171 research outputs found
Simulation of stellar instabilities with vastly different timescales using domain decomposition
Strange mode instabilities in the envelopes of massive stars lead to shock
waves, which can oscillate on a much shorter timescale than that associated
with the primary instability. The phenomenon is studied by direct numerical
simulation using a, with respect to time, implicit Lagrangian scheme, which
allows for the variation by several orders of magnitude of the dependent
variables. The timestep for the simulation of the system is reduced appreciably
by the shock oscillations and prevents its long term study. A procedure based
on domain decomposition is proposed to surmount the difficulty of vastly
different timescales in various regions of the stellar envelope and thus to
enable the desired long term simulations. Criteria for domain decomposition are
derived and the proper treatment of the resulting inner boundaries is
discussed. Tests of the approach are presented and its viability is
demonstrated by application to a model for the star P Cygni. In this
investigation primarily the feasibility of domain decomposition for the problem
considered is studied. We intend to use the results as the basis of an
extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA
Zinc oxide as an ozone sensor
Journal of Applied Physics, Vol. 96, nº3This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet sUVd photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf
magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4 mW/cm2. On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose,
with a response frequency above 15 kHz, thus showing improved ozone ac sensing
discrimination
On the stability of very massive primordial stars
The stability of metal-free very massive stars ( = 0; M = 120 - 500
\msol) is analyzed and compared with metal-enriched stars. Such zero-metal
stars are unstable to nuclear-powered radial pulsations on the main sequence,
but the growth time scale for these instabilities is much longer than for their
metal-rich counterparts. Since they stabilize quickly after evolving off the
ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss
in Z = 0 stars. For reasonable assumptions regarding the efficiency of
converting pulsational energy into mass loss, we find that, even for the larger
masses considered, the star may die without losing a large fraction of its
mass. We find a transition between the - and -mechanisms for
pulsational instability at Z\sim 2\E{-4} - 2\E{-3}. For the most metal-rich
stars, the -mechanism yields much shorter -folding times, indicating
the presence of a strong instability. We thus stress the fundamental difference
of the stability and late stages of evolution between very massive stars born
in the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1,
accepted for publication in Ap
Instability of LBV-stars against radial oscillations
In this study we consider the nonlinear radial oscillations exciting in
LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric
luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <=
49.1 M_odot. Hydrodynamic computations were carried out with initial conditions
obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with
initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show
instability against radial oscillations with amplitude growth time comparable
with dynamical time scale of the star. Radial oscillations exist in the form of
nonlinear running waves propagating from the boundary of the compact core to
the upper boundary of the hydrodynamical model. The velocity amplitude of outer
layers is of several hundreds of km/s while the bolometric light amplitude does
not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism
and are due to the instability of the gas with adiabatic exponent close to the
critical value Gamma_1 = 4/3 due to the large contribution of radiation in the
total pressure. The range of the light variation periods (6 day <= P <= 31 day)
of hydrodynamical models agrees with periods of microvariability observed in
LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter
Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)
The {\it Microvariability and Oscillations of Stars (MOST)} satellite
observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and
detected 48 frequencies \la 2.8 c d with amplitudes of a few
milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode
pulsations. It was generally thought that no g-modes are excited in less
luminous B supergiants because strong radiative damping is expected in the
core. Our theoretical models, however, show that such g-modes are excited in
massive post-main-sequence stars, in accordance with these observations. The
nonradial pulsations excited in models between at and at are roughly
consistent with the observed frequency range. Excitation by the Fe-bump in
opacity is possible because g-modes can be partially reflected at a convective
zone associated with the hydrogen-burning shell, which significantly reduces
radiative damping in the core. The {\it MOST} light curve of HD 163899 shows
that such a reflection of g-modes actually occurs, and reveals the existence of
a previously unrecognized type of variable, slowly pulsating B supergiants
(SPBsg) distinct from Cyg variables. Such g-modes have great potential
for asteroseismology.Comment: 24 pages, 9 figures, Astrophysical Journal in pres
Alterations in the gut microbiome implicate key taxa and metabolic pathways across inflammatory arthritis phenotypes
Musculoskeletal diseases affect up to 20% of adults worldwide. The gut microbiome has been implicated in inflammatory conditions, but large-scale metagenomic evaluations have not yet traced the routes by which immunity in the gut affects inflammatory arthritis. To characterize the community structure and associated functional processes driving gut microbial involvement in arthritis, the Inflammatory Arthritis Microbiome Consortium investigated 440 stool shotgun metagenomes comprising 221 adults diagnosed with rheumatoid arthritis, ankylosing spondylitis, or psoriatic arthritis and 219 healthy controls and individuals with joint pain without an underlying inflammatory cause. Diagnosis explained about 2% of gut taxonomic variability, which is comparable in magnitude to inflammatory bowel disease. We identified several candidate microbes with differential carriage patterns in patients with elevated blood markers for inflammation. Our results confirm and extend previous findings of increased carriage of typically oral and inflammatory taxa and decreased abundance and prevalence of typical gut clades, indicating that distal inflammatory conditions, as well as local conditions, correspond to alterations to the gut microbial composition. We identified several differentially encoded pathways in the gut microbiome of patients with inflammatory arthritis, including changes in vitamin B salvage and biosynthesis and enrichment of iron sequestration. Although several of these changes characteristic of inflammation could have causal roles, we hypothesize that they are mainly positive feedback responses to changes in host physiology and immune homeostasis. By connecting taxonomic alternations to functional alterations, this work expands our understanding of the shifts in the gut ecosystem that occur in response to systemic inflammation during arthritis
