227 research outputs found
Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell
Convection is one of the most important mixing processes in stellar
interiors. Hydrodynamic mass entrainment can bring fresh fuel from neighboring
stable layers into a convection zone, modifying the structure and evolution of
the star. Under some conditions, strong magnetic fields can be sustained by the
action of a turbulent dynamo, adding another layer of complexity and possibly
altering the dynamics in the convection zone and at its boundaries. In this
study, we used our fully compressible Seven-League Hydro code to run detailed
and highly resolved three-dimensional magnetohydrodynamic simulations of
turbulent convection, dynamo amplification, and convective boundary mixing in a
simplified setup whose stratification is similar to that of an oxygen-burning
shell in a star with an initial mass of . We find that the random
stretching of magnetic field lines by fluid motions in the inertial range of
the turbulent spectrum (i.e., a small-scale dynamo) naturally amplifies the
seed field by several orders of magnitude in a few convective turnover
timescales. During the subsequent saturated regime, the magnetic-to-kinetic
energy ratio inside the convective shell reaches values as high as , and
the average magnetic field strength is . Such strong
fields efficiently suppress shear instabilities, which feed the turbulent
cascade of kinetic energy, on a wide range of spatial scales. The resulting
convective flows are characterized by thread-like structures that extend over a
large fraction of the convective shell. The reduced flow speeds and the
presence of magnetic fields with strengths up to of the equipartition
value at the upper convective boundary diminish the rate of mass entrainment
from the stable layer by as compared to the purely
hydrodynamic case
A finite-volume scheme for modeling compressible magnetohydrodynamic flows at low Mach numbers in stellar interiors
Fully compressible magnetohydrodynamic (MHD) simulations are a fundamental
tool for investigating the role of dynamo amplification in the generation of
magnetic fields in deep convective layers of stars. The flows that arise in
such environments are characterized by low (sonic) Mach numbers (M_son < 0.01
). In these regimes, conventional MHD codes typically show excessive
dissipation and tend to be inefficient as the Courant-Friedrichs-Lewy (CFL)
constraint on the time step becomes too strict. In this work we present a new
method for efficiently simulating MHD flows at low Mach numbers in a
space-dependent gravitational potential while still retaining all effects of
compressibility. The proposed scheme is implemented in the finite-volume
Seven-League Hydro (SLH) code, and it makes use of a low-Mach version of the
five-wave Harten-Lax-van Leer discontinuities (HLLD) solver to reduce numerical
dissipation, an implicit-explicit time discretization technique based on Strang
splitting to overcome the overly strict CFL constraint, and a well-balancing
method that dramatically reduces the magnitude of spatial discretization errors
in strongly stratified setups. The solenoidal constraint on the magnetic field
is enforced by using a constrained transport method on a staggered grid. We
carry out five verification tests, including the simulation of a small-scale
dynamo in a star-like environment at M_son ~ 0.001 . We demonstrate that the
proposed scheme can be used to accurately simulate compressible MHD flows in
regimes of low Mach numbers and strongly stratified setups even with moderately
coarse grids
Towards a self-consistent model of the convective core boundary in upper-main-sequence stars
There is strong observational evidence that convective cores of
intermediate-mass and massive main-sequence stars are substantially larger than
standard stellar-evolution models predict. However, it is unclear what physical
processes cause this phenomenon or how to predict the extent and stratification
of stellar convective boundary layers. Convective penetration is a
thermal-time-scale process that is likely to be particularly relevant during
the slow evolution on the main sequence. We use our low-Mach-number
Seven-League Hydro (SLH) code to study this process in 2.5D and 3D geometries.
Starting with a chemically homogeneous model of a M zero-age
main-sequence star, we construct a series of simulations with the luminosity
increased and opacity decreased by the same factor ranging from to
. After reaching thermal equilibrium, all of our models show a clear
penetration layer. Its thickness becomes statistically constant in time and it
is shown to converge upon grid refinement. As the luminosity is decreased, the
penetration layer becomes nearly adiabatic with a steep transition to a
radiative stratification. This structure corresponds to the adiabatic ,,step
overshoot'' model often employed in stellar-evolution calculations. The
thickness of the penetration layer slowly decreases with decreasing luminosity.
