6,429 research outputs found
X-ray, UV and optical analysis of supergiants: Ori
We present a multi-wavelength (X-ray to optical) analysis, based on non-local
thermodynamic equilibrium photospheric+wind models, of the B0 Ia-supergiant:
~Ori. The aim is to test the consistency of physical parameters, such
as the mass-loss rate and CNO abundances, derived from different spectral
bands. The derived mass-loss rate is
1.610 M yr where
is the volume filling factor. However, the S IV
1062,1073 profiles are too strong in the models; to fit the
observed profiles it is necessary to use 0.01. This value is a
factor of 5 to 10 lower than inferred from other diagnostics, and implies
M yr. The discrepancy could
be related to porosity-vorosity effects or a problem with the ionization of
sulfur in the wind. To fit the UV profiles of N V and O VI it was necessary to
include emission from an interclump medium with a density contrast
() of 100. X-ray emission in H-He like and Fe L
lines was modeled using four plasma components located within the wind. We
derive plasma temperatures from to K, with
lower temperatures starting in the outer regions (R3-6 R), and a
hot component starting closer to the star (R2.9 R). From X-ray
line profiles we infer M yr. The
X-ray spectrum (0.1 kev) yields an X-ray luminosity , consistent with the superion line profiles.
X-ray abundances are in agreement with those derived from the UV and optical
analysis: Ori is slightly enhanced in nitrogen and depleted in
carbon and oxygen, evidence for CNO processed material.Comment: 33 pages, 25 figures. Accepted for publication in MNRA
Construction of a Tethered Poly(ethylene glycol) Surface Gradient For Studies of Cell Adhesion Kinetics
Surface gradients can be used to perform a wide range of functions and represent a novel experimental platform for combinatorial discovery and analysis. In this work, a gradient in the coverage of a surface-immobilized poly(ethylene glycol) (PEG) layer is constructed to interrogate cell adhesion on a solid surface. Variation of surface coverage is achieved by controlled transport of a reactive PEG precursor from a point source through a hydrated gel. Immobilization of PEG is achieved by covalent attachment of the PEG molecule via direct coupling chemistry to a cystamine self-assembled monolayer on gold. This represents a simple method for creating spatial gradients in surface chemistry that does not require special instrumentation or microfabrication procedures. The structure and spatial distribution of the PEG gradient are evaluated via ellipsometry and atomic force microscopy. A cell adhesion assay using bovine arteriole endothelium cells is used to study the influence of PEG thickness and chain density on biocompatibility. The kinetics of cell adhesion are quantified as a function of the thickness of the PEG layer. Results depict a surface in which the variation in layer thickness along the PEG gradient strongly modifies the biological response
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
The qWR star HD 45166. II. Fundamental stellar parameters and evidence of a latitude-dependent wind
The enigmatic object HD 45166 is a qWR star in a binary system with an
orbital period of 1.596 day, and presents a rich emission-line spectrum in
addition to absorption lines from the companion star (B7 V). As the system
inclination is very small (i=0.77 +- 0.09 deg), HD 45166 is an ideal laboratory
for wind-structure studies. The goal of the present paper is to determine the
fundamental stellar and wind parameters of the qWR star. A radiative transfer
model for the wind and photosphere of the qWR star was calculated using the
non-LTE code CMFGEN. The wind asymmetry was also analyzed using a
recently-developed version of CMFGEN to compute the emerging spectrum in
two-dimensional geometry. The temporal-variance spectrum (TVS) was calculated
for studying the line-profile variations. Abundances, stellar and wind
parameters of the qWR star were obtained. The qWR star has an effective
temperature of Teff=50000 +- 2000 K, a luminosity of log(L/Lsun)=3.75 +- 0.08,
and a corresponding photospheric radius of Rphot=1.00 Rsun. The star is
helium-rich (N(H)/N(He) = 2.0), while the CNO abundances are anomalous when
compared either to solar values, to planetary nebulae, or to WR stars. The
mass-loss rate is Mdot = 2.2 . 10^{-7} Msun/yr, and the wind terminal velocity
is vinf=425 km/s. The comparison between the observed line profiles and models
computed under different latitude-dependent wind densities strongly suggests
the presence of an oblate wind density enhancement, with a density contrast of
at least 8:1 from equator to pole. If a high velocity polar wind is present
(~1200 km/s), the minimum density contrast is reduced to 4:1. The wind
parameters determined are unusual when compared to O-type stars or to typical
WR stars. (abridged)Comment: 16 pages, 13 figures, accepted for publication in A&
Comparative study of the centrosymmetric and non-centrosymmetric superconducting phases of Re3W using muon-spin spectroscopy and heat capacity measurements
We compare the low-temperature electronic properties of the centrosymmetric
(CS) and non-centrosymmetric (NCS) phases of Re3W using muon-spin spectroscopy
and heat capacity measurements. The zero-field muSR results indicate that
time-reversal symmetry is preserved for both structures of Re3W.
