217 research outputs found
Three-component modeling of C-rich AGB star winds I. Method and first results
Radiative acceleration of newly-formed dust grains and transfer of momentum
from the dust to the gas plays an important role for driving winds of AGB
stars. Therefore a detailed description of the interaction of gas and dust is a
prerequisite for realistic models of such winds. In this paper we present the
method and first results of a three-component time-dependent model of
dust-driven AGB star winds. With the model we plan to study the role and
effects of the gas-dust interaction on the mass loss and wind formation. The
wind model includes separate conservation laws for each of the three components
of gas, dust and the radiation field and is developed from an existing model
which assumes position coupling between the gas and the dust. As a new feature
we introduce a separate equation of motion for the dust component in order to
fully separate the dust phase from the gas phase. The transfer of mass, energy
and momentum between the phases is treated by interaction terms. We also carry
out a detailed study of the physical form and influence of the momentum
transfer term (the drag force) and three approximations to it. In the present
study we are interested mainly in the effect of the new treatment of the dust
velocity on dust-induced instabilities in the wind. As we want to study the
consequences of the additional freedom of the dust velocity on the model we
calculate winds both with and without the separate dust equation of motion. The
wind models are calculated for several sets of stellar parameters. We find that
there is a higher threshold in the carbon/oxygen abundance ratio at which winds
form in the new model. The winds of the new models, which include drift, differ
from the previously stationary winds, and the winds with the lowest mass loss
rates no longer form.Comment: 15 pages, 5 figures, accepted by A&
Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models
The heavy mass loss observed in evolved asymptotic giant branch stars is
usually attributed to a two-step process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. Detailed wind models suggest that the outflows of M-type
AGB stars may be triggered by photon scattering on Fe-free silicates with grain
sizes of about 0.1 - 1 m. Due to the low grain temperature, these Fe-free
silicates can condense close to the star, but they do not produce the
characteristic mid-IR features that are often observed in M-type AGB stars.
However, it is probable that the silicate grains are gradually enriched with Fe
as they move away from the star, to a degree where the grain temperature stays
below the sublimation temperature, but is high enough to produce emission
features. We investigate whether differences in grain temperature in the inner
wind region, which are related to changes in the grain composition, can be
detected with current interferometric techniques, in order to put constraints
on the wind mechanism. To investigate this we use radial structures of the
atmosphere and wind of an M-type AGB star, produced with the 1D
radiation-hydrodynamical code DARWIN. The spectral energy distribution is found
to be a poor indicator of different temperature profiles and therefore is not a
good tool for distinguishing different scenarios of changing grain composition.
However, spatially resolved interferometric observations have promising
potential. They show signatures even for Fe-free silicates (found at 2-3
stellar radii), in contrast to the spectral energy distribution. Observations
with baselines that probe spatial scales of about 4 stellar radii and beyond
are suitable for tracing changes in grain composition, since this is where
effects of Fe enrichment should be found.Comment: Accepted for publication in Section 8. Stellar atmospheres of
Astronomy and Astrophysics. The official date of acceptance is 07/09/2017. 9
pages, 7 figures, 4 figures in appendi
Winds of M- and S-type AGB stars: an unorthodox suggestion for the driving mechanism
Current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from C-rich AGB stars. For M-type objects, however, recent detailed models demonstrate that radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations. In this paper, we suggest an alternative mechanism for the mass-loss of M-type AGB stars, involving the formation of both carbon and silicate grains due to non-equilibrium effects, and we study the viability of this scenario. We model the dynamical atmospheres and winds of AGB stars by solving the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation, using a parameterized description of non-equilibrium effects in the gas phase. This approach allows 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 properties of the resulting wind models, such as mass loss rates and outflow velocities, are well within the observed limits for M-type AGB stars. Furthermore, according to our results, it is quite unlikely that significant amounts of silicate grains will condense in a wind driven by a force totally unrelated to dust formation, as the conditions in the upper atmosphere and wind acceleration region put strong constraints on grain growth. 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 long-standing problem of the driving mechanism for stars with C/O close to one
Measuring The Mass Loss Evolution at The Tip of The Asymptotic Giant Branch
In the final stages of stellar evolution low- to intermediate-mass stars lose
their envelope in increasingly massive stellar winds. Such winds affect the
interstellar medium and the galactic chemical evolution as well as the
circumstellar envelope where planetary nebulae form subsequently.
