326 research outputs found
Der Grünauslauf als Futterquelle
Der Grünauslauf verbessert nicht nur die Gesundheit der Tiere und die Qualität der Eier. Er kann, wenn er optimal eingerichtet ist, auch zu einer Einsparung am Mischfutter führen
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
Autowaves in a dc complex plasma confined behind a de Laval nozzle
Experiments to explore stability conditions and topology of a dense
microparticle cloud supported against gravity by a gas flow were carried out.
By using a nozzle shaped glass insert within the glass tube of a dc discharge
plasma chamber a weakly ionized gas flow through a de Laval nozzle was
produced. The experiments were performed using neon gas at a pressure of 100 Pa
and melamine-formaldehyde particles with a diameter of 3.43 {\mu}m. The
capturing and stable global confining of the particles behind the nozzle in the
plasma were demonstrated. The particles inside the cloud behaved as a single
convection cell inhomogeneously structured along the nozzle axis in a tube-like
manner. The pulsed acceleration localized in the very head of the cloud
mediated by collective plasma-particle interactions and the resulting wave
pattern were studied in detail.Comment: 6 pages, 4 figure
A random laser as a dynamical network
The mode dynamics of a random laser is investigated in experiment and theory. The laser consists of a ZnCdO/ZnO multiple quantum well with air-holes that provide the necessary feedback. Time-resolved measurements reveal multi-mode spectra with individually developing features but no variation from shot to shot. These findings are qualitatively reproduced with a model that exploits the specifics of a dilute system of weak scatterers and can be interpreted in terms of a lasing network. Introducing the phase-sensitive node coherence reveals new aspects of the self-organization of the laser field. Lasing is carried by connected links between a subset of scatterers, the fields on which are oscillating coherently in phase. In addition, perturbing feedback with possibly unfitting phases from frustrated other scatterers is suppressed by destructive superposition. We believe that our findings are representative at least for weakly scattering random lasers. A generalization to random laser with dense and strong scatterers seems to be possible when using a more complex scattering theory for this case.Peer Reviewe
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&
The mass-loss return from evolved stars to the Large Magellanic Cloud V. The GRAMS carbon-star model grid
The total dust return rate from AGB and RSG star outflows is an important
constraint to galactic chemical evolution models. However, this requires
detailed radiative transfer (RT) modeling of individual stars, which becomes
impractical for large data sets. Another approach is to select the best-fit
spectral energy distribution (SED) from a grid of dust shell models, allowing
for a faster determination of the luminosities and mass-loss rates for entire
samples. We have developed the Grid of RSG and AGB ModelS (GRAMS) to measure
the mass-loss return from evolved stars. The models span the range of stellar,
dust shell and grain properties relevant to evolved stars. In this paper we
present the carbon-star grid and compare our results with data of Large
Magellanic Cloud (LMC) carbon stars from the SAGE and SAGE-Spec programs. We
generate spherically symmetric dust shell models using the 2Dust code, with
hydrostatic models for the central stars. We explore five values of the inner
radius R_in of the dust shell (1.5, 3, 4.5, 7 and 12 R_star). We use amorphous
carbon dust mixed with 10% silicon carbide by mass. The grain sizes follows a
KMH distribution. The models span 26 values of 11.3 um optical depth, ranging
from 0.001 to 4. For each model, 2Dust calculates the output SED from 0.2 to
200 um. Over 12,000 models have dust temperatures below 1800 K. The GRAMS
synthetic photometry is in good agreement with SAGE photometry for LMC
carbon-rich and extreme AGB star candidates, as well as spectroscopically
confirmed carbon stars from the SAGE-Spec study. Our models reproduce the IRAC
colors of most of the extreme AGB star candidates, consistent with the
expectation that a majority of these enshrouded stars have carbon-rich dust.
Finally, we fit the SEDs of some well-studied carbon stars and compare the
resulting luminosities and mass-loss rates with those from previous studies.Comment: 16 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
The time variation in infrared water-vapour bands in Mira variables
The time variation in the water-vapour bands in oxygen-rich Mira variables
has been investigated using multi-epoch ISO/SWS spectra of four Mira variables
in the 2.5-4.0 micron region. All four stars show H2O bands in absorption
around minimum in the visual light curve. At maximum, H2O emission features
appear in the ~3.5-4.0 micronm region, while the features at shorter
wavelengths remain in absorption. These H2O bands in the 2.5-4.0 micron region
originate from the extended atmosphere.
The analysis has been carried out with a disk shape, slab geometry model. The
observed H2O bands are reproduced by two layers; a `hot' layer with an
excitation temperature of 2000 K and a `cool' layer with an excitation
temperature of 1000-1400 K. The radii of the `hot' layer (R_hot) are ~1 R_* at
visual minimum and 2 R_* at maximum, where R_* is a radius of background source
of the model. The time variation of R_hot/R_* from 1 to 2 is attributed to the
actual variation in the radius of the H2O layer. A high H2O density shell
occurs near the surface of the star around minimum, and moves out with the
stellar pulsation. This shell gradually fades away after maximum, and a new
high H2O density shell is formed in the inner region again at the next minimum.
Due to large optical depth of H2O, the near-infrared variability is dominated
by the H2O layer, and the L'-band flux correlates with the area of the H2O
shell. The infrared molecular bands trace the structure of the extended
atmosphere and impose appreciable effects on near-infrared light curve of Mira
variables.Comment: 15 pages, 16 figures, accepted by A&
Radio and IR interferometry of SiO maser stars
Radio and infrared interferometry of SiO maser stars provide complementary
information on the atmosphere and circumstellar environment at comparable
spatial resolution. Here, we present the latest results on the atmospheric
structure and the dust condensation region of AGB stars based on our recent
infrared spectro-interferometric observations, which represent the environment
of SiO masers. We discuss, as an example, new results from simultaneous VLTI
and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near-
and mid-infrared interferometry, as well as VLBA observations of the SiO maser
emission toward this source. We present preliminary results from a monitoring
campaign of high-frequency SiO maser emission toward evolved stars obtained
with the APEX telescope, which also serves as a precursor of ALMA images of the
SiO emitting region. We speculate that large-scale long-period chaotic motion
in the extended molecular atmosphere may be the physical reason for observed
deviations from point symmetry of atmospheric molecular layers, and for the
observed erratic variability of high-frequency SiO maser emissionComment: 8 pages, 4 figures, submitted to Proc. IAU Symp. 287 "Cosmic masers -
from OH to H_0", R.S. Booth, E.M.L. Humphreys, W.H.T. Vlemmings (eds.),
invited pape
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