218 research outputs found
An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds
The purpose of this work is to present an extensive grid of dynamical
atmosphere and wind models for M-type AGB stars, covering a wide range of
relevant stellar parameters. We used the DARWIN code, which includes
frequency-dependent radiation-hydrodynamics and a time-dependent description of
dust condensation and evaporation, to simulate the dynamical atmosphere. The
wind-driving mechanism is photon scattering on submicron-sized MgSiO
grains. The grid consists of models, with luminosities from
to and
effective temperatures from 2200K to 3400K. For the first time different
current stellar masses are explored with M-type DARWIN models, ranging from
0.75M to 3M. The modelling results are radial atmospheric
structures, dynamical properties such as mass-loss rates and wind velocities,
and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of
condensed Si). We find that the mass-loss rates of the models correlate
strongly with luminosity. They also correlate with the ratio :
increasing by an order of magnitude increases the mass-loss rates by
about three orders of magnitude, which may naturally create a superwind regime
in evolution models. There is, however, no discernible trend of mass-loss rate
with effective temperature, in contrast to what is found for C-type AGB stars.
We also find that the mass-loss rates level off at luminosities higher than
, and consequently at pulsation periods longer
than days. The final grain radii range from 0.25 micron to 0.6
micron. The amount of condensed Si is typically between 10% and 40%, with
gas-to-dust mass ratios between 500 and 4000.Comment: Accepted to A&A, 17 pages, 15 figure
Evidence for mass ejection associated with long secondary periods in red giants
Approximately 30% of luminous red giants exhibit a Long Secondary Period
(LSP) of variation in their light curves, in addition to a shorter primary
period of oscillation. The cause of the LSP has so far defied explanation:
leading possibilities are binarity and a nonradial mode of oscillation. Here,
large samples of red giants in the Large Magellanic Cloud both with and without
LSPs are examined for evidence of an 8 or 24 m mid-IR excess caused by
circumstellar dust. It is found that stars with LSPs show a significant mid-IR
excess compared to stars without LSPs. Furthermore, the near-IR - color
seems unaffected by the presence of the 24 m excess. These findings
indicate that LSPs cause mass ejection from red giants and that the lost mass
and circumstellar dust is most likely in either a clumpy or a disk-like
configuration. The underlying cause of the LSP and the mass ejection remains
unknown.Comment: 6 pages, accepted for publication in Ap
Condensation of MgS in outflows from carbon stars
The basic mechanism responsible for the widespread condensation of MgS in the
outflows from carbon rich stars on the tip of the AGB is discussed with the aim
of developing a condensation model that can be applied in model calculations of
dust formation in stellar winds.
The different possibilities how MgS may be formed in the chemical environment
of outflows from carbon stars are explored by some thermochemical calculations
and by a detailed analysis of the growth kinetics of grains in stellar winds.
The optical properties of core-mantle grains with a MgS mantle are calculated
to demonstrate that such grains reproduce the structure of the observed 30
m feature. These considerations are complemented by model calculations of
circumstellar dust shells around carbon stars.
It is argued that MgS is formed via precipitation on silicon carbide grains.
This formation mechanism explains some of the basic observed features of MgS
condensation in dust shells around carbon stars. A weak secondary peak at about
33 ... 36 m is shown to exist in certain cases if MgS forms a coating on
SiC.Comment: 9 pages, 7 figure
A close halo of large transparent grains around extreme red giant stars
Intermediate-mass stars end their lives by ejecting the bulk of their
envelope via a slow dense wind back into the interstellar medium, to form the
next generation of stars and planets. Stellar pulsations are thought to elevate
gas to an altitude cool enough for the condensation of dust, which is then
accelerated by radiation pressure from starlight, entraining the gas and
driving the wind. However accounting for the mass loss has been a problem due
to the difficulty in observing tenuous gas and dust tens of milliarcseconds
from the star, and there is accordingly no consensus on the way sufficient
momentum is transferred from the starlight to the outflow. Here, we present
spatially-resolved, multi-wavelength observations of circumstellar dust shells
of three stars on the asymptotic giant branch of the HR diagram. When imaged in
scattered light, dust shells were found at remarkably small radii (<~ 2 stellar
radii) and with unexpectedly large grains (~300 nm radius). This proximity to
the photosphere argues for dust species that are transparent to starlight and
therefore resistant to sublimation by the intense radiation field. While
transparency usually implies insufficient radiative pressure to drive a wind,
the radiation field can accelerate these large grains via photon scattering
rather than absorption - a plausible mass-loss mechanism for lower-amplitude
pulsating stars.Comment: 13 pages, 1 table, 6 figure
Too little radiation pressure on dust in the winds of oxygen-rich AGB stars
New dynamical models for dust-driven winds of oxygen-rich AGB stars are
presented which include frequency-dependent Monte Carlo radiative transfer by
means of a sparse opacity distribution technique and a time-dependent treatment
of the nucleation, growth and evaporation of inhomogeneous dust grains composed
of a mixture of Mg2SiO4, SiO2, Al2O3, TiO2, and solid Fe. The
frequency-dependent treatment of radiative transfer reveals that the gas is
cold close to the star (700-900 K at 1.5-2 R*) which facilitates the nucleation
process. The dust temperatures are strongly material-dependent, with
differences as large as 1000 K for different pure materials, which has an
important influence on the dust formation sequence. Two dust layers are formed
in the dynamical models: almost pure glassy Al2O3 close to the star (r > 1.5
R*) and the more opaque Fe-poor Mg-Fe-silicates further out. Solid Fe or
Fe-rich silicates are found to be the only condensates that can efficiently
absorb the stellar light in the near IR. Consequently, they play a crucial role
for the wind driving mechanism and act as thermostat. Only small amounts of Fe
can be incorporated into the grains, because otherwise the grains get too hot.
Thus, the models reveal almost no mass loss, and no dust shells.Comment: 4 pages, 3 figures. accepted as A&A letter after minor revision
Dynamical Opacity-Sampling Models of Mira Variables. I: Modelling Description and Analysis of Approximations
We describe the Cool Opacity-sampling Dynamic EXtended (CODEX) atmosphere
models of Mira variable stars, and examine in detail the physical and numerical
approximations that go in to the model creation. The CODEX atmospheric models
are obtained by computing the temperature and the chemical and radiative states
of the atmospheric layers, assuming gas pressure and velocity profiles from
Mira pulsation models, which extend from near the H-burning shell to the outer
layers of the atmosphere. Although the code uses the approximation of Local
Thermodynamic Equilibrium (LTE) and a grey approximation in the dynamical
atmosphere code, many key observable quantities, such as infrared diameters and
low-resolution spectra, are predicted robustly in spite of these
approximations. We show that in visible light, radiation from Mira variables is
dominated by fluorescence scattering processes, and that the LTE approximation
likely under-predicts visible-band fluxes by a factor of two.Comment: 9 pages, 10 figures, accepted for MNRA
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