163 research outputs found
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 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
Numerical simulations of stellar SiO maser variability. Investigation of the effect of shocks
A stellar hydrodynamic pulsation model has been combined with a SiO maser
model in an attempt to calculate the temporal variability of SiO maser emission
in the circumstellar envelope (CE) of a model AGB star. This study investigates
whether the variations in local physical conditions brought about by shocks are
the predominant contributing factor to SiO maser variability because, in this
work, the radiative part of the pump is constant. We find that some aspects of
the variability are not consistent with a pump provided by shock-enhanced
collisions alone. In these simulations, gas parcels of relatively enhanced SiO
abundance are distributed in a model CE by a Monte Carlo method, at a single
epoch of the stellar cycle. From this epoch on, Lagrangian motions of
individual parcels are calculated according to the velocity fields encountered
in the model CE during the stellar pulsation cycle. The potentially masing gas
parcels therefore experience different densities and temperatures, and have
varying line-of-sight velocity gradients throughout the stellar cycle, which
may or may not be suitable to produce maser emission. At each epoch (separated
by 16.6 days), emission lines from the parcels are combined to produce
synthetic spectra and VLBI-type images. We report here the results for v=1,
J=1-0 (43-GHz) and J=2-1 (86-GHz) masers.Comment: 16 pages, 8 figures, accepted by A&
The extended molecular envelope of the asymptotic giant branch star Gruis as seen by ALMA II. The spiral-outflow observed at high-angular resolution
The AGB star Gruis has a known companion (at a separation of ~400
AU) which cannot explain the strong deviations from the spherical symmetry of
the CSE. Recently, hydrodynamic simulations of mass transfer in closer binary
systems have successfully reproduced the spiral-shaped CSEs found around a
handful of sources. There is growing evidence for an even closer, undetected
companion complicating the case of Gruis further. The improved
spatial resolution allows for the investigation of the complex circumstellar
morphology and the search for imprints on the CSE of the third component. We
have observed the 12CO J=3-2 line emission from Gruis using both the
compact and extended array of Atacama Large Millimeter/submillimeter Array
(ALMA). The interferometric data has furthermore been combined with data from
the ALMA total power (TP) array. The imaged brightness distribution has been
used to constrain a non-local, non-LTE 3D radiative transfer model of the CSE.
The high-angular resolution ALMA data have revealed the first example of a
source on the AGB where both a faster bipolar outflow and a spiral pattern
along the orbital plane can be seen in the gas envelope. The spiral can be
traced in the low- to intermediate velocity, from 13 to 25 km s,
equatorial torus. The largest spiral-arm separation is 5".5 and
consistent with a companion with an orbital period of 330 yrs and a
separation of less than 70 AU. The kinematics of the bipolar outflow is
consistent with it being created during a mass-loss eruption where the
mass-loss rate from the system increased by at least a factor of 5 during 10-15
yrs. The spiral pattern is the result of an undetected companion. The bipolar
outflow is the result of a rather recent mass-loss eruption event.Comment: 12 pages, 11 figure
Homochiral growth through enantiomeric cross-inhibition
The stability and conservation properties of a recently proposed
polymerization model are studied. The achiral (racemic) solution is linearly
unstable once the relevant control parameter (here the fidelity of the
catalyst) exceeds a critical value. The growth rate is calculated for different
fidelity parameters and cross-inhibition rates. A chirality parameter is
defined and shown to be conserved by the nonlinear terms of the model. Finally,
a truncated version of the model is used to derive a set of two ordinary
differential equations and it is argued that these equations are more realistic
than those used in earlier models of that form.Comment: 20 pages, 6 figures, Orig. Life Evol. Biosph. (accepted
Dust-driven Winds and Mass Loss of C-rich AGB Stars with subsolar Metallicities
We investigate the mass loss of highly evolved, low- and intermediate mass
stars and stellar samples with subsolar metallicity. We give a qualitative as
well as quantitative description which can be applied to LMC/SMC-type stellar
populations. For that purpose we apply the same approach as we did for solar
metallicity stars and calculate hydrodynamical wind models including dust
formation with LMC and SMC abundances under consideration of an adapted model
assumption. In particular, we improved the treatment of the radiative transfer
problem in order to accommodate larger non-local contributions occurring with
smaller opacities. For each wind model we determine an averaged mass-loss rate.
