Letter to the Editor Dynamical response of a stellar atmosphere to pressure perturbations: numerical simulations

The time dependent reactions of an isothermal spherically symmetric stellar atmosphere to perturbations of the external (interstellar) pressure are analysed by means of computer simulations. The system is seen to evolve, through the phases of wind, breeze, accretion and back, according to an hysteresis type cycle with two catastrophe points: the value of the external pressure relative to a static atmosphere and that corresponding to the fastest (critical) breeze. This behaviour is proved to be due to the instability of the outflow breeze solutions (due to their unfavourable stratification), while subsonic accretion is stable. A crucial factor of this instability is the position of the outer boundary: if this is placed too close to the base of the atmosphere the inflow/outflow breeze stability is reversed. These simulations confirm a scenario first proposed by Velli (1994).