Logotropic distributions

Abstract

In all spatial dimensions $d$, we study the static and dynamical properties of a generalized Smoluchowski equation which describes the evolution of a gas obeying a logotropic equation of state, $p=A\ln\rho$. A logotrope can be viewed as a limiting form of polytrope ($p=K\rho^{\gamma}$, $\gamma=1+1/n$), with index $\gamma=0$ or $n=-1$. In the language of generalized thermodynamics, it corresponds to a Tsallis distribution with index $q=0$. We solve the dynamical logotropic Smoluchowski equation in the presence of a fixed external force deriving from a quadratic potential, and for a gas of particles subjected to their mutual gravitational force. In the latter case, the collapse dynamics is studied for any negative index $n$, and the density scaling function is found to decay as $r^{-\alpha}$, with $\alpha=\frac{2n}{n-1}$ for $n<-\frac{d}{2}$, and $\alpha=\frac{2d}{d+2}$ for $-\frac{d}{2}\leq n<0$.