排序方式： 共有15条查询结果，搜索用时 515 毫秒
1.
Modern spectropolarimeters are capable of detecting subkilogauss field strengths using the Zeeman effect in line profiles from the static photosphere, but supersonic Doppler broadening makes it more difficult to detect the Zeeman effect in the wind lines of hot stars. Nevertheless, the recent advances in observational capability motivate an assessment of the potential for detecting the magnetic fields threading such winds. We incorporate the weak-field longitudinal Zeeman effect in the Sobolev approximation to yield integral expressions for the flux of circularly polarized emission. To illustrate the results, two specific wind flows are considered: (i) spherical constant expansion with v ( r ) = v ∞ and (ii) homologous expansion with v ( r ) ∝ r . Axial and split monopole magnetic fields are used to schematically illustrate the polarized profiles. For constant expansion, optically thin lines yield the well-known 'flat-topped' total intensity emission profiles and an antisymmetric circularly polarized profile. For homologous expansion, we include occultation and wind absorption to provide a more realistic observational comparison. Occultation severely reduces the circularly polarized flux in the redshifted component, and in the blueshifted component, the polarization is reduced by partially offsetting emission and absorption contributions. We find that for a surface field of approximately 100 G, the largest polarizations result for thin but strong recombination emission lines. Peak polarizations are approximately 0.05 per cent, which presents a substantial although not inconceivable sensitivity challenge for modern instrumentation. 相似文献
2.
R. Ignace J. E. Bjorkman H. M. Bryce 《Monthly notices of the Royal Astronomical Society》2006,366(1):92-100
In recent years, it has been shown that microlensing is a powerful tool for examining the atmospheres of stars in the Galactic bulge and Magellanic Clouds. The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarization signal: arising from differential magnification across the otherwise symmetric source. Earlier work has addressed the signal from a scattering photosphere for both point mass lenses and caustic crossings. In a previous paper, polarimetric variations from point lensing of a circumstellar envelope were considered, as would be suitable for an extended envelope around a red giant. In this work, we examine the polarization in the context of caustic crossing events, the scenario that represents the most easily accessible situation for actually observing a polarization signal in Galactic microlensing. Furthermore, we present an analysis of the effectiveness of using the polarimetric data to determine the envelope properties, illustrating the potential of employing polarimetry in addition to photometry and spectroscopy with microlensing follow-up campaigns. 相似文献
3.
Pierre Nehlig Fawsia Asfirane Antonin Genna Catherine Guerrot Nadège Nicol Ignace Salpeteur Mohammed Shanti & Denis Thiéblemont 《地学学报》2001,13(5):347-353
A new aeromagnetic map together with new geological and geochronological data has led to a reinterpretation of the geological history of the Arabian Shield.
The magnetic anomalies outline an orogenic complex containing a network of mostly left-lateral strike-slip faults, including the Nabitah Belt and several peripheral mountain ranges. Oblique accretion resulted in obliteration of early volcanic-arc magnetic fabrics, which were almost completely replaced by a NW–SE magnetic fabric in the northern Shield; the southern Shield, however, reveals extensive E–W anomalies related to post-accretion magmatic intrusions. This complex web of orogenic zones is intimately associated with synchronous molasse basins that formed 680–610 Ma.
The distribution and chronology of orogenic zones, related to the closing of East and West Gondwana, brings into question several earlier assumptions, such as high continental growth rates, palaeogeodynamic reconstructions, the definitions of the Nabitah and Najd faults, and the significance of molasse basins. 相似文献
The magnetic anomalies outline an orogenic complex containing a network of mostly left-lateral strike-slip faults, including the Nabitah Belt and several peripheral mountain ranges. Oblique accretion resulted in obliteration of early volcanic-arc magnetic fabrics, which were almost completely replaced by a NW–SE magnetic fabric in the northern Shield; the southern Shield, however, reveals extensive E–W anomalies related to post-accretion magmatic intrusions. This complex web of orogenic zones is intimately associated with synchronous molasse basins that formed 680–610 Ma.
The distribution and chronology of orogenic zones, related to the closing of East and West Gondwana, brings into question several earlier assumptions, such as high continental growth rates, palaeogeodynamic reconstructions, the definitions of the Nabitah and Najd faults, and the significance of molasse basins. 相似文献
4.
The St Bavo's Cathedral tower in Ghent, Belgium, is currently under restoration. This tower was built during the fifteenth and sixteenth centuries over a period of 73 years, using Eocene white stone. Restorations over the last two centuries introduced different Jurassic limestones from the Paris Basin. Documentation of these interventions, however, is very poor for the period before the twentieth century, and documentation on the original construction is non‐existing. Nevertheless, the use of the different stones in their historical context forms a typical example of the historical mindset on the use of white stone, representative for Ghent and by extension northwestern Belgium. 相似文献
5.
