2,287 research outputs found
Pulsation-induced atmospheric dynamics in M-type AGB stars. Effects on wind properties, photometric variations and near-IR CO line profiles
Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to
a two-step process. First, matter in the stellar atmosphere is levitated by
shock waves, induced by stellar pulsation, and second, this matter is
accelerated by radiation pressure on dust, resulting in a wind. In dynamical
atmosphere and wind models the effects of the stellar pulsation are often
simulated by a simplistic prescription at the inner boundary. We test a sample
of dynamical models for M-type AGB stars, for which we kept the stellar
parameters fixed to values characteristic of a typical Mira variable but varied
the inner boundary condition. The aim was to evaluate the effect on the
resulting atmosphere structure and wind properties. The results of the models
are compared to observed mass-loss rates and wind velocities, photometry, and
radial velocity curves, and to results from 1D radial pulsation models.
Dynamical atmosphere models are calculated, using the DARWIN code for different
combinations of photospheric velocities and luminosity variations. The inner
boundary is changed by introducing an offset between maximum expansion of the
stellar surface and the luminosity and/or by using an asymmetric shape for the
luminosity variation. Models that resulted in realistic wind velocities and
mass-loss rates, when compared to observations, also produced realistic
photometric variations. For the models to also reproduce the characteristic
radial velocity curve present in Mira stars (derived from CO
lines), an overall phase shift of 0.2 between the maxima of the luminosity and
radial variation had to be introduced. We find that a group of models with
different boundary conditions (29 models, including the model with standard
boundary conditions) results in realistic velocities and mass-loss rates, and
in photometric variations
Dust-driven winds of AGB stars: The critical interplay of atmospheric shocks and luminosity variations
Winds of AGB stars are thought to be driven by a combination of
pulsation-induced shock waves and radiation pressure on dust. In dynamic
atmosphere and wind models, the stellar pulsation is often simulated by
prescribing a simple sinusoidal variation in velocity and luminosity at the
inner boundary of the model atmosphere. We experiment with different forms of
the luminosity variation in order to assess the effects on the wind velocity
and mass-loss rate, when progressing from the simple sinusoidal recipe towards
more realistic descriptions. Using state-of-the-art dynamical models of C-rich
AGB stars, a range of different asymmetric shapes of the luminosity variation
and a range of phase shifts of the luminosity variation relative to the radial
variation are tested. These tests are performed on two stellar atmosphere
models. The first model has dust condensation and, as a consequence, a stellar
wind is triggered, while the second model lacks both dust and wind. The first
model with dust and stellar wind is very sensitive to moderate changes in the
luminosity variation. There is a complex relationship between the luminosity
minimum, and dust condensation: changing the phase corresponding to minimum
luminosity can either increase or decrease mass-loss rate and wind velocity.
The luminosity maximum dominates the radiative pressure on the dust, which in
turn, is important for driving the wind. These effects of changed luminosity
variation are coupled with the dust formation. In contrast there is very little
change to the structure of the model without dust. Changing the luminosity
variation, both by introducing a phase shift and by modifying the shape,
influences wind velocity and the mass-loss rate. To improve wind models it
would probably be desirable to extract boundary conditions from 3D dynamical
interior models or stellar pulsation models.Comment: 11 pages, 13 figures, accepted for publication in A&
Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars
The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars
is usually attributed to a two-stage process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. The dust transfers momentum to the surrounding gas through
collisions and thereby triggers a general outflow. Radiation-hydrodynamical
models of M-type AGB stars suggest that these winds can be driven by photon
scattering -- in contrast to absorption -- on Fe-free silicate grains of sizes
0.1--1\,m. In this paper we study photometric constraints for wind-driving
dust species in M-type AGB stars, as part of an ongoing effort to identify
likely candidates among the grain materials observed in circumstellar
envelopes. To investigate the scenario of stellar winds driven by photon
scattering on dust, and to explore how different optical and chemical
properties of wind-driving dust species affect photometry we focus on two sets
of dynamical models atmospheres: (i) models using a detailed description for
the growth of MgSiO grains, taking into account both scattering and
absorption cross-sections when calculating the radiative acceleration, and (ii)
models using a parameterized dust description, constructed to represent
different chemical and optical dust properties. By comparing synthetic
photometry from these two sets of models to observations of M-type AGB stars we
can provide constraints on the properties of wind-driving dust species.
Photometry from wind models with a detailed description for the growth of
MgSiO grains reproduces well both the values and the time-dependent
behavior of observations of M-type AGB stars, providing further support for the
scenario of winds driven by photon scattering on dust.Comment: Accepted for publication in A&A. 15 pages, 14 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&
(Quantum) Space-Time as a Statistical Geometry of Fuzzy Lumps and the Connection with Random Metric Spaces
We develop a kind of pregeometry consisting of a web of overlapping fuzzy
lumps which interact with each other. The individual lumps are understood as
certain closely entangled subgraphs (cliques) in a dynamically evolving network
which, in a certain approximation, can be visualized as a time-dependent random
graph. This strand of ideas is merged with another one, deriving from ideas,
developed some time ago by Menger et al, that is, the concept of probabilistic-
or random metric spaces, representing a natural extension of the metrical
continuum into a more microscopic regime. It is our general goal to find a
better adapted geometric environment for the description of microphysics. In
this sense one may it also view as a dynamical randomisation of the causal-set
framework developed by e.g. Sorkin et al. In doing this we incorporate, as a
perhaps new aspect, various concepts from fuzzy set theory.Comment: 25 pages, Latex, no figures, some references added, some minor
changes added relating to previous wor
Multi-neuronal refractory period adapts centrally generated behaviour to reward
Oscillating neuronal circuits, known as central pattern generators (CPGs), are responsible for generating rhythmic behaviours such as walking, breathing and chewing. The CPG model alone however does not account for the ability of animals to adapt their future behaviour to changes in the sensory environment that signal reward. Here, using multi-electrode array (MEA) recording in an established experimental model of centrally generated rhythmic behaviour we show that the feeding CPG of Lymnaea stagnalis is itself associated with another, and hitherto unidentified, oscillating neuronal population. This extra-CPG oscillator is characterised by high population-wide activity alternating with population-wide quiescence. During the quiescent periods the CPG is refractory to activation by food-associated stimuli. Furthermore, the duration of the refractory period predicts the timing of the next activation of the CPG, which may be minutes into the future. Rewarding food stimuli and dopamine accelerate the frequency of the extra-CPG oscillator and reduce the duration of its quiescent periods. These findings indicate that dopamine adapts future feeding behaviour to the availability of food by significantly reducing the refractory period of the brain's feeding circuitry
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