60 research outputs found
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
Dynamic model atmospheres of cool giants
Cool giant stars are highly dynamical objects, and complex micro-physical
processes play an important role in their extended atmospheres and winds. The
interpretation of observations, and in particular of high-resolution IR
spectra, requires realistic self-consistent model atmospheres. Current
dynamical models include rather detailed micro-physics, and the resulting
synthetic spectra compare reasonably well with observations. A transition from
qualitative to quantitative modelling is taking place at present. We give an
overview of existing dynamical model atmospheres for AGB stars, discussing
recent advances and current trends in modelling, including 3D 'star-in-a-box'
models. When comparing synthetic spectra and other observable properties
resulting from dynamical models with observations we focus on the near- and
mid-IR wavelength range.Comment: 12 pages, 3 figures, to be published in Proc. of ESO Workshop on High
Resolution Infrared Spectroscopy in Astronomy, eds. Kaeufl H.U., Siebenmorgen
R., Moorwood A., ESO Astrophysics Symposia, Springe
Intense Mass Loss from C-rich AGB Stars at low Metallicity?
We argue that the energy injection of pulsations may be of greater importance
to the mass-loss rate of AGB stars than metallicity, and that the mass-loss
trend with metallicity is not as simple as sometimes assumed. Using our
detailed radiation hydrodynamical models that include dust formation, we
illustrate the effects of pulsation energy on wind properties. We find that the
mass-loss rate scales with the kinetic energy input by pulsations as long as a
dust-saturated wind does not occur, and all other stellar parameters are kept
constant. This includes the absolute abundance of condensible carbon (not bound
in CO), which is more relevant than keeping the C/O-ratio constant when
comparing stars of different metallicity. The pressure and temperature
gradients in the atmospheres of stars, become steeper and flatter,
respectively, when the metallicity is reduced, while the radius where the
atmosphere becomes opaque is typically associated with a higher gas pressure.
This effect can be compensated for by adjusting the velocity amplitude of the
variable inner boundary (piston), which is used to simulate the effects of
pulsation, to obtain models with comparable kinetic-energy input. Hence, it is
more relevant to compare models with similar energy-injections than of similar
velocity amplitude. Since there is no evidence for weaker pulsations in
low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity
C-stars have a lower mass-loss rate, than their more metal-rich counterparts
with similar stellar parameters, as long as they have a comparable amount of
condensible carbon.Comment: 4 pages, 3 figures. Accepted for publication in A&A. Updated after
language editing. Additional typos fixe
Atmospheric dynamics in carbon-rich Miras. I. Model atmospheres and synthetic line profiles
Atmospheres of evolved AGB stars are heavily affected by pulsation, dust
formation and mass loss, and they can become very extended. Time series of
observed high-resolution spectra proved to be a useful tool to study
atmospheric dynamics throughout the outer layers of these pulsating red giants.
Originating at various depths, different molecular spectral lines observed in
the near-infrared can be used to probe gas velocities there for different
phases during the lightcycle. Dynamic model atmospheres are needed to represent
the complicated structures of Mira variables properly. An important aspect
which should be reproduced by the models is the variation of line profiles due
to the influence of gas velocities. Based on a dynamic model, synthetic spectra
(containing CO and CN lines) were calculated, using an LTE radiative transfer
code that includes velocity effects. It is shown that profiles of lines that
sample different depths qualitatively reproduce the behaviour expected from
observations.Comment: accepted by A&A, 12 pages, 9 figure
Exploring wind-driving dust species in cool luminous giants I. Basic criteria and dynamical models of M-type AGB stars
This work is part of an ongoing effort aiming at identifying the actual
wind-drivers among the dust species observed in circumstellar envelopes. In
particular, we focus on the interplay between a strong stellar radiation field
and the dust formation process. To identify critical properties of potential
wind-driving dust species we use detailed radiation-hydrodynamical models which
include a parameterized dust description, complemented by simple analytical
estimates to help with the physical interpretation of the numerical results.
The adopted dust description is constructed to mimic different chemical and
optical dust properties in order to systematically study the effects of a
realistic radiation field on the second stage of the mass loss mechanism. We
see distinct trends in which combinations of optical and chemical dust
properties are needed to trigger an outflow. Dust species with a low
condensation temperature and a NIR absorption coefficient that decreases
strongly with wavelength will not condense close enough to the stellar surface
to be considered as potential wind-drivers. Our models confirm that metallic
iron and Fe-bearing silicates are not viable as wind-drivers due to their
near-infrared optical properties and resulting large condensation distances.
TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other
species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties
in the optical and chemical data and further work is needed. A strong candidate
is Mg2SiO4 with grain sizes of 0.1-1 micron, where scattering contributes
significantly to the radiative acceleration, as suggested by earlier
theoretical work and supported by recent observations.Comment: 15 pages, 12 figure
Abundance analysis for long period variables. Velocity effects studied with O-rich dynamic model atmospheres
(abbreviated) Measuring the surface abundances of AGB stars is an important
tool for studying the effects of nucleosynthesis and mixing in the interior of
low- to intermediate mass stars during their final evolutionary phases. The
atmospheres of AGB stars can be strongly affected by stellar pulsation and the
development of a stellar wind, though, and the abundance determination of these
objects should therefore be based on dynamic model atmospheres. We investigate
the effects of stellar pulsation and mass loss on the appearance of selected
spectral features (line profiles, line intensities) and on the derived
elemental abundances by performing a systematic comparison of hydrostatic and
dynamic model atmospheres. High-resolution synthetic spectra in the near
infrared range were calculated based on two dynamic model atmospheres (at
various phases during the pulsation cycle) as well as a grid of hydrostatic
COMARCS models. Equivalent widths of a selection of atomic and molecular lines
were derived in both cases and compared with each other. In the case of the
dynamic models, the equivalent widths of all investigated features vary over
the pulsation cycle. A consistent reproduction of the derived variations with a
set of hydrostatic models is not possible, but several individual phases and
spectral features can be reproduced well with the help of specific hydrostatic
atmospheric models. In addition, we show that the variations in equivalent
width that we found on the basis of the adopted dynamic model atmospheres agree
qualitatively with observational results for the Mira R Cas over its light
cycle. The findings of our modelling form a starting point to deal with the
problem of abundance determination in strongly dynamic AGB stars (i.e.,
long-period variables).Comment: 13 pages, 22 figures, accepted for publication in A&
Mass Loss Evolution and the Formation of Detached Shells around TP-AGB Stars
The origin of the so called 'detached shells' around AGB stars is not fully
understood, but two common hypotheses state that these shells form either
through the interaction of distinct wind phases or an eruptive mass loss
associated with a He-shell flash. We present a model of the formation of
detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars,
based on detailed modelling of mass loss and stellar evolution, leading to a
combination of eruptive mass loss and wind interaction.
The purpose of this paper is first of all to connect stellar evolution with
wind and mass loss evolution and demonstrate its consistency with observations,
but also to show how thin detached shells around TP-AGB stars can be formed.
Previous attempts to link mass loss evolution with the formation of detached
shells were based on approximate prescriptions for the mass loss and have not
included detailed modelling of the wind formation as we do here. (abridged)Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Interferometric properties of pulsating C-rich AGB stars I. Intensity profiles and uniform disc diameters of dynamic model atmospheres
We present the first theoretical study on center-to-limb variation (CLV)
properties and relative radius interpretation for narrow and broad-band
filters, on the basis of a set of dynamic model atmospheres of C-rich AGB
stars. We computed visibility profiles and the equivalent uniform disc radii
(UD-radii) in order to investigate the dependence of these quantities upon the
wavelength and pulsation phase. After an accurate morphological analysis of the
visibility and intensity profiles determined in narrow and broad-band filter,
we fitted our visibility profiles with a UD function simulating the
observational approach. UD-radii have been computed using three different
fitting-methods to investigate the influence of the sampling of the visibility
profile: single point, two points and least square method. The intensity and
visibility profiles of models characterized by mass loss show a behaviour very
different from a UD. We found that UD-radii are wavelength dependent and this
dependence is stronger if mass loss is present. Strong opacity contributions
from C2H2 affect all radius measurements at 3 micron and in the N-band,
resulting in higher values for the UD-radii. The predicted behaviour of
UD-radii versus phase is complicated in the case of models with mass loss,
while the radial changes are almost sinusoidal for the models without mass
loss. Compared to the M-type stars, for the C-stars no windows for measuring
the pure continuum are available.Comment: 13 pages, 9 figures, accepted for publication in A&
Circumstellar molecular composition of the oxygen-rich AGB star IK Tau: I. Observations and LTE chemical abundance analysis
The aim of this paper is to study the molecular composition in the
circumstellar envelope around the oxygen-rich star IK Tau. We observed IK Tau
in several (sub)millimeter bands using the APEX telescope during three
observing periods. To determine the spatial distribution of the
emission, mapping observations were performed. To
constrain the physical conditions in the circumstellar envelope, multiple
rotational CO emission lines were modeled using a non local thermodynamic
equilibrium radiative transfer code. The rotational temperatures and the
abundances of the other molecules were obtained assuming local thermodynamic
equilibrium. An oxygen-rich Asymptotic Giant Branch star has been surveyed in
the submillimeter wavelength range. Thirty four transitions of twelve molecular
species, including maser lines, were detected. The kinetic temperature of the
envelope was determined and the molecular abundance fractions of the molecules
were estimated. The deduced molecular abundances were compared with
observations and modeling from the literature and agree within a factor of 10,
except for SO, which is found to be almost a factor 100 stronger than
predicted by chemical models. From this study, we found that IK Tau is a good
laboratory to study the conditions in circumstellar envelopes around
oxygen-rich stars with (sub)millimeter-wavelength molecular lines. We could
also expect from this study that the molecules in the circumstellar envelope
can be explained more faithful by non-LTE analysis with lower and higher
transition lines than by simple LTE analysis with only lower transition lines.
In particular, the observed CO line profiles could be well reproduced by a
simple expanding envelope model with a power law structure.Comment: 13 pages, 14 figures, 8 tables *Accepted for publication in Astronomy
and Astrophysic
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