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Astron. Astrophys. 330, L13-L16 (1998)

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Letter to the Editor

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

L. Del Zanna 1, M. Velli 1 and P. Londrillo 2

1 Dipartimento di Astronomia e Scienze dello Spazio, Università di Firenze, Largo E. Fermi 5, I-50125 Firenze, Italy
2 Osservatorio Astronomico, Via Zamboni 33, I-40126 Bologna, Italy

Received 23 October 1997 / Accepted 7 November 1997

Abstract

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).

Key words: hydrodynamics – instabilities – (Sun:) solar wind – sun: atmosphere – stars: atmospheres – ISM: jets and outflows

Present address: International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland

Send offprint requests to: L. Del Zanna (e-mail: ldz@arcetri.astro.it)

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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