Abstract
A fraction of the thermal protons in the outer envelope of an x-ray cluster have velocities that exceed the local escape speed from the cluster gravitational potential. The Coulomb mean free path of these protons is larger than the virial radius of the cluster at temperatures ≳2.5 keV. The resulting leakage of suprathermal particles generates a collisionless shock in neighbouring voids and fills them with heat and magnetic fields. If suppression by magnetic fields is ignored, then over a Hubble time thermal evaporation could drain a tenth of the cluster gas at its virial temperature. The evaporated fraction could increase dramatically if additional heat is deposited into the gas by cluster mergers, active galactic nuclei or supernovae. Thermal evaporation is not included in existing cosmological simulations since they are based on the fluid approximation. Measurements of the baryon mass fraction in the outer envelopes of hot clusters (through their Sunyaev–Zel'dovich effect or x-ray emission) can be used to empirically constrain their evaporation rate.