Transverse-field muon spin rotation has been used to study the temperature
dependence of the penetration depth lambda(T) in the mixed state. For both
phases of Re3W, lambda(T) can be explained using a single-gap s-wave BCS model.
The magnetic penetration depth at zero temperature, lambda(0), is 164(7) and
418(6) nm for the centrosymmetric and the non-centrosymmetric phases of Re3W
respectively. Low-temperature specific heat data also provide evidence for an
s-wave gap-symmetry for the two phases of Re3W. Both the muSR and heat capacity
data show that the CS material has a higher Tc and a larger superconducting gap
Delta(0) at 0 K than the NCS compound. The ratio Delta(0)/kBTc indicates that
both phases of Re3W should be considered as strong-coupling superconductors.Comment: 7 pages, to appear in Physical Review
Detection of He II 4686 in eta Carinae
We report the detection of the emission line He II 4686 A in eta Carinae. The
equivalent width of this line is ~100 mA along most of the 5.5-yr cycle and
jumps to ~900 mA just before phase 1.0, followed by a brief disappearance. The
similarity between the intensity variations of this line and of the X-ray light
curve is remarkable, suggesting that they are physically connected. We show
that the number of ionizing photons in the ultraviolet and soft X-rays,
expected to be emitted in the shock wave from the colliding winds, is of the
order of magnitude required to produce the He II emission via photoionization.
The emission is clearly blueshifted when the line is strong. The radial
velocity of the line is generally -100 Km/s, decreases steadily just before the
event, and reaches -400 Km/s at ph = 1.001. At this point, the velocity
gradient suddenly changes sign, at the same time that the emission intensity
drops to nearly zero. Possible scenarios for explaining this emission are
briefly discussed. The timing of the peak of He II intensity is likely to be
associated to the periastron and may be a reliable fiduciary mark, important
for constraining the orbital parameters.Comment: accepted in ApJLetters, 4 pages, 3 figure
Use of Sildenafil to Facilitate Weaning From Inhaled Nitric Oxide in Children With Pulmonary Hypertension Following Surgery for Congenital Heart Disease
Pulmonary hypertension frequently complicates the postoperative management of patients after congenital cardiac surgery. Inhaled nitric oxide is an effective treatment option, but rebound pulmonary hypertension can occur upon its withdrawal. Sildenafil may facilitate its withdrawal by restoring cyclic guanosine monophosphate availability via phosphodiesterase-5 inhibition. The purpose of this study was to evaluate the use of sildenafil in facilitating weaning from inhaled nitric oxide after congenital cardiac surgery in patients who had previously failed weaning, and to compare the effects of sildenafil on pulmonary and systemic hemodynamics. Children who received sildenafil after cardiovascular surgery during a 23-month period at Riley Hospital for Children were identified. Medical records were retrospectively reviewed to determine sildenafil and nitric oxide dosing, pulmonary and systemic blood pressures, and adverse effects. Oral sildenafil was administered to 7 children who had failed attempts at inhaled nitric oxide weaning. Inhaled nitric oxide was weaned from 29.8 ± 5.9 ppm prior to sildenafil initiation to 12.2 ± 3.4 ppm (mean ± SE; P = .024) in the 24 hours after sildenafil. Mean pulmonary artery and systemic arterial pressure were unchanged from baseline when measured 1 hour after sildenafil dosing (mean pulmonary artery pressure, 29 ± 1 to 27 ± 0.7 mm Hg, P = .066; mean systemic arterial pressure, 56 ± 1.2 to 54 ± 1.2 mm Hg, P = .202). Sildenafil may facilitate withdrawal of inhaled nitric oxide and prevent rebound pulmonary hypertension in patients previously failing inhaled nitric oxide weaning attempts
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