Depending on how we extrapolate our 3D data to the actual luminosity of the
initial stellar model, we obtain penetration distances ranging from to
pressure scale heights, which are broadly compatible with observations.Comment: 10 pages, 12 figures, submitted to A&
Well-balanced treatment of gravity in astrophysical fluid dynamics simulations at low Mach numbers
Accurate simulations of flows in stellar interiors are crucial to improving
our understanding of stellar structure and evolution. Because the typically
slow flows are merely tiny perturbations on top of a close balance between
gravity and the pressure gradient, such simulations place heavy demands on
numerical hydrodynamics schemes. We demonstrate how discretization errors on
grids of reasonable size can lead to spurious flows orders of magnitude faster
than the physical flow. Well-balanced numerical schemes can deal with this
problem. Three such schemes were applied in the implicit, finite-volume
Seven-League Hydro (SLH) code in combination with a low-Mach-number numerical
flux function. We compare how the schemes perform in four numerical experiments
addressing some of the challenges imposed by typical problems in stellar
hydrodynamics. We find that the - and deviation well-balancing
methods can accurately maintain hydrostatic solutions provided that
gravitational potential energy is included in the total energy balance. They
accurately conserve minuscule entropy fluctuations advected in an isentropic
stratification, which enables the methods to reproduce the expected scaling of
convective flow speed with the heating rate. The deviation method also
substantially increases accuracy of maintaining stationary orbital motions in a
Keplerian disk on long timescales. The Cargo-LeRoux method fares substantially
worse in our tests, although its simplicity may still offer some merits in
certain situations. Overall, we find the well-balanced treatment of gravity in
combination with low Mach number flux functions essential to reproducing
correct physical solutions to challenging stellar slow-flow problems on
affordable collocated grids.Comment: Accepted for publication in A&
Comparison of Four Approaches to Age and Gender Recognition for Telephone Applications
This paper presents a comparative study of four different ap-proaches to automatic age and gender classification using seven classes on a telephony speech task and also compares the results with Human performance on the same data. The automatic approaches compared are based on (1) a parallel phone recognizer, derived from an automatic language identification system; (2) a system using dy-namic Bayesian networks to combine several prosodic features; (3) a system based solely on linear prediction analysis; and (4) Gaus-sian mixture models based on MFCCs for separate recognition of age and gender. On average, the parallel phone recognizer performs as well as Human listeners do, while loosing performance on short utterances. The system based on prosodic features however shows very little dependence on the length of the utterance. Index Terms — speech processing, acoustic signal analysis, speaker classification, age, gender 1
Community Research Report Wisbech
The Institute for Sustainability Leadership at Cambridge University (CISL) is carrying out research on how supermarkets can support community health and wellbeing. The CISL researchers were keen to understand what support really looked like on the ground, and what difference it made. It was really important to them that the voice and experience of community members was included in the research and so a team from the Centre for Health Promotion Research at Leeds Beckett University supported a group of residents from a town in Cambridgeshire to do a community research project. To explore their research topic, Leeds Beckett researchers used a peer research approach. They trained community members to carry out a research project and the Community Researchers chose the topic of the research: experiences of living in the Cambridgeshire town. This report shares the findings from the research project
Oxidation of HMGB1 Causes Attenuation of Its Pro-Inflammatory Activity and Occurs during Liver Ischemia and Reperfusion
High mobility group box 1 (HMGB1) is a nuclear transcription factor. Once HMGB1 is released by damaged cells or activated immune cells, it acts as danger molecule and triggers the inflammatory signaling cascade. Currently, evidence is accumulating that posttranslational modifications such as oxidation may modulate the pro-inflammatory potential of danger signals. We hypothesized that oxidation of HMGB1 may reduce its pro-inflammatory potential and could take place during prolonged ischemia and upon reperfusion
