24,690 research outputs found

    A 3D radiative transfer framework: IV. spherical & cylindrical coordinate systems

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    We extend our framework for 3D radiative transfer calculations with a non-local operator splitting methods along (full) characteristics to spherical and cylindrical coordinate systems. These coordinate systems are better suited to a number of physical problems than Cartesian coordinates. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate Λ\Lambda operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter for the different coordinate systems. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.Comment: A&A, in pres

    A 3D radiative transfer framework: VII. Arbitrary velocity fields in the Eulerian frame

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    A solution of the radiative-transfer problem in 3D with arbitrary velocity fields in the Eulerian frame is presented. The method is implemented in our 3D radiative transfer framework and used in the PHOENIX/3D code. It is tested by comparison to our well- tested 1D co-moving frame radiative transfer code, where the treatment of a monotonic velocity field is implemented in the Lagrangian frame. The Eulerian formulation does not need much additional memory and is useable on state-of-the-art computers, even large-scale applications with 1000's of wavelength points are feasible

    Numerical Solution of the Expanding Stellar Atmosphere Problem

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    In this paper we discuss numerical methods and algorithms for the solution of NLTE stellar atmosphere problems involving expanding atmospheres, e.g., found in novae, supernovae and stellar winds. We show how a scheme of nested iterations can be used to reduce the high dimension of the problem to a number of problems with smaller dimensions. As examples of these sub-problems, we discuss the numerical solution of the radiative transfer equation for relativistically expanding media with spherical symmetry, the solution of the multi-level non-LTE statistical equilibrium problem for extremely large model atoms, and our temperature correction procedure. Although modern iteration schemes are very efficient, parallel algorithms are essential in making large scale calculations feasible, therefore we discuss some parallelization schemes that we have developed.Comment: JCAM, in press. 28 pages, also available at ftp://calvin.physast.uga.edu:/pub/preprints/CompAstro.ps.g

    General Relativistic Radiative Transfer

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    We present a general method to calculate radiative transfer including scattering in the continuum as well as in lines in spherically symmetric systems that are influenced by the effects of general relativity (GR). We utilize a comoving wavelength ansatz that allows to resolve spectral lines throughout the atmosphere. The used numerical solution is an operator splitting (OS) technique that uses a characteristic formal solution. The bending of photon paths and the wavelength shifts due to the effects of GR are fully taken into account, as is the treatment of image generation in a curved spacetime. We describe the algorithm we use and demonstrate the effects of GR on the radiative transport of a two level atom line in a neutron star like atmosphere for various combinations of continuous and line scattering coefficients. In addition, we present grey continuum models and discuss the effects of different scattering albedos on the emergent spectra and the determination of effective temperatures and radii of neutron star atmospheres

    A 3D radiative transfer framework IX. Time dependence

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    Context. Time-dependent, 3D radiation transfer calculations are important for the modeling of a variety of objects, from supernovae and novae to simulations of stellar variability and activity. Furthermore, time-dependent calculations can be used to obtain a 3D radiative equilibrium model structure via relaxation in time. Aims. We extend our 3D radiative transfer framework to include direct time dependence of the radiation field; i.e., the I/t\partial I/ \partial t terms are fully considered in the solution of radiative transfer problems. Methods. We build on the framework that we have described in previous papers in this series and develop a subvoxel method for the I/t\partial I/\partial t terms. Results. We test the implementation by comparing the 3D results to our well tested 1D time dependent radiative transfer code in spherical symmetry. A simple 3D test model is also presented. Conclusions. The 3D time dependent radiative transfer method is now included in our 3D RT framework and in PHOENIX/3D.Comment: A&A in press, 7 pages, 14 figure

    Obstetrics and gynaecology

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    A review of publications relating to significant advances in the specialty of Obstetrics and Gynaecology over the past four years will be discussed: topics reviewed will have an important impact on reducing maternal/fetal morbidity and mortality and should improve on woman's health care.peer-reviewe

    Skin ageing

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    Cutaneous ageing manifests itself as a progressive reduction in maximum function and reserve capacity of skin tissue. It is not a unique and uniform biological event. Skin comprises three layers: epidermis, dermis and subcutaneous tissue. Collagen atrophy is a major factor in skin ageing. There is a strong correlation between skin collagen loss and estrogen deficiency due to the menopause. Skin ageing, especially in the face, is associated with a progressive increase in extensibility and a reduction in elasticity. With increasing age, the skin also becomes more fragile and susceptible to trauma, leading to more lacerations and bruising. Furthermore, wound healing is impaired in older women. Estrogen use after the menopause increases collagen content, dermal thickness and elasticity, and it decreases the likelihood of senile dry skin. Large-scale clinical trials are necessary to help make informed recommendations regarding postmenopausal estrogen use and its role in the prevention of skin ageing.peer-reviewe
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