Orientation studies were carried out by the BRGM mission in Saudi Arabia between 1982–1986 to investigate the dispersion of gold in soils and wadi sediments and to define the optimum sample medium around eight prospects in the central Arabian pediplain. A comparison of gold distribution in several sieved fractions with various pedologic horizons shows that the distribution of gold changes abruptly from the coarser sizes near the source to the finest sizes 150 m downstream.The coarse fraction recommended by previous workers is not representative and yields erratic results in the specific environment of the pediplain. Gold is enriched and more homogeneously distributed in the minus 80 μm fraction of the skeletal soils and wadi sediments.Regional geochemical survey can be effective using a sensitive analytical method for gold and the minus 80 μm fraction of the brown gravelly sediment with a minimum density of 2 samples per km2. Geochemical halos in the 30–50 ppb range indicate gold mineralization 500 to 1000 m upstream, depending on relief.In a first follow-up stage, continued use of the same size of the upper argillaceous layer with a regular reg sampling grid gives more contrasted and more extensive anomalies than using a coarse material. These anomalies may be slightly offset by the present arid erosion. A second follow-up stage is then recommended at a closer grid and sampling the brown blocky layer below the surficial reg pavement. Again, the use of the finest fraction at this exploration stage has given the best probability of finding a blind gold target.Optical determination of gold on a nonmagnetic fraction of heavy panconcentrate is not recommended, because nugget growth is practically absent in the present arid conditions and flour-sized gold particles are lost in desliming samples. 相似文献
6.
Ignace Loris Guust Nolet Ingrid Daubechies F. A. Dahlen 《Geophysical Journal International》2007,170(1):359-370
We propose the use of ℓ1 regularization in a wavelet basis for the solution of linearized seismic tomography problems A m = d , allowing for the possibility of sharp discontinuities superimposed on a smoothly varying background. An iterative method is used to find a sparse solution m that contains no more fine-scale structure than is necessary to fit the data d to within its assigned errors. 相似文献
7.
Plant community structure in the southwestern United States co-varies with soil surface characteristics due to their role in controlling water availability. At the University of Arizona Desert Laboratory on Tumamoc Hill, we evaluated winter and summer season dynamics of photosynthesis in a dominant shrub species, Larrea tridentata (creosotebush), across soils with contrasting development of surface and sub-surface horizons. We measured pre-dawn water potential (Ψpd), stomatal conductance (gs), leaf nitrogen content (%N), and determined stable carbon isotope discrimination (Δ). There were no differences in these parameters throughout the winter, although Δ was higher and %N was lower on the clay than sandy soil early in the dry summer period, and gs was higher on the clay than sandy soil late in the summer season while precipitation decreased. We used path analysis to determine the relative influences of %N, gs and Ψpd on photosynthetic function (Δ) during the winter and summer seasons. There was no influence during the summer, although gs and %N had positive and negative impacts on Δ within the sandy soil type respectively during the winter. Across soil type, gs was the main driver of %N and Δ. Our study highlights the need to include soil type for understanding how physiological performance of L. tridentata varies over seasons within arid and semi-arid ecosystems. 相似文献
8.
9.
A number of strong infrared forbidden lines have been observed in several evolved Wolf–Rayet (WR) star winds, and these are important for deriving metal abundances and testing stellar evolution models. In addition, because these optically thin lines form at large radius in the wind, their resolved profiles carry an imprint of the asymptotic structure of the wind flow. This work presents model forbidden line profile shapes formed in axisymmetric winds. It is well known that an optically thin emission line formed in a spherical wind expanding at constant velocity yields a flat-topped emission profile shape. Simulated forbidden lines are produced for a model stellar wind with an axisymmetric density distribution that treats the latitudinal ionization self-consistently and examines the influence of the ion stage on the profile shape. The resulting line profiles are symmetric about line centre. Within a given atomic species, profile shapes can vary between centrally peaked, doubly peaked, and approximately flat-topped in appearance depending on the ion stage (relative to the dominant ion) and viewing inclination. Although application to WR star winds is emphasized, the concepts are also relevant to other classes of hot stars such as luminous blue variables and Be/B[e] stars. 相似文献
10.
R. Ignace R. Bessey † C. S. Price 《Monthly notices of the Royal Astronomical Society》2009,395(2):962-972
This paper presents calculations for forbidden emission-line profile shapes arising from colliding wind binaries. The main application is for systems involving a Wolf–Rayet (WR) star and an OB star companion. The WR wind is assumed to dominate the forbidden line emission. The colliding wind interaction is treated as an Archimedean spiral with an inner boundary. Under the assumptions of the model, the major findings are as follows. (i) The redistribution of the WR wind as a result of the wind collision is not flux conservative but typically produces an excess of line emission; however, this excess is modest at around the 10 per cent level. (ii) Deviations from a flat-toped profile shape for a spherical wind are greatest for viewing inclinations that are more nearly face-on to the orbital plane. At intermediate viewing inclinations, profiles display only mild deviations from a flat-toped shape. (iii) The profile shape can be used to constrain the colliding wind bow shock opening angle. (iv) Structure in the line profile tends to be suppressed in binaries of shorter periods. (v) Obtaining data for multiple forbidden lines is important since different lines probe different characteristic radial scales. Our models are discussed in relation to Infrared Space Observatory data for WR 147 and γ Vel (WR 11). The lines for WR 147 are probably not accurate enough to draw firm conclusions. For γ Vel, individual line morphologies are broadly reproducible but not simultaneously so for the claimed wind and orbital parameters. Overall, the effort demonstrates how lines that are sensitive to the large-scale wind can help to deduce binary system properties and provide new tests of numerical simulations. 相似文献