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Astron. Astrophys. 322, 756-763 (1997)

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The effect of dynamical friction on a young stellar cluster prior to the gas removal

A. Saiyadpour , B.M. Deiss and W.H. Kegel

Institut für Theoretische Physik, J.W.Goethe-Universität, Robert-Mayer-Straße 10, D-60054 Frankfurt, Germany

Received 15 July 1996 / Accepted 3 December 1996


Stars are formed preferably in a collective manner in the dense cores of molecular clouds, leading to young stellar clusters, embedded in large amounts of residual interstellar gas. This residual gas will be expelled from the system after some [FORMULA] yr, mainly due to stellar winds from young protostars. The star formation efficiency, SFE, ([FORMULA], defined as the ratio of the total stellar mass to the original cloud core mass) is a crucial parameter, which determines, whether the 'naked' stellar system will be a bound one, known as open cluster, or an unbound ever expanding one, known as association. For the stellar cluster to remain bound [FORMULA] is a lower limit, given the case of a rapid gas removal (gas removal time [FORMULA] crossing time), while a SFE of [FORMULA] will suffice if the gas is removed slowly (Wilking & Lada 1985).

Following the ideas of Verschueren (1990) we demonstrate by means of a simple model that it is possible to get these lower limits for the star formation efficiency shifted towards smaller values by accounting for the dynamical friction between the stellar system and the interstellar gas. Due to this friction the stars will decelerate within a time scale short compared to the time scale for the onset of stellar winds. In this way we need not to assume the stars being born with a smaller velocity dispersion than that of the gas, as is done by Verschueren. Subsequent contraction of the stellar system and the revirialization of the stars is followed by the gas removal. As a result we find that bound open clusters for [FORMULA] as a lower limit are possible.

Key words: clusters: open, and associations – ISM: clouds – ISM: kinetics and dynamics – stars: formation – stellar dynamics

Send offprint requests to: A. Saiyadpour

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998