Astronomisches Rechen-Institut Heidelberg

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ARI-Heidelberg Mitteilungen Serie A, No. 9

Author(s): von Hoerner, S.
Title: Die Auflösungszeit offener Sternhaufen
Source: Z. Astrophys. 44, 221-242
Year: 1958
Abstract: Model 1. By the exchange of energy, in each time of relaxation the fraction q=0.0074 of the cluster stars get velocities high enough to escape from the cluster, carrying away the fraction p=0.0054 of the cluster energy. Mass and radius of the cluster decrease first slowly and at last very rapidly; the time of relaxation decreases linearly with time. With T0=original time of relaxation, the whole lifetime of a cluster is about 30 T0. Model 2. At the same time, the cluster looses some mass by the development of the more massive cluster stars. The resulting influence on the rate of disintegration has been calculated; this influence is important only in young clusters. Model 3. Each cluster has a limiting radius of stability due to the tidal force of the Galaxy. Already those stars escape which fly beyond this limit. The lifetime of a cluster as a function of its original radius and mass has been tabulated; for an average cluster the lifetime is about 109 years. This agrees fairly well with a lifetime of 5*108 years, estimated from the frequencies of open clusters according to the spectral types of their earliest stars. But the discussion of some weak points of our models shows that a new treatment of the energy exchange is needed. High-velocity stars. During the disintegration of a cluster, the velocities of the remaining stars increase. At last some few stars escape with very high velocities. The disintegration being completed, about 0.5% of the former cluster stars have velocities >= 60 km/sec, thus being high-velocity stars of population I. They should be found on the main sequence from about A1 on, while the main part of the high-velocity stars begin at about F5.
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