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A survey and comparison of various models of gravitational dynamical friction Elson, Rebecca


Under certain circumstances, a massive object travelling through a medium may experience a net average deceleration due to the gravitational interaction between it and the medium. This slowing down effect is called dynamical friction. There are a variety of ways in which dynamical friction may be modeled, depending primarily on the nature of the medium, and on what approximations are deemed to be reasonable. This thesis is devoted to reviewing the various models, with an emphasis on the assumptions underlying each, to pinpointing the source of any discrepancies between the models, and to assessing the validity of each. For this purpose the models are grouped into two categories. The first comprises those in which the medium consists of dust or gas. The models in the second describe the interaction between a test object and a medium consisting of other objects of its own mass. These latter are of two general types: those which employ the two-body approximation, and those which describe the interaction in terms of a stochastically fluctuating force arising from the varying distribution of field objects surrounding the test object. The models lead to three different expressions for the dynamical friction experienced by the object: on proportional to TlogX (X is the distance from the object), one proportional to TlogvT (v is the average velocity of the members of the system), and one finite one proportional to TlogD[sub=o], (D[sub=o] is the mean inter-object spacing). It is argued that the TlogD[sub=o] result is based on an analysis which does not take into account the long range nature of gravitational forces. The TlogX models rely on the behaviour of the medium being stationary, but it is demonstrated that it takes an infinite amount of time to establish such behaviour, so these models are not strictly applicable to any real situation.

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