Relaxation-limited evaporation of globular clusters

Evaporative evolution of stellar clusters is shown to be relaxation limited when the number of stars satisfies N >> N-c, where N-c similar or equal to 1600. For a Maxwell velocity distribution that extends beyond the escape velocity, this process is bright in that the Kelvin-Helmholtz time scale, f(H)(-1)t(relax) is shorter than the Ambartsumian-Spitzer time scale, f(N)(-1)t(relax), where f(H), > f(N) denote the fractional changes in total energy and number of stars per relaxation time, t(relax). The resulting evaporative lifetime t(ev) similar or equal to 20.5t(relax) for isolated clusters is consistent with Fokker-Planck and N-body simulations, where t(relax) is expressed in terms of the half-mass radius. We calculate the gray body factor by averaging over the anisotropic perturbation of the potential barrier across the tidal sphere, and derive the tidal sensitivity dlnt(ev)dy similar or equal to -1.9 to -0.7 as a function of the ratio y of the virial-to-tidal radius. Relaxation limited evaporation applies to the majority of globular clusters of the Milky Way with N = 10(4)-10(6) that are in a pre-collapse phase. It drives streams of stars into the tidal field with a mean kinetic energy of 0.71 relative to temperature of the cluster. Their S shape morphology leads in sub-orbital and a trails in super-orbital streams separated by 3.4 sigma/Omega in the radial direction of the orbit, where Omega denotes the orbital angular velocity and sigma the stellar velocity dispersion in the cluster. These correlations may be tested by advanced wide field photometry and spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.