Large-scale wind structures in OB supergiants: a search for rotationally modulated H\alpha variability
We present the results of a long-term monitoring campaign of the H\alpha line
in a sample of bright OB-supergiants (O7.5-B9) that aims at detecting
rotationally modulated changes potentially related to the existence of
large-scale wind structures. A total of 22 objects were monitor ed during 36
nights spread over 6 months in 2001-2002. Coordinated broad-band photometric
observations were also obtained for some targets. Conspicuous evidence for
variability in H\alpha is found for the stars displaying a feature contaminated
by wind emission. Most changes take place on a daily time-scale, although
hourly variations are also occasionally detected. Convincing evidence for a
cyclical pattern of variability in H\alpha has been found in 2 stars: HD 14134
and HD 42087 (periodic signals are also detected in other stars, but
independent confirmation is required). Rotational modulation is suggested from
the similarity between the observed recurrence time-scales (in the range 13-25
days) and estimated periods of stellar rotation. We call attention to the
atypical case of HD 14134 which exhibits a clear 12.8-d periodicity both in the
photometric and in the spectroscopic data sets. This places this object among a
handful of early-type stars where one may observe a clear link between extended
wind structures and photospheric disturbances. Further modelling may test the
hypothesis that azimuthally-extended wind streams are responsible for the
patterns of spectral variability in our target stars.Comment: 18 pages, accepted for publication in MNRA
Spatio-temporal Models of Lymphangiogenesis in Wound Healing
Several studies suggest that one possible cause of impaired wound healing is
failed or insufficient lymphangiogenesis, that is the formation of new
lymphatic capillaries. Although many mathematical models have been developed to
describe the formation of blood capillaries (angiogenesis), very few have been
proposed for the regeneration of the lymphatic network. Lymphangiogenesis is a
markedly different process from angiogenesis, occurring at different times and
in response to different chemical stimuli. Two main hypotheses have been
proposed: 1) lymphatic capillaries sprout from existing interrupted ones at the
edge of the wound in analogy to the blood angiogenesis case; 2) lymphatic
endothelial cells first pool in the wound region following the lymph flow and
then, once sufficiently populated, start to form a network. Here we present two
PDE models describing lymphangiogenesis according to these two different
hypotheses. Further, we include the effect of advection due to interstitial
flow and lymph flow coming from open capillaries. The variables represent
different cell densities and growth factor concentrations, and where possible
the parameters are estimated from biological data. The models are then solved
numerically and the results are compared with the available biological
literature.Comment: 29 pages, 9 Figures, 6 Tables (39 figure files in total
Alterations in the gut microbiome implicate key taxa and metabolic pathways across inflammatory arthritis phenotypes
Musculoskeletal diseases affect up to 20% of adults worldwide. The gut microbiome has been implicated in inflammatory conditions, but large-scale metagenomic evaluations have not yet traced the routes by which immunity in the gut affects inflammatory arthritis. To characterize the community structure and associated functional processes driving gut microbial involvement in arthritis, the Inflammatory Arthritis Microbiome Consortium investigated 440 stool shotgun metagenomes comprising 221 adults diagnosed with rheumatoid arthritis, ankylosing spondylitis, or psoriatic arthritis and 219 healthy controls and individuals with joint pain without an underlying inflammatory cause. Diagnosis explained about 2% of gut taxonomic variability, which is comparable in magnitude to inflammatory bowel disease. We identified several candidate microbes with differential carriage patterns in patients with elevated blood markers for inflammation. Our results confirm and extend previous findings of increased carriage of typically oral and inflammatory taxa and decreased abundance and prevalence of typical gut clades, indicating that distal inflammatory conditions, as well as local conditions, correspond to alterations to the gut microbial composition. We identified several differentially encoded pathways in the gut microbiome of patients with inflammatory arthritis, including changes in vitamin B salvage and biosynthesis and enrichment of iron sequestration. Although several of these changes characteristic of inflammation could have causal roles, we hypothesize that they are mainly positive feedback responses to changes in host physiology and immune homeostasis. By connecting taxonomic alternations to functional alterations, this work expands our understanding of the shifts in the gut ecosystem that occur in response to systemic inflammation during arthritis
TNF autovaccination induces self anti-TNF antibodies and inhibits metastasis in a murine melanoma model
TNF is a proinflammatory cytokine involved in the pathogenesis of chronic inflammatory diseases, but also in metastasis in certain types of cancer. In terms of therapy, TNF is targeted by anti-TNF neutralising monoclonal antibodies or soluble TNF receptors. Recently, a novel strategy based on the generation of self anti-TNF antibodies (TNF autovaccination) has been developed. We have previously shown that TNF autovaccination successfully generates high anti-TNF antibody titres, blocks TNF and ameliorates collagen-induced arthritis in DBA/1 mice. In this study, we examined the ability of TNF autovaccination to generate anti-TNF antibody titres and block metastasis in the murine B16F10 melanoma model. We found that immunisation of C57BL/6 mice with TNF autovaccine produces a 100-fold antibody response to TNF compared to immunisation with phosphate-buffered saline vehicle control and significantly reduces both the number (P<0.01) and size of metastases (P<0.01) of B16F10 melanoma cells. This effect is also observed when an anti-TNF neutralising monoclonal antibody is administered, confirming the essential role TNF plays in metastasis in this model. This study suggests that TNF autovaccination is a cheaper and highly efficient alternative that can block TNF and reduce metastasis in vivo and trials with TNF autovaccination are already underway in patients with metastatic cancer
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