The mass of the Pleiades

Abstract

A membership catalogue for the Pleiades is divided into four mass bins, and a tidally truncated King profile is fitted to each bin with good agreement with the data. The tidal radius of the cluster is found to be 13.1 pc, and the total mass of the cluster down to the stellar limit is calculated to be 735 M⊙. The spread of stars in each bin, as well as the relaxation and crossing times, shows the Pleiades to be an approximately relaxed cluster with equilibrium between the density and velocity distributions. The cluster kinetic energy and binding energy are consistent with the virial theorem, indicating no large unseen population of brown dwarfs. However, the 1σ errors in the cluster parameters provide an upper limit to the mass of any brown dwarf population of 131 M⊙, which would show up in deep CCD surveys as ≤5.5 brown dwarfs per 10 × 10 arcminute field in the cluster centre.