270 research outputs found
Policy Considerations in Revitalizing Local and Regional Food Systems
Agricultural Finance, Political Economy,
Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars
It is well established that mass loss from AGB stars due to dust driven winds
cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars
using detailed frequency-dependent radiation hydrodynamics including dust
formation. We present a study of the thresholds for the mass loss rate as a
function of stellar parameters based on a subset of a larger grid of such
models and compare these results to previous observational and theoretical
work. Furthermore, we demonstrate the impact of the pulsation mechanism and
dust formation for the creation of a stellar wind and how it affects these
thresholds and briefly discuss the consequences for stellar evolution.Comment: 2 pages, 1 figure. To be published in the proceedings of IAU Symp.
241 on Stellar Populations as Building Blocks of Galaxies, ed. A. Vazdekis et
al. (2007). Replaced to match edited versio
Dust grain properties in atmospheres of AGB stars
We present self-consistent dynamical models for dust driven winds of
carbon-rich AGB stars. The models are based on the coupled system of
frequency-dependent radiation hydrodynamics and time-dependent dust formation.
We investigate in detail how the wind properties of the models are influenced
by the micro-physical properties of the dust grains that enter as parameters.
The models are now at a level where it is necessary to be quantitatively
consistent when choosing the dust properties that enters as input into the
models. At our current level of sophistication the choice of dust parameters is
significant for the derived outflow velocity, the degree of condensation and
the estimated mass loss rates of the models. In the transition between models
with and without mass-loss the choice ofmicro-physical parameters turns out to
be very significant for whether a particular set of stellar parameters will
give rise to a dust-driven mass loss or not.Comment: 10 pages, 3 figures. To appear in: Modelling of Stellar Atmospheres,
N.E. Piskunov, W.W. Weiss, D.F. Gray (eds.), IAU Symposium Vol. xxx.
Proceedings for the IAU Symposium 210, Uppsala, June 200
The influence of dust properties on the mass loss in pulsating AGB stars
We are currently studying carbon based dust types of relevance for
carbon-rich AGB stars, to obtain a better understanding of the influence of the
optical and chemical properties of the grains on the mass loss of the star. An
investigation of the complex interplay between hydrodynamics,radiative transfer
and chemistry has to be based on a better knowledge of the micro-physics of the
relevant dust species.Comment: 4 pages, 2 figures. Proceedings for IAU Colloquium 185 "Radial and
Nonradial Pulsations as Probes of Stellar Physics
Uni-directional polymerization leading to homochirality in the RNA world
The differences between uni-directional and bi-directional polymerization are
considered. The uni-directional case is discussed in the framework of the RNA
world. Similar to earlier models of this type, where polymerization was assumed
to proceed in a bi-directional fashion (presumed to be relevant to peptide
nucleic acids), left-handed and right-handed monomers are produced via an
autocatalysis from an achiral substrate. The details of the bifurcation from a
racemic solution to a homochiral state of either handedness is shown to be
remarkably independent of whether the polymerization in uni-directional or
bi-directional. Slightly larger differences are seen when dissociation is
allowed and the dissociation fragments are being recycled into the achiral
substrate.Comment: 9 pages, 4 figures, submitted to Astrobiolog
Three-component modeling of C-rich AGB star winds. II. The effects of drift in long-period variables
We present three-component wind models for carbon rich pulsating AGB stars.
In particular we study the effects of drift in models of long-period variables,
meaning that the dust is allowed to move relative to the gas (drift models). In
addition we investigate the importance of the degree of variability of the wind
structures. The wind model contains separate conservation laws for each of the
three components of gas, dust and the radiation field. We use two different
representations for the gas opacity, resulting in models with different gas
densities in the wind. The effects which we investigate here are important for
the understanding of the wind mechanism and mass loss of AGB stars. This study
is hereby a necessary step towards more reliable interpretations of
observations. We find that the effects of drift generally are significant. They
cannot be predicted from models calculated without drift. Moreover, the
non-drift models showing the lowest mass loss rates, outflow velocities, and
the smallest variability in the degree of condensation do not form drift model
winds. The wind formation in drift models is, except for a few cases, generally
less efficient and the mass loss consequently lower than in the corresponding
non-drift models. The effects of drift are generally larger in the more
realistic models using that representation of the gas opacity which results in
lower densities. The outflow properties of these models are also -- for all
cases we have studied -- sensitive to the period of the stellar pulsations. A
check of the mass loss rates against a (recent) fit formula shows
systematically lower values, in particular in the more realistic models with a
low density. The fit is in its current form inapplicable to the new models
presented here.Comment: 19 pages, 8 figures, accepted by A&
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