Coffee, Alcohol, Smoking, Physical Activity and QT Interval Duration: Results from the Third National Health and Nutrition Examination Survey
Abnormalities in the electrocardiographic QT interval duration have been associated with an increased risk of ventricular arrhythmias and sudden cardiac death. However, there is substantial uncertainty about the effect of modifiable factors such as coffee intake, cigarette smoking, alcohol consumption, and physical activity on QT interval duration.We studied 7795 men and women from the Third National Health and Nutrition Survey (NHANES III, 1988-1994). Baseline QT interval was measured from the standard 12-lead electrocardiogram. Coffee and tea intake, alcohol consumption, leisure-time physical activities over the past month, and lifetime smoking habits were determined using validated questionnaires during the home interview.In the fully adjusted model, the average differences in QT interval comparing participants drinking ≥6 cups/day to those who did not drink any were -1.2 ms (95% CI -4.4 to 2.0) for coffee, and -2.0 ms (-11.2 to 7.3) for tea, respectively. The average differences in QT interval duration comparing current to never smokers was 1.2 ms (-0.6 to 2.9) while the average difference in QT interval duration comparing participants drinking ≥7 drinks/week to non-drinkers was 1.8 ms (-0.5 to 4.0). The age, race/ethnicity, and RR-interval adjusted differences in average QT interval duration comparing men with binge drinking episodes to non-drinkers or drinkers without binge drinking were 2.8 ms (0.4 to 5.3) and 4.0 ms (1.6 to 6.4), respectively. The corresponding differences in women were 1.1 (-2.9 to 5.2) and 1.7 ms (-2.3 to 5.7). Finally, the average differences in QT interval comparing the highest vs. the lowest categories of total physical activity was -0.8 ms (-3.0 to 1.4).Binge drinking was associated with longer QT interval in men but not in women. QT interval duration was not associated with other modifiable factors including coffee and tea intake, smoking, and physical activity
The metastasis-associated protein S100A4 exists in several charged variants suggesting the presence of posttranslational modifications
<p>Abstract</p> <p>Background</p> <p>S100A4 is a metastasis-associated protein which has been linked to multiple cellular events, and has been identified extracellularly, in the cytoplasm and in the nucleus of tumor cells; however, the biological implications of subcellular location are unknown. Associations between a variety of posttranslational protein modifications and altered biological functions of proteins are becoming increasingly evident. Identification and characterization of posttranslationally modified S100A4 variants could thus contribute to elucidating the mechanisms for the many cellular functions that have been reported for this protein, and might eventually lead to the identification of novel drugable targets.</p> <p>Methods</p> <p>S100A4 was immuoprecipitated from a panel of <it>in vitro </it>and <it>in vivo </it>sources using a monoclonal antibody and the samples were separated by 2D-PAGE. Gels were analyzed by western blot and silver staining, and subsequently, several of the observed spots were identified as S100A4 by the use of MALDI-TOF and MALDI-TOF/TOF.</p> <p>Results</p> <p>A characteristic pattern of spots was observed when S100A4 was separated by 2D-PAGE suggesting the presence of at least three charge variants. These charge variants were verified as S100A4 both by western immunoblotting and mass spectrometry, and almost identical patterns were observed in samples from different tissues and subcellular compartments. Interestingly, recombinant S100A4 displayed a similar pattern on 2D-PAGE, but with different quantitative distribution between the observed spots.</p> <p>Conclusion</p> <p>Endogenously expressed S100A4 were shown to exist in several charge variants, which indicates the presence of posttranslational modifications altering the net charge of the protein. The different variants were present in all subcellular compartments and tissues/cell lines examined, suggesting that the described charge variants is a universal phenomenon, and cannot explain the localization of S100A4 in different subcellular compartments. However, the identity of the specific posttranslational modification and its potential contribution to the many reported biological events induced by S100A4, are subject to further studies.</p
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