6,033 research outputs found
3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars
Context: State of the art quantitative spectroscopy of OB-stars compares
synthetic spectra (calculated by means of 1D, spherically symmetric computer
codes) with observations. Certain stellar atmospheres, however, show strong
deviations from spherical symmetry, and need to be treated in 3D. Aims: We
present a newly developed 3D radiative transfer code, tailored to the solution
of the radiation field in rapidly expanding stellar atmospheres. We apply our
code to the continuum transfer in wind-ablation models, and to the UV resonance
line formation in magnetic winds. Methods: We have used a 3D finite-volume
method for the solution of the equation of radiative transfer, to study
continuum- and line-scattering problems. Convergence has been accelerated by a
non-local approximate Lambda-iteration scheme. Particular emphasis has been put
on careful (spherically symmetric) test cases. Results: Typical errors of the
source functions, when compared to 1D solutions, are of the order of 10-20 %,
and increase for optically thick continua. In circumstellar discs, the
radiation temperatures in the (optically thin) transition region from wind to
disc are quite similar to corresponding values in the wind. For MHD simulations
of dynamical magnetospheres, the line profiles, calculated with our 3D code,
agree well with previous solutions using a 3D-SEI method. When compared with
profiles resulting from the `analytic dynamical magnetosphere' (ADM) model,
significant differences become apparent. Conclusions: Due to similar radiation
temperatures in the wind and the transition region to the disc, the same
line-strength distribution can be applied within radiation hydrodynamic
calculations for circumstellar discs in `accreting high-mass stars'. To
properly describe the UV line formation in dynamical magnetospheres, the ADM
model needs to be further developed, at least in a large part of the outer
wind
Radiative Ablation of Disks Around Massive Stars
Hot, massive stars (spectral types O and B) have extreme luminosities () that drive strong stellar winds through UV line-scattering.
Some massive stars also have disks, formed by either decretion from the star
(as in the rapidly rotating "Classical Be stars"), or accretion during the
star's formation. This dissertation examines the role of stellar radiation in
driving (ablating) material away from these circumstellar disks.
A key result is that the observed month to year decay of Classical Be disks
can be explained by line-driven ablation without, as previously done, appealing
to anomalously strong viscous diffusion. Moreover, the higher luminosity of O
stars leads to ablation of optically thin disks on dynamical timescales of
order a day, providing a natural explanation for the lack of observed Oe stars.
In addition to the destruction of Be disks, this dissertation also introduces a
model for their formation by coupling observationally inferred non-radial
pulsation modes and rapid stellar rotation to launch material into orbiting
Keplerian disks.
In contrast to such Be decretion disks, star-forming accretion disks are much
denser and so are generally optically thick to continuum processes. To
circumvent the computational challenges associated with radiation hydrodynamics
through optically thick media, we develop an approximate method for treating
continuum absorption in the limit of geometrically thin disks. The comparison
of ablation with and without continuum absorption shows that accounting for
disk optical thickness leads to less than a 50 reduction in ablation rate,
implying that ablation rate depends mainly on stellar properties like
luminosity.Comment: PhD dissertation, Univ Delaware, Fall 201
Portal vein thrombosis after laparoscopic splenectomy: an ongoing clinical challenge.
