A class of spherical, truncated, anisotropic models for application to globular clusters

¹ Università degli Studi di MilanoDipartimento di Fisica, via Celoria 16, 20133 Milano, Italy
e-mail: ruggero.devita@gmail.com
² University of Surrey, Department of Physics, Guildford, Surrey GU2 7XH, UK

Received: 8 February 2016
Accepted: 14 March 2016

Abstract

Recently, a class of non-truncated, radially anisotropic models (the so-called f^(ν)-models), originally constructed in the context of violent relaxation and modelling of elliptical galaxies, has been found to possess interesting qualities in relation to observed and simulated globular clusters. In view of new applications to globular clusters, we improve this class of models along two directions. To make them more suitable for the description of small stellar systems hosted by galaxies, we introduce a “tidal” truncation by means of a procedure that guarantees full continuity of the distribution function. The new f_T^(ν)-models are shown to provide a better fit to the observed photometric and spectroscopic profiles for a sample of 13 globular clusters studied earlier by means of non-truncated models; interestingly, the best-fit models also perform better with respect to the radial-orbit instability. Then, we design a flexible but simple two-component family of truncated models to study the separate issues of mass segregation and multiple populations. We do not aim at a fully realistic description of globular clusters to compete with the description currently obtained by means of dedicated simulations. The goal here is to try to identify the simplest models, that is, those with the smallest number of free parameters, but still have the capacity to provide a reasonable description for clusters that are evidently beyond the reach of one-component models. With this tool, we aim at identifying the key factors that characterize mass segregation or the presence of multiple populations. To reduce the relevant parameter space, we formulate a few physical arguments based on recent observations and simulations. A first application to two well-studied globular clusters is briefly described and discussed.