Characteristics of this mass loss depend on both stellar properties and
properties of gas and dust in the wind formation region. In this paper we
present an approach towards studies of mass loss using both observations and
models, focusing on the stage where the stellar envelope is nearly empty of
mass. In a recent study we measure the mass-loss evolution, and other
properties, of four planetary nebulae in the Galactic Disk. Specifically we use
the method of integral field spectroscopy on faint halos, which are found
outside the much brighter central parts of a planetary nebula. We begin with a
brief comparison between our and other observational methods to determine
mass-loss rates in order to illustrate how they differ and complement each
other. An advantage of our method is that it measures the gas component
directly requiring no assumptions of properties of dust in the wind. Thereafter
we present our observational approach in more detail in terms of its validity
and its assumptions. In the second part of this paper we discuss capabilities
and assumptions of current models of stellar winds. We propose and discuss
improvements to such models that will allow meaningful comparisons with our
observations. Currently the physically most complete models include too little
mass in the model domain to permit a formation of winds with as high mass-loss
rates as our observations show.Comment: 7 pages, workshop in honour of Agnes Acker, Legacies of the
Macquarie/AAO/Strasbourg Halpha Planetary Nebula project, ed. Q.Parker and
D.Frew, PASA, in press; clarified some parts and added some additional
reference
Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution
We study the circumstellar environment of the carbon-rich star R Scl using
the near- and mid-infrared high spatial resolution observations from the
ESO-VLTI instruments VINCI and MIDI. These observations aim at increasing our
knowledge of the dynamic processes in play within the very close circumstellar
environment where the mass loss of AGB stars is initiated. Data are interpreted
using a self-consistent dynamic model. Interferometric observations do not show
any significant variability effect at the 16 m baseline between phases 0.17 and
0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97
and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find
fairly good agreement between the dynamic model and the spectrophotometric data
from 0.4 to 25 m. The model agrees well with the time-dependent flux data
at 8.5 m, whereas it is too faint at 11.3 and 12.5 m. The VINCI
visibilities are reproduced well, meaning that the extension of the model is
suitable in the K-band. In the mid-infrared, the model has the proper extension
to reveal molecular structures of C2H2 and HCN located above the stellar
photosphere. However, the windless model used is not able to reproduce the more
extended and dense dusty environment. Among the different explanations for the
discrepancy between the model and the measurements, the strong nonequilibrium
process of dust formation is one of the most probable. The complete dynamic
coupling of gas and dust and the approximation of grain opacities with the
small-particle limit in the dynamic calculation could also contribute to the
difference between the model and the data
Dust formation in winds of long-period variables. V. The influence of micro-physical dust properties in carbon 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 are required by the
description of grain formation. The choice of dust parameters is significant
for the derived outflow velocity, the degree of condensation and the resulting
mass loss rates of the models. In the transition region 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. We also calculate near-infrared colors to
test how the dust parameters influence the observable properties of the models,
however, at this point we do not attempt to fit particular stars.Comment: 13 pages, 8 figures, A&A in pres
Synthesis and biological evaluation of α- and β-hydroxy substituted amino acid derivatives as potential mGAT1-4 inhibitors
<jats:title>Abstract</jats:title><jats:p>In this study, we report the synthesis and biological evaluation of a variety of α- and β-hydroxy substituted amino acid derivatives as potential amino acid subunits in inhibitors of GABA uptake transporters (GATs). In order to ensure that the test compounds adopt a binding pose similar to that presumed for related larger GAT inhibitors, lipophilic residues were introduced either at the amino nitrogen atom or at the alcohol function. Several of the synthesized compounds were found to exhibit similar inhibitory activity at the GAT subtypes mGAT2, mGAT3, and mGAT4, respectively, as compared with the reference N-butylnipecotic acid. Hence, these compounds might serve as starting point for future developments of more complex GAT inhibitors.</jats:p>
The mid-infrared diameter of W Hydrae
Mid-infrared (8-13 microns) interferometric data of W Hya were obtained with
MIDI/VLTI between April 2007 and September 2009, covering nearly three
pulsation cycles. The spectrally dispersed visibility data of all 75
observations were analyzed by fitting a circular fully limb-darkened disk (FDD)
model to all data and individual pulsation phases. Asymmetries were studied
with an elliptical FDD. Modeling results in an apparent angular FDD diameter of
W Hya of about (80 +/- 1.2) mas (7.8 AU) between 8 and 10 microns, which
corresponds to an about 1.9 times larger diameter than the photospheric one.
The diameter gradually increases up to (105 +/- 1.2) mas (10.3 AU) at 12
microns. In contrast, the FDD relative flux fraction decreases from (0.85 +/-
0.02) to (0.77 +/- 0.02), reflecting the increased flux contribution from a
fully resolved surrounding silicate dust shell. The asymmetric character of the
extended structure could be confirmed. An elliptical FDD yields a position
angle of (11 +/- 20) deg and an axis ratio of (0.87 +/- 0.07). A weak pulsation
dependency is revealed with a diameter increase of (5.4 +/- 1.8) mas between
visual minimum and maximum, while detected cycle-to-cycle variations are
smaller. W Hya's diameter shows a behavior that is very similar to the Mira
stars RR Sco and S Ori and can be described by an analogous model. The constant
diameter part results from a partially resolved stellar disk, including a close
molecular layer of H2O, while the increase beyond 10 microns can most likely be
attributed to the contribution of a spatially resolved nearby Al2O3 dust shell.Comment: 18 pages, 16 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
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&
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