"M" for "Mystery": What drives the winds of M-type AGB stars?
by
Susanne Höfner(Uppsala Astronomical Observatory)
→
Europe/Stockholm
FA31
FA31
Description
The intricate interplay of atmospheric shock waves and a complex,
variable radiation field with non-equilibrium dust formation
turns self-consistent modelling of atmospheres and winds of AGB stars
into a challenging task. Nevertheless it is clear that realistic models
predicting mass-loss rates and synthetic spectra are crucial for our
understanding of this important phase of stellar evolution.
Models for atmospheres and winds of C-type stars have reached a level
which allows direct comparison with observations. In the case of stars
with C/O < 1, however, recent models demonstrate that radiation pressure
on silicate grains is not sufficient to drive the observed winds,
contrary to previous expectations (Woitke 2006, A&A 460, L9).
This has led us to suggest an alternative scenario for the mass loss of
M-type AGB stars, involving the formation of both carbon and silicate grains
due to non-equilibrium effects (Höfner & Andersen 2007, A&A 465, L39).
Our models allow us to assess under which circumstances it is possible to
drive winds with small amounts of carbon dust and to get silicate grains
forming in these outflows at the same time.
The proposed scenario provides
a natural explanation for the observed similarities in wind properties of
M-type and C-type AGB stars and implies a smooth transition for stars with
increasing carbon abundance, from solar-composition to C-rich AGB stars,
possibly solving the longstanding problem of the driving mechanism for
stars with C/O close to one.
