175 research outputs found
Turbulent convection: comparing the moment equations to numerical simulations
The non-local hydrodynamic moment equations for compressible convection are
compared to numerical simulations. Convective and radiative flux typically
deviate less than 20% from the 3D simulations, while mean thermodynamic
quantities are accurate to at least 2% for the cases we have investigated. The
moment equations are solved in minutes rather than days on standard
workstations. We conclude that this convection model has the potential to
considerably improve the modelling of convection zones in stellar envelopes and
cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures),
Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ
Detection of Water Vapor in the Photosphere of Arcturus
We report detections of pure rotation lines of OH and H2O in the K1.5 III
red-giant star Arcturus (alpha Bootis) using high-resolution, infrared spectra
covering the regions 806-822 cm-1 (12.2-12.4 um) and 884-923 cm-1 (10.8-11.3
um). Arcturus is the hottest star yet to show water-vapor features in its
disk-averaged spectrum. We argue that the water vapor lines originate from the
photosphere, albeit in the outer layers. We are able to predict the observed
strengths of OH and H2O lines satisfactorily after lowering the temperature
structure of the very outer parts of the photosphere (log tau_500=-3.8 and
beyond) compared to a flux-constant, hydrostatic, standard MARCS model
photosphere. Our new model is consistently calculated including chemical
equilibrium and radiative transfer from the given temperature structure.
Possible reasons for a temperature decrease in the outer-most parts of the
photosphere and the assumed break-down of the assumptions made in classical
model-atmosphere codes are discussed.Comment: To appear in ApJ. See also http://www.astro.uu.se/~ryde/ART
Central immune tolerance depends on crosstalk between the classical and alternative NF-κB pathways in medullary thymic epithelial cells
Medullary thymic epithelial cells (mTECs) contribute to self-tolerance by expressing and presenting peripheral tissue antigens for negative selection of autoreactive T cells and differentiation of natural regulatory T cells. The molecular control of mTEC development remains incompletely understood. We here demonstrate by TEC-specific gene manipulation in mice that the NF-κB transcription factor subunit RelB, which is activated by the alternative NF-κB pathway, regulates development of mature mTECs in a dose-dependent manner. Mice with conditional deletion of Relb lacked mature mTECs and developed spontaneous autoimmunity. In addition, the NF-κB subunits RelA and c-Rel, which are both activated by classical NF-κB signaling, were jointly required for mTEC differentiation by directly regulating the transcription of Relb. Our data reveal a crosstalk mechanism between classical and alternative NF-κB pathways that tightly controls the development of mature mTECs to ensure self-tolerance
The s Process: Nuclear Physics, Stellar Models, Observations
Nucleosynthesis in the s process takes place in the He burning layers of low
mass AGB stars and during the He and C burning phases of massive stars. The s
process contributes about half of the element abundances between Cu and Bi in
solar system material. Depending on stellar mass and metallicity the resulting
s-abundance patterns exhibit characteristic features, which provide
comprehensive information for our understanding of the stellar life cycle and
for the chemical evolution of galaxies. The rapidly growing body of detailed
abundance observations, in particular for AGB and post-AGB stars, for objects
in binary systems, and for the very faint metal-poor population represents
exciting challenges and constraints for stellar model calculations. Based on
updated and improved nuclear physics data for the s-process reaction network,
current models are aiming at ab initio solution for the stellar physics related
to convection and mixing processes. Progress in the intimately related areas of
observations, nuclear and atomic physics, and stellar modeling is reviewed and
the corresponding interplay is illustrated by the general abundance patterns of
the elements beyond iron and by the effect of sensitive branching points along
the s-process path. The strong variations of the s-process efficiency with
metallicity bear also interesting consequences for Galactic chemical evolution.Comment: 53 pages, 20 figures, 3 tables; Reviews of Modern Physics, accepte
Water Vapor on Betelgeuse as Revealed by TEXES High-Resolution 12 Micron Spectra
The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published
observations of different kinds have shed light on different aspects of the
atmosphere, but no unified picture has emerged. They have shown, for example,
evidence of a water envelope (MOLsphere) that in some studies is found to be
optically thick in the mid-infrared. In this paper, we present high-resolution,
mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The
spectra clearly show absorption features of water vapor and OH. We show that a
spectrum based on a spherical, hydrostatic model photosphere with T_eff = 3600
K, an effective temperature often assumed for Betelgeuse, fails to model the
observed lines. Furthermore, we show that published MOLspheres scenarios are
unable to explain our data. However, we are able to model the observed spectrum
reasonably well by adopting a cooler outer photospheric structure corresponding
to T_mod = 3250 K. The success of this model may indicate the observed
mid-infrared lines are formed in cool photospheric surface regions. Given the
uncertainties of the temperature structure and the likely presence of
inhomogeneities, we cannot rule out the possibility that our spectrum could be
mostly photospheric, albeit non-classical. Our data put new, strong constraints
on atmospheric models of Betelgeuse and we conclude that continued
investigation requires consideration of non-classical model photospheres as
well as possible effects of a MOLsphere. We show that the mid-infrared
water-vapor features have great diagnostic value for the environments of K and
M (super-) giant star atmospheres.Comment: Accepted by Ap
s-Process Nucleosynthesis in AGB Stars: A Test for Stellar Evolution
[abridged] We study the s-process in AGB stars using three different stellar
