Circumstellar dust shells around long-period variables

IX. Molecular layers resulting from shocked carbon-rich atmospheres

¹ Institut für Astronomie und Astrophysik, TU Berlin, Sekr. PN 8-1, Hardenbergstraße 36, 10623 Berlin, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ DEMIRM, UMR 8540, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France

Corresponding author: Ch. Helling, chris@astro.physik.tu-berlin.de

Received: 5 April 2000
Accepted: 6 October 2000

Abstract

The existence of warm zones of an enhanced molecular content surrounding the photospheres of AGB stars has been postulated to explain observed molecular absorption/emission components which are in excess of the results obtained from classical hydrostatic model atmospheres. We have analyzed the chemical equilibrium molecular composition of time-dependent models of carbon-rich circumstellar envelopes for various combinations of the stellar parameters and found that groups of molecules appear to be present in different zones with temporally varying temperature and density. The regions of enhanced molecular abundances are situated between the stellar photosphere and the circumstellar dust shell. In the models, these zones are produced by the levitation of the atmospheric gas due to the dissipation of shock waves and/or due to radiation pressure on molecules and dust.