1,621 research outputs found
The Effect of Magnetic Field Tilt and Divergence on the Mass Flux and Flow Speed in a Line-Driven Stellar Wind
We carry out an extended analytic study of how the tilt and
faster-than-radial expansion from a magnetic field affect the mass flux and
flow speed of a line-driven stellar wind. A key motivation is to reconcile
results of numerical MHD simulations with previous analyses that had predicted
non-spherical expansion would lead to a strong speed enhancement. By including
finite-disk correction effects, a dynamically more consistent form for the
non-spherical expansion, and a moderate value of the line-driving power index
, we infer more modest speed enhancements that are in good quantitative
agreement with MHD simulations, and also are more consistent with observational
results. Our analysis also explains simulation results that show the
latitudinal variation of the surface mass flux scales with the square of the
cosine of the local tilt angle between the magnetic field and the radial
direction. Finally, we present a perturbation analysis of the effects of a
finite gas pressure on the wind mass loss rate and flow speed in both spherical
and magnetic wind models, showing that these scale with the ratio of the sound
speed to surface escape speed, , and are typically 10-20% compared
to an idealized, zero-gas-pressure model.Comment: Accepted for publication in ApJ, for the full version of the paper go
to: http://www.bartol.udel.edu/~owocki/preprints/btiltdiv-mdotvinf.pd
Morbidity following Surgical Management of Vulval Cancer.
The objective of this study was to know the complications following vulvectomy and inguinofemoral lymphadenectomy including the time taken to complete wound healing. 42 patients who were subjected to either radical or modified radical vulvectomy for primary and inguinofemoral lymphadenectomy (80 groins) for groin metastases were analysed retrospectively. The complications analysed were wound breakdown, wound cellulitis or infection, lymphocyst, limb edema and the time to wound healing. In a total of 80 inguinofemoral lymphadenectomies 55% had wound breakdown, 17.5% had wound infection/cellulitis, lymphocyst in 31%, limb edema in 36% and time taken for complete wound healing ranged from 10-134 (average 46 days). Overall post operative morbidity was 85%
A Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars
We introduce a new Rigid-Field Hydrodynamics approach to modeling the
magnetospheres of massive stars in the limit of very-strong magnetic fields.
Treating the field lines as effectively rigid, we develop hydrodynamical
equations describing the 1-dimensional flow along each, subject to pressure,
radiative, gravitational, and centrifugal forces. We solve these equations
numerically for a large ensemble of field lines, to build up a 3-dimensional
time-dependent simulation of a model star with parameters similar to the
archetypal Bp star sigma Ori E. Since the flow along each field line can be
solved for independently of other field lines, the computational cost of this
approach is a fraction of an equivalent magnetohydrodynamical treatment.
The simulations confirm many of the predictions of previous analytical and
numerical studies. Collisions between wind streams from opposing magnetic
hemispheres lead to strong shock heating. The post-shock plasma cools initially
via X-ray emission, and eventually accumulates into a warped, rigidly rotating
disk defined by the locus of minima of the effective (gravitational plus
centrifugal) potential. But a number of novel results also emerge. For field
lines extending far from the star, the rapid area divergence enhances the
radiative acceleration of the wind, resulting in high shock velocities (up to
~3,000 km/s) and hard X-rays. Moreover, the release of centrifugal potential
energy continues to heat the wind plasma after the shocks, up to temperatures
around twice those achieved at the shocks themselves. Finally, in some
circumstances the cool plasma in the accumulating disk can oscillate about its
equilibrium position, possibly due to radiative cooling instabilities in the
adjacent post-shock regions.Comment: 21 pages, 12 figures w/ color, accepted by MNRA
A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612
The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such "dynamical magnetospheres" (DMs) with the more steady-state
"centrifugal magnetospheres" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation
Building upon our previous MHD simulation study of magnetic channeling in
radiatively driven stellar winds, we examine here the additional dynamical
effects of stellar {\em rotation} in the (still) 2-D axisymmetric case of an
aligned dipole surface field. In addition to the magnetic confinement parameter
introduced in Paper I, we characterize the stellar rotation in
terms of a parameter (the ratio of the
equatorial surface rotation speed to orbital speed), examining specifically
models with moderately strong rotation 0.25 and 0.5, and comparing these
to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius
R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar and Kepler corotation radius
R_{\rm{K}} \approx W^{-2/3} \Rstar, we find rotation effects are weak for
models with , but can be substantial and even dominant
for models with R_{\rm{A}} \gtwig R_{\rm{K}}. In particular, by extending our
simulations to magnetic confinement parameters (up to )
that are well above those () considered in Paper I, we are
able to study cases with ; we find that these do
indeed show clear formation of the {\em rigid-body} disk predicted in previous
analytic models, with however a rather complex, dynamic behavior characterized
by both episodes of downward infall and outward breakout that limit the buildup
of disk mass. Overall, the results provide an intriguing glimpse into the
complex interplay between rotation and magnetic confinement, and form the basis
for a full MHD description of the rigid-body disks expected in strongly
magnetic Bp stars like Ori E.Comment: 14 pp, visit this
http://shayol.bartol.udel.edu/massivewiki-media/publications/rotation.pdf for
full figure version of the paper. MNRAS, in pres
Wind Channeling, Magnetospheres, And Spindown Of Magnetic Massive Stars
A subpopulation (~10%) of hot, luminous, massive stars have been revealed through spectropolarimetry to harbor strong (hundreds to tens of thousand Gauss), steady, large-scale (often significantly dipolar) magnetic fields. This review focuses on the role of such fields in channeling and trapping the radiatively driven wind of massive stars, including both in the strongly perturbed outflow from open field regions, and the wind-fed âmagnetospheresâ that develop from closed magnetic loops. For B-type stars with weak winds and moderately fast rotation, one finds âcentrifugal magnetospheresâ, in which rotational support allows magnetically trapped wind to accumulate to a large density, with quite distinctive observational signatures, e.g. in Balmer line emission. In contrast, more luminous O-type stars have generally been spun down by magnetic braking from angular momentum loss in their much stronger winds. The lack of centrifugal support means their closed loops form a âdynamical magnetosphereâ, with trapped material falling back to the star on a dynamical timescale; nonetheless, the much stronger wind feeding leads to a circumstellar density that is still high enough to give substantial Balmer emission. Overall, this review describes MHD simulations and semi-analytic dynamical methods for modeling the magnetospheres, the magnetically channeled wind outflows, and the associated spin-down of these magnetic massive stars
An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars
Slowly rotating magnetic massive stars develop "dynamical magnetospheres"
(DM's), characterized by trapping of stellar wind outflow in closed magnetic
loops, shock heating from collision of the upflow from opposite loop
footpoints, and subsequent gravitational infall of radiatively cooled material.
