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Magnetic field is believed to play an important role in the collapse of a molecular cloud. In particular, due to the properties of magnetic forces, collapse should be easier along magnetic field lines. This is supported by the large-scale sheet-like structures observed in the Taurus giant molecular cloud for instance. Here we investigate whether such a preferred orientation for collapse is present at a much smaller scale, that of individual objects, i.e., about 100AU. We use recent high-angular resolution images of T Tauri stars located in the Taurus star-forming region to find the orientation of the symmetry axis of each star+jet+disk system and compare it with that of the local magnetic field. We find that (i) T Tauri stars that are associated to a jet or an outflow are generally oriented parallel to the magnetic field, as previously demonstrated. More surprising, given our current knowledge of these objects, we also find that (ii) T Tauri stars that are not at present believed to be associated to a jet or an outflow are oriented very differently, i.e., mostly perpendicular to the magnetic field. We present some implications of this puzzling new result. 相似文献
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Pierre Bastien François Ménard Patrice Corporon Nadine Manset Frédérick Poidevin Gaspard Duchêne Jean-Louis Monin 《Astrophysics and Space Science》2004,292(1-4):427-433
We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field. 相似文献
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At high latitudes, the albedo and energy budget of shrub‐tundra landscapes is determined by the relationship between the fractional snow cover and the fraction of vegetation protruding above the snowpack. The exposed vegetation fraction is affected by the bending and/or burial of shrubs in winter and their spring‐up during melt. Little is known about the meteorological conditions and snowpack and shrub properties required to cause bending, and few quantitative measurements of bending processes exist. Here, a model combining the few, mostly qualitative, observations available with a biomechanical model representing branches as cantilevers is proposed to provide a first approximation of bending mechanisms. The exposed vegetation fraction is then calculated using structural parameters of shrubs measured at two sites in Canada: the Granger Basin in the Yukon Territory and Trail Valley Creek in the Northwest Territories. The exposed vegetation fraction is in turn used to calculate albedo, which is evaluated against measurements at the two sites. The model considerably improves modelled albedo compared to a model which only buries but does not bend shrubs at TVC, where shrubs become completely buried. However, the model overestimates albedo at GB where only a few shrubs get buried. The bending model is then used to calculate a compression factor for use in a simple parameterization of the exposed vegetation fraction proposed by previous investigators. The parameterization, which is simpler and computationally less expensive than the full model, is evaluated and found to perform well. Despite the need for further developments, the model provides a first approximation of bending processes and contributes to the identification of measurements that are needed in order to improve the model and our understanding of the bending of shrubs. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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The Jason-1 Mission 总被引：1，自引：2，他引：1
Yves M nard Lee-Lueng Fu P. Escudier F. Parisot J. Perbos P. Vincent S. Desai B. Haines G. Kunstmann 《Marine Geodesy》2003,26(3):131-146
On December 7, 2001, the Jason-1 satellite was successfully launched by a Boeing Delta II rocket from the Vandenberg site in California, USA. Its main mission was to maintain the high accuracy altimeter measurements, provided since 1992 by TOPEX/Poseidon (T/P), ensuring continuity in observing and monitoring the ocean for intraseasonal to interannual changes, mean sea level, tides, and so forth. Despite four times less mass and power, the Jason-1 system has been designed to have the same performances as T/P, measuring sea surface topography at the centimeter level. This new Centre National d'Etudes Spatiales/National Aeronautics and Space Administration (CNES/NASA) mission also provides near real-time data for sea state and ocean forecast. The first 10 months of the Jason mission were dedicated to the verification of the system performance and cross-calibration with T/P measurements. A complete CALVAL plan was conducted by the Science and Project Teams of the mission based on in situ and regional experiments, global statistical approaches, and multisatellite comparisons, taking advantage of the T/P-Jason overlap during the first months of the mission. CALVAL and first science results showed that the Jason-1 performances were compliant with prelaunch specifications. This was a needed preamble before starting the routine phase of the mission in July 2003 with generation and distribution of validated geophysical data records to the whole user community. 相似文献
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Ménard A. Dana Houser Chris Brander Robert W. Trimble Sarah Scaman Alexandra 《Natural Hazards》2018,94(2):953-973
Natural Hazards - The rip current hazard on beaches is a global public health issue. While physical controls on rip current formation and flow behavior are relatively well understood, there has... 相似文献