2,140 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&
Satchitananda' : meditation and dance around the still point, in the form of a raga performance for piano solo : inaugural lecture/recital delivered at Rhodes University
Inaugural lecture delivered at Rhodes UniversityRhodes University Libraries (Digitisation
The theory and practice of "Klangreihenkomposition"
Diese Arbeit befaßt sich mit der "Klangreihenlehre'', einer Musiktheorie, die von Josef Matthias Hauer aufgestellt und von meinem Lehrer Othmar Steinbauer weiterentwickelt wurde. Diese auf der Gesetzmäßigkeit der zwölf Töne beruhende Satzlehre hat jedoch mit der "Reihentechnik" jener Musik, die schlechthin als "Zwölftonmusik" bezeichnet wird, nichts gemein. Obwohl eine beachtliche Literatur über J.M. Hauer vorliegt, ist sie doch, was Hauers Musiktheorie betrifft, nicht zufriedenstellend, da sie hauptsächlich auf sein Leben und seine Musikphilosophie eingeht
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
Modelling the atmosphere of the carbon-rich Mira RU Vir
Context. We study the atmosphere of the carbon-rich Mira RU Vir using the
mid-infrared high spatial resolution interferometric observations from
VLTI/MIDI. Aims. The aim of this work is to analyse the atmosphere of the
carbon-rich Mira RU Vir, with state of the art models, in this way deepening
the knowledge of the dynamic processes at work in carbon-rich Miras. Methods.
We compare spectro-photometric and interferometric measurements of this
carbon-rich Mira AGB star, with the predictions of different kinds of modelling
approaches (hydrostatic model atmospheres plus MOD-More Of Dusty,
self-consistent dynamic model atmospheres). A geometric model fitting tool is
used for a first interpretation of the interferometric data. Results. The
results show that a joint use of different kind of observations (photometry,
spectroscopy, interferometry) is essential to shed light on the structure of
the atmosphere of a carbon-rich Mira. The dynamic model atmospheres fit well
the ISO spectrum in the wavelength range {\lambda} = [2.9, 25.0] {\mu}m.
Nevertheless, a discrepancy is noticeable both in the SED (visible), and in the
visibilities (shape and level). A possible explanation are intra-/inter-cycle
variations in the dynamic model atmospheres as well as in the observations. The
presence of a companion star and/or a disk or a decrease of mass loss within
the last few hundred years cannot be excluded but are considered unlikely.Comment: 15 pages. Accepted in A&
Neuronal synchrony: peculiarity and generality
Synchronization in neuronal systems is a new and intriguing application of dynamical systems theory. Why are neuronal systems different as a subject for synchronization? (1) Neurons in themselves are multidimensional nonlinear systems that are able to exhibit a wide variety of different activity patterns. Their “dynamical repertoire” includes regular or chaotic spiking, regular or chaotic bursting, multistability, and complex transient regimes. (2) Usually, neuronal oscillations are the result of the cooperative activity of many synaptically connected neurons (a neuronal circuit). Thus, it is necessary to consider synchronization between different neuronal circuits as well. (3) The synapses that implement the coupling between neurons are also dynamical elements and their intrinsic dynamics influences the process of synchronization or entrainment significantly. In this review we will focus on four new problems: (i) the synchronization in minimal neuronal networks with plastic synapses (synchronization with activity dependent coupling), (ii) synchronization of bursts that are generated by a group of nonsymmetrically coupled inhibitory neurons (heteroclinic synchronization), (iii) the coordination of activities of two coupled neuronal networks (partial synchronization of small composite structures), and (iv) coarse grained synchronization in larger systems (synchronization on a mesoscopic scale
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&
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