evolutionary models computed for a 3Msun and solar metallicity star. First we
investigate the formation and the efficiency of the main neutron source. We
parametrically vary the number of protons mixed from the envelope into the C12
rich core. For p/C12 > 0.3, mainly N14 is produced, which represent a major
neutron poison. The amount of C12 in the He intershell and the maximum value of
the time-integrated neutron flux are proportional. Then we generate detailed
s-process calculations on the basis of stellar evolutionary models constructed
with three different codes. One code considers convective hydrodynamic
overshoot that depends on a free parameter f, and results in partial mixing
beyond convective boundaries, the most efficient third dredge up and the
formation of the C13 pocket. For the other two codes an identical C13 pocket is
introduced in the post-processing nucleosynthesis calculations. The models
generally reproduce the spectroscopically observed s-process enhancements. The
results of the cases without overshoot are remarkably similar. The code
including hydrodynamic overshoot produces a He intershell composition near to
that observed in H-deficient central stars of planetary nebulae. As a result of
this intershell dredge up the neutron fluxes have a higher efficiency, both
during the interpulse periods and within thermal pulses. The s-element
distribution is pushed toward the heavier s-process elements and large
abundances of neutron-rich isotopes fed by branching points in the s-process
path are produced. Several observational constraints are better matched by the
models without overshoot. Our study need to be extended to different masses and
metallicities and in the space of the free overshoot parameter f.Comment: 44 pages, incl 10 figures, accepted for publication in Ap
Atmospheric Heating and Wind Acceleration: Results for Cool Evolved Stars based on Proposed Processes
A chromosphere is a universal attribute of stars of spectral type later than
~F5. Evolved (K and M) giants and supergiants (including the zeta Aurigae
binaries) show extended and highly turbulent chromospheres, which develop into
slow massive winds. The associated continuous mass loss has a significant
impact on stellar evolution, and thence on the chemical evolution of galaxies.
Yet despite the fundamental importance of those winds in astrophysics, the
question of their origin(s) remains unsolved. What sources heat a chromosphere?
What is the role of the chromosphere in the formation of stellar winds? This
chapter provides a review of the observational requirements and theoretical
approaches for modeling chromospheric heating and the acceleration of winds in
single cool, evolved stars and in eclipsing binary stars, including physical
models that have recently been proposed. It describes the successes that have
been achieved so far by invoking acoustic and MHD waves to provide a physical
description of plasma heating and wind acceleration, and discusses the
challenges that still remain.Comment: 46 pages, 9 figures, 1 table; modified and unedited manuscript;
accepted version to appear in: Giants of Eclipse, eds. E. Griffin and T. Ake
(Berlin: Springer
The Element Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot hydrogen-deficient
post-AGB stars of spectral type [WC] and PG1159. Their H-deficiency is probably
caused by a (very) late helium-shell flash or a AGB final thermal pulse, laying
bare interior stellar regions which are usually kept hidden below the hydrogen
envelope. Thus, the photospheric element abundances of these stars allow to
draw conclusions about details of nuclear burning and mixing processes in the
precursor AGB stars. We summarize the state-of-the-art of stellar evolution
models which simulate AGB evolution and the occurrence of a late He-shell
flash. We compare predicted element abundances to those determined by
quantitative spectral analyses performed with advanced non-LTE model
atmospheres. A good qualitative and quantitative agreement is found. Future
work can contribute to an even more complete picture of the nuclear processes
in AGB stars.Comment: Review, accepted for publication in PASP, Febr. 06 issue. For high
resolution versions of Figures 1 and 6 see preprint on
http://astro.uni-tuebingen.de/publications/paper_05_05.shtm
A Gap Analysis Methodology for Collecting Crop Genepools: A Case Study with Phaseolus Beans
Background The wild relatives of crops represent a major source of valuable traits for crop improvement. These resources are threatened by habitat destruction, land use changes, and other factors, requiring their urgent collection and long-term availability for research and breeding from ex situ collections. We propose a method to identify gaps in ex situ collections (i.e. gap analysis) of crop wild relatives as a means to guide efficient and effective collecting activities. Methodology/Principal Findings The methodology prioritizes among taxa based on a combination of sampling, geographic, and environmental gaps. We apply the gap analysis methodology to wild taxa of the Phaseolus genepool. Of 85 taxa, 48 (56.5%) are assigned high priority for collecting due to lack of, or under-representation, in genebanks, 17 taxa are given medium priority for collecting, 15 low priority, and 5 species are assessed as adequately represented in ex situ collections. Gap “hotspots”, representing priority target areas for collecting, are concentrated in central Mexico, although the narrow endemic nature of a suite of priority species adds a number of specific additional regions to spatial collecting priorities. Conclusions/Significance Results of the gap analysis method mostly align very well with expert opinion of gaps in ex situ collections, with only a few exceptions. A more detailed prioritization of taxa and geographic areas for collection can be achieved by including in the analysis predictive threat factors, such as climate change or habitat destruction, or by adding additional prioritization filters, such as the degree of relatedness to cultivated species (i.e. ease of use in crop breeding). Furthermore, results for multiple crop genepools may be overlaid, which would allow a global analysis of gaps in ex situ collections of the world's plant genetic resource
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