Extended King Models for Star Clusters

Abstract

Models applied to galaxies in equilibrium configuration are based on the solution of the collisionless Boltzmann equation and the Poisson equation for gravitational interaction which are related to each other by a smoothed-out mass density. This mass density is related to a distribution function $f(\mathbf{x},\mathbf{v},t)$ and produces a gravitational potential $ϕ(\mathbf{x},t)$. The most popular models to describe spherically symmetric systems are King models. These models are based on a truncated isothermal sphere. They fit the surface brightness of globular clusters and some elliptical galaxies well. A well-known example of a galaxy that King models does not fit well is NGC 3379. In this work, we extend King models. Our models are based on the Tsallis distribution function. There is a parameter $q$ in this function and we can recover the King distribution function in the limit $q\to 1$. We discuss the general behaviors of our models and, moreover, we use them to fit the surface brightness of NGC 3379 and 47 TUC.