ObjectivesPortal vein thrombosis (PVT) following open splenectomy is a potentially lethal complication with an incidence of up to 6%. The objective of this report is to describe our management of a recent laparoscopic case, discuss current therapies, and consider antiplatelet therapy for prophylaxis.MethodsMedical records, laboratory studies, and imaging studies pertaining to a recent case of a laparoscopic splenectomy were examined. Current literature related to this topic was reviewed.ResultsA 16-year-old girl underwent laparoscopic splenectomy for idiopathic thrombocytopenic purpura. Her preoperative platelet count was 96K. She was discharged on postoperative day 1 after an uneventful operation including division of the splenic hilum with an endoscopic linear stapler. On postoperative day 20, she presented with a 5-day history of epigastric pain, nausea, and low-grade fevers without peritoneal signs. Her white blood cell count was 17.3; her platelets were 476K. Computed tomography demonstrated thrombosis of the splenic, superior mesenteric, and portal veins propagating into the liver. Heparinization was begun followed by an unsuccessful attempt at pharmacologic and mechanical thrombolysis by interventional radiology. Over the next 5 days, her pain resolved, she tolerated a full diet, was converted to oral anticoagulation and sent home. Follow-up radiographic studies demonstrated the development of venous collaterals and cavernous transformation of the portal vein.DiscussionNo standard therapy for PVT exists; several approaches have been described. These include systemic anticoagulation, systemic or regional medical thrombolysis, mechanical thrombolysis, and surgical thrombectomy. Unanswered questions exist about the most effective acute therapy, duration of anticoagulation, and the potential efficacy of routine prophylaxis with perioperative antiplatelet agents. PVT following splenectomy occurs with both the open and laparoscopic approach
Investigating the origin of cyclical wind variability in hot, massive stars - II. Hydrodynamical simulations of co-rotating interaction regions using realistic spot parameters for the O giant Persei
OB stars exhibit various types of spectral variability historically
associated with wind structures, including the apparently ubiquitous discrete
absorption components (DACs). These features have been proposed to be caused
either by magnetic fields or non-radial pulsations. In this second paper of
this series, we revisit the canonical phenomenological hydrodynamical modelling
used to explain the formation of DACs by taking into account modern
observations and more realistic theoretical predictions. Using constraints on
putative bright spots located on the surface of the O giant Persei
derived from high precision space-based broadband optical photometry obtained
with the Microvariability and Oscillations of STars (MOST) space telescope, we
generate two-dimensional hydrodynamical simulations of co-rotating interaction
regions in its wind. We then compute synthetic ultraviolet (UV) resonance line
profiles using Sobolev Exact Integration and compare them with historical
timeseries obtained by the International Ultraviolet Explorer (IUE) to evaluate
if the observed behaviour of Persei's DACs is reproduced. Testing three
different models of spot size and strength, we find that the classical pattern
of variability can be successfully reproduced for two of them: the model with
the smallest spots yields absorption features that are incompatible with
observations. Furthermore, we test the effect of the radial dependence of
ionization levels on line driving, but cannot conclusively assess the
importance of this factor. In conclusion, this study self-consistently links
optical photometry and UV spectroscopy, paving the way to a better
understanding of cyclical wind variability in massive stars in the context of
the bright spot paradigm.Comment: 16 pages, 10 figures, accepted for publication by MNRA
Interplay between Fermi surface topology and ordering in URuSi revealed through abrupt Hall coefficient changes in strong magnetic fields
Temperature- and field-dependent measurements of the Hall effect of pure and
4 % Rh-doped URuSi reveal low density (0.03 hole/U) high mobility
carriers to be unique to the `hidden order' phase and consistent with an
itinerant density-wave order parameter. The Fermi surface undergoes a series of
abrupt changes as the magnetic field is increased. When combined with existing
de Haas-van Alphen data, the Hall data expose a strong interplay between the
stability of the `hidden order,' the degree of polarization of the Fermi liquid
and the Fermi surface topology.Comment: 4 pages, 4 figures, Accepted to Phys. Rev. Let
The Use of Story Map in Improving the Year Four Pupils' Ability in Reading Comprehension
This is an action research on improving reading comprehension through story map among Year 4 pupils in one of the primary schools in Malaysia. My pupils were unable to answer WH-questions of the narrative texts. The research aimed to use story map to improve pupils' reading comprehension and improve my teaching practices in reading comprehension. I am the researcher with 24 pupils, 12 males and 12 females. I utilized three data collection methods: pre-reading comprehension test and post-reading comprehension test, pupils' work and teacher's reflective journal. I introduced story map through demonstration. Then the participants completed the story through literary elements identification based on the narrative texts. Findings showed that story map helped to improve pupils' reading comprehension by identifying literary elements and relating the literary elements with the comprehension questions. The story map gave positive impact to reading comprehension. The research has impacted the teaching and learning environment, pupils' learning, and my teaching practices. I found out that story map was a useful method in attracting pupils' attention and teaching literary elements. Furthermore, I was developed to be a reflective teacher in terms of strengths and weaknesses of the teaching and learning process. Story map can be an effective tool in improving reading comprehension among pupils. In addition, a more interactive story map which incorporates Information and Communication Technology (ICT) may also be employed in further research
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