The resulting, approximate mass-loss formulae are then applied to well-tested
and calibrated stellar evolution calculations in order to quantify the stellar
mass loss. The dynamical models for LMC and SMC metallicity result in mass-loss
rates of the same order of magnitude as the solar metallicity models which is
in this basic approach in agreement with observations. The hydrodynamical
properties like e.g. the outflow velocity differ (for fixed C/O abundance
ratio) noticeably, though. While critical luminosities of LMC and solar
metallicity models fairly coincide, the SMC models need higher luminosities to
develop dust-driven winds.Comment: 8 pages, 4 figures. Accepted for publication in A&
Evolved star water maser cloud size determined by star size
Cool, evolved stars undergo copious mass loss but the details of how the
matter is returned to the ISM are still under debate. We investigated the
structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic
Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved
stars were imaged using MERLIN, at sub-AU resolution. Each source was observed
at between 2 and 7 epochs (several stellar periods). We compared our results
with long-term Pushchino single dish monitoring. The 22-GHz emission is located
in ~spherical, thick, unevenly filled shells. The outflow velocity doubles
between the inner and outer shell limits. Water maser clumps could be matched
at successive epochs separated by <2 years for AGB stars, or at least 5 years
for RSG. This is much shorter than the decades taken for the wind to cross the
maser shell, and comparison with spectral monitoring shows that some features
fade and reappear. In 5 sources, most of the matched features brighten or dim
in concert from one epoch to the next. One cloud in W Hya was caught in the act
of passing in front of a background cloud leading to 50-fold, transient
amplification. The masing clouds are 1-2 orders of magnitude denser than the
wind average and contain a substantial fraction of the mass loss in this
region, with a filling factor <1%. The RSG clouds are ~10x bigger than those
round the AGB stars. Proper motions are dominated by expansion, with no
systematic rotation. The maser clouds survive for decades (the shell crossing
time) but the masers are not always beamed in our direction. Radiative effects
cause changes in flux density throughout the maser shells on short timescales.
Cloud size is proportional to parent star size; clouds have a similar radius to
the star in the 22-GHz maser shell. Stellar properties such as convection cells
must determine the clumping scale.Comment: Accepted by A&A 2012 July 10 Main text 29 pages, 62 figures Appendix
44 pages, 23 figure
The propagation of uncertainties in stellar population synthesis modeling III: model calibration, comparison, and evaluation
Stellar population synthesis (SPS) provides the link between the stellar and
dust content of galaxies and their observed spectral energy distributions. In
the present work we perform a comprehensive calibration of our own flexible SPS
(FSPS) model against a suite of data. Several public SPS models are
intercompared, including the models of Bruzual & Charlot (BC03), Maraston (M05)
and FSPS. The relative strengths and weaknesses of these models are evaluated,
with the following conclusions: 1) The FSPS and BC03 models compare favorably
with MC data at all ages, whereas M05 colors are too red and the age-dependence
is incorrect; 2) All models yield similar optical and near-IR colors for old
metal-poor systems, and yet they all provide poor fits to the integrated J-K
and V-K colors of both MW and M31 star clusters; 4) All models predict ugr
colors too red, D4000 strengths too strong and Hdelta strengths too weak
compared to massive red sequence galaxies, under the assumption that such
galaxies are composed solely of old metal-rich stars; 5) FSPS and, to a lesser
extent, BC03 can reproduce the optical and near-IR colors of post-starburst
galaxies, while M05 cannot. Reasons for these discrepancies are explored. The
failure at predicting the ugr colors, D4000, and Hdelta strengths can be
explained by some combination of a minority population of metal-poor stars,
young stars, blue straggler and/or blue horizontal branch stars, but not by
appealing to inadequacies in either theoretical stellar atmospheres or
canonical evolutionary phases (e.g., the main sequence turn-off). We emphasize
that due to a lack of calibrating star cluster data in regions of the
metallicity-age plane relevant for galaxies, all of these models continue to
suffer from serious uncertainties that are difficult to quantify. (ABRIDGED)Comment: 26 pages, 16 figures, submitted to ApJ. The FSPS code can be
downloaded at http://www.astro.princeton.edu/~cconroy/SPS
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
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