The Chaotic Winds of Cool Giants

Peter Woitke (c) 2006

University of Vienna
Department of Astronomy
Tuerkenschanzstrasse 17
A-1180 Vienna
Austria
Tel: +43 1 4277 53815
peter.woitke&univie.ac.at

1. Observations

More than about 90% of all stars, including our sun, will evolve into so-called Asymptotic Giant Branch (AGB) stars at the end of their life time, just before they run out of fuel for nuclear burning. The "Asymptotic Giant Branch" is a certain region in the Hertzsprung-Russel diagram (c John Lattanzio 2004), occupied by these stars, which become about 10000 times more luminous and about a factor of 2 cooler at the surface than our present sun. AGB stars may become as big as the orbit of Earth.

At such large radii, matter can easily be lifted off from the stellar surface and expelled into space. In fact, massive stellar winds appear in this stage which enforce a rather sudden end of the stellar evolution due to mass loss. The mass loss rate of these winds ranges about from 10^-8 to 10^-4 solar masses per year (for comparison, the sun loses only about 10^-14 solar masses per year).

IRC+10216 at 2.2 micro-meter, evolution 1995-2001
(Weigelt et al. 2002, Astronomy and Astrophysics 392, p.131-141)

As a result of these massive stellar winds, the AGB stars are usually surrounded by large quantities of gas and dust which partly absorb the stellar radiation and thereby modify the spectral appearance of the stars. Modern observations with high angular resolution reveal that this surrounding matter often has an irregular clumpy structure, as e.g. in the case of the infrared carbon star IRC+10216, which is the brightest object on the sky at mid infrared wavelengths (except for the sun). This object shows direct evidence for a clumpy distribution of dust around the star which changes on timescales of only a few years.

The final destiny of AGB stars is to expel their complete outer shell into space as so-called superwind, except for a tiny but hot white dwarf in the center, the former core of the star, which remains. As soon as this stellar remnant becomes visible and ionizes the surrounding matter ejected before, the so-called Planetary Nebula (PN) phase is reached. PNs are often asymmetrical, and in particular, show spectacular internal structures. Here are a few examples:


The Eskimo Nebula, Hubble Space Telescope, WFPC2	The Bug nebula, Hubble Space Telescope, WFPC2	A zoomed-in picture from the Helix nebula, WFPC2

The richness of spatial structures of gas and dust in the fossil envelope of PNs is surprising and not very well understood so far. One possible explanation is that the fromer AGB star wind was not ejected in a spherically symmetric way, but already had a clumpy internal structure at that time. As the central star gets bright in the optical wavelength region, we can now see and study the details in the fossil AGB star wind, which were dark and unobservable before.

Beside this relevance for related objects like PNs, the winds of AGB stars are important

to understand late stages of stellar evolution,
for the replenishment of the Interstellar Medium with mass and heavy elements,
for the formation of the first solid particles in space, in particular the seed particles which can only be formed in dense and warm environments, and
to analyze and interpret AGB and post-AGB star observations.

2. Multi-dimensional models for AGB star winds

The details of the mass loss mechanism of AGB stars are still puzzling. Small solid particles ("dust grains") are expected to form close to the star, where the gas is dense and cool enough. These dust particles absorb the stellar photons and their momentum, which accelerates them outward, and drag the gas along due to frictional forces. Previous models have assumed that all these complicated chemical, radiative and dynamical processes occur in a stable, spherically symmetry way. However, observations seem to tell a different story, and evidence for chaotic, event-like dust formation epochs in only limited volumes are numerous.

How can we understand the chaotic winds of AGB stars?

We have performed axi-symmetric (2D) model calculations for AGB star winds which combine hydrodynamics with radiation pressure on dust grains, thermodynamics, chemistry, time-dependent dust formation, and radiative transfer. These model calculations show rather unexpected flow patterns, far away from spherical symmetry:

log density	radial velocity	degree of condensation

This model shows the innermost dust formation and wind acceleration region of a carbon-rich AGB star out to a distance of 7 stellar radii after about 31 years of evolution. An .mpeg-animation of the model (degree of condensation) can be downloaded here (18MB).

3. Conclusions

We propose the following chaotic scenario for dust-driven AGB star winds:

Excited by hydrodynamical, radiative or thermal instabilities, dust clouds are formed from time to time in the dust formation zone close to the star, which are then accelerated outward due to radiation pressure on dust grains. At the same time, dust-free matter falls back towards this zone at different places. A highly dynamical and turbulent dust formation zone is created in this way, which - in return - again bears a strongly inhomogeneous dust distribution. Further away from the star, where flow instabilities have time to amplify small disturbances (as e.g. the Rayleigh-Taylor instability), the dust arcs, lanes and clouds produced in the dust formation zone are further shaped and modified.

In the presented model, the central star is assumed to be in hydrostatic equilibrium. However, observations show that almost all AGB stars are pulsating variables, which produces waves which steepen up into shock waves in the large density gradient of the stellar atmosphere. These pulsational shocks are well-known to influence the dust formation zone. Moreover, the convective envelope of the AGB star is a source of sound waves with a wide spectrum of wavelengths, which can also affect the dust and wind formation. Therefore, besides improving the methods for radiative transfer in the current model, we intend to study the impact of convection and pulsation in future investigations.

The aim of this research project is to understand the mass-loss mechanism of AGB stars, to identify various instabilities and structure formation processes occurring in these outflows, and to make qualified interpretations of high resolution observational data.