In 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these
three components is spatially complex and temporally variable, making it
difficult to derive observational signatures and discern their overall scaling
trends.Within a simplified, steady-state analysis based on overall conservation
principles, we present here an "analytic dynamical magnetosphere" (ADM) model
that provides explicit formulae for density, temperature and flow speed in each
of these three components -- wind outflow, hot post-shock gas, and cooled
inflow -- as a function of colatitude and radius within the closed (presumed
dipole) field lines of the magnetosphere. We compare these scalings with
time-averaged results from MHD simulations, and provide initial examples of
application of this ADM model for deriving two key observational diagnostics,
namely hydrogen H-alpha emission line profiles from the cooled infall, and
X-ray emission from the hot post-shock gas. We conclude with a discussion of
key issues and advantages in applying this ADM formalism toward derivation of a
broader set of observational diagnostics and scaling trends for massive stars
with such dynamical magnetospheres.Comment: 15 pages, 11 figures, accepted for MNRA
Protodiscs around Hot Magnetic Rotator Stars
We develop equations and obtain solutions for the structure and evolution of
a protodisc region that is initially formed with no radial motion and
super-Keplerian rotation speed when wind material from a hot rotating star is
channelled towards its equatorial plane by a dipole-type magnetic field. Its
temperature is around K because of shock heating and the inflow of wind
material causes its equatorial density to increase with time. The centrifugal
force and thermal pressure increase relative to the magnetic force and material
escapes at its outer edge. The protodisc region of a uniformly rotating star
has almost uniform rotation and will shrink radially unless some instability
intervenes. In a star with angular velocity increasing along its surface
towards the equator, the angular velocity of the protodisc region decreases
radially outwards and magnetorotational instability (MRI) can occur within a
few hours or days. Viscosity resulting from MRI will readjust the angular
velocity distribution of the protodisc material and may assist in the formation
of a quasi-steady disc. Thus, the centrifugal breakout found in numerical
simulations for uniformly rotating stars does not imply that quasi-steady discs
with slow outflow cannot form around magnetic rotator stars with solar-type
differential rotation.Comment: Accepted for publication in MNRAS. 16 pages, 1 figure, 7 table
Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds
We present 2D MHD simulations of the radiatively driven outflow from a
rotating hot star with a dipole magnetic field aligned with the star's rotation
axis. We focus primarily on a model with moderately rapid rotation (half the
critical value), and also a large magnetic confinement parameter, . The magnetic field
channels and torques the wind outflow into an equatorial, rigidly rotating disk
extending from near the Kepler corotation radius outwards. Even with
fine-tuning at lower magnetic confinement, none of the MHD models produce a
stable Keplerian disk. Instead, material below the Kepler radius falls back on
to the stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outwards, leading to
episodic breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly K, high
enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection}
represents a novel mechanism for driving magnetic reconnection, and seems a
very promising basis for modeling X-ray flares recently observed in rotating
magnetic Bp stars like Ori E.Comment: 5 pages, 3 figures, accepted by ApJ
AN OVERVIEW ON BILAYERED TABLET TECHNOLOGY
Bilayer tablet is new novel of tablet for the successful development of controlled release formulation along with many ĂÂ features to provide a way of successful drugdelivery system. Bi-layer tablet is suitable for sequential release of two drugs incombination, separate two incompatible substances and also for sustained release tablet in which one Layer is immediaterelease as initial dose and second layer is maintenance dose. So use ofbi-layer tablets is a very differentaspectfor anti-hypertensive, diabetic, anti-inflammatory and analgesic drugs where combination therapy is often used. ĂÂ Bi-layer tablets have been developed to achieve modified release of drug. Bilayer tabletis improved beneficial technology to overcome the shortcoming of the single layered tablet. In the case of bilayered tablets drug release can be rendered almost unidirectional if the drug can be incorporated in the upper non adhesive layer its delivery occurs into the whole oral cavity.To reduce capital investment, quite often existing but modified tablet presses are used to develop and produce such tablets. The present article explains why the production and development of quality bi-layer tablets needs to be carried out on purpose-built tablet presses to overcome common bi-layer problems, such as hardness, insufficient, layer-separation, inaccurate individual layer weight control, reduced yield, cross-contamination between the layers, etc. Using a modified tablet press may therefore not be your best approach to producing a quality bi-layer tablet under GMP-conditions.ĂÂ Keywords:Approaches, Bilayer tablets, OROS push pull technology, DUROS technology,immediate release, GMP requirements for bilayer tablet
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