it is still not completed yet. We analyzed the incompleteness of our sample with Monte Carlo simulations, based on the observation results of open clusters near the Sun. Under the consideration of fixed and random positions of open clusters, we derive the scale length of the Galactic disk to be and , respectively.