Thermal conduction in systems out of hydrostatic equilibrium

yArea de F´ ´isica Teorica, Facultad de Ciencias, Universidad de Salamanca, 37008, Salamanca,´SpainzGrupo de F´isica Estad ´istica, Departamento de F ´isica, Universidad Aut onoma de Barcelona,´08193 Bellaterra, Barcelona, SpainReceived 24 October 1996, in final form 24 March 1997Abstract. We analyse the effects of thermal conduction in a relativistic fluid, just after itsdeparture from hydrostatic equilibrium, on a time scale of the order of the thermal relaxationtime. It is found that the resulting evolution will depend critically on a parameter defined interms of thermodynamic variables, which is constrained by causality requirements.PACS numbers: 0440D, 0440, 4775, 9530L, 9530T, 9760