THE YOUNG OPEN STAR-CLUSTERS - STABILITY AND STRUCTURE

It is found that the differential extinction ΔE(B - V) in open clusters, which may be due to the presence of gas and dust, decreases systematically with the age of the cluster. Consequently we can infer that the average gas removal time must be about 108 yr. The present work supports the findings of Elmegreen (1983) that bound clusters are formed in low-mass clouds (script M sign≤104 script M sign⊙) while the unbound OB associations are formed in clouds having higher masses (script M sign>105 script M sign⊙). It is also concluded that if formation of bound clusters takes place in low-mass clouds (script M sign∼104script M sign⊙) only, then the observed low cluster formation rate and higher space density of low-mass clouds suggest that star formation in open clusters must be a continuous process for about 108 yr. The present work also supports the existence of a corona around open clusters and it is concluded that the coronal regions in bound open clusters are dynamically stable in the tidal forces of the Galaxy.