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981.
We show that hydrostatically equilibrium models for the thin photospheres of helium stars based on new opacities κR (OPAL and OP) can be constructed only for masses M<5M ⊙. The parameter Г=κL/4πGMc, defined as the ratio of light pressure to gravity, exceeds a critical value of 1.0 for larger masses, which must result in mass outflow under light pressure. This mass limit matches the observed lower limit for the masses of Wolf-Rayet stars (M WR>5M ⊙)), which is an additional argument that the Wolf-Rayet stellar cores are actually helium stars. By solving the equation of radiative transfer in extended atmospheres, we construct a semiempirical model for a WN5 star (M WN5=10M ⊙)) with a helium core and an expanding envelope, whose physical and geometric parameters are known mainly from light-curve solution for the eclipsing binary V444 Cyg (WN5+06): outflow rate $\dot M \approx 1.0 \times 10^{ - 5} M_ \odot yr^{ - 1} $ , terminal velocity V ∞≈2000 km s?1, and expanding-envelope optical depth τenv≈25. The temperature at the outer boundary of the photosphere of a helium star surrounded by such an envelope is approximately 130 kK higher than that in the absence of an envelope, being T ph≈240 kK. Because of the high temperatures, the absorption coefficients at the corresponding photospheric levels are smaller than those in models with no envelope; therefore, the photosphere turns out to be in hydrostatic equilibrium and stable against light pressure (Гmax≈0.9). As a way out of this conflicting situation (an expanding envelope together with a hydrostatically equilibrium photosphere), we propose a model of discrete mass outflow, which is also supported by the observed cloudy structure of the envelopes in this type of stars. To quantitatively estimate parameters of the nonuniform outflow model requires detailed gas-dynamical calculations. 相似文献
982.
Camps A. Skou N. Torres F. Corbella I. Duffo N. Vall-llossera M. 《Geoscience and Remote Sensing Letters, IEEE》2006,3(2):259-261
Accurate measurement of the antenna voltage patterns of large-aperture synthesis radiometers is critical in order to achieve good radiometric accuracy, and a very time consuming and expensive task. Measurement requirements and a tradeoff study relating radiometric accuracy degradation and number of elements to be measured are presented. 相似文献
983.
N. R. Bigil’deeva A. P. Makhinya I. A. Usatova D. A. Makhinya V. N. Petkevich 《Russian Meteorology and Hydrology》2007,32(11):694-697
Some results of use of snow survey monitoring data within the system of the Privolzhie Administration for Hydrometeorological Service for studying the degree of snow cover pollution and mapping pollution distribution in several urban and rural areas of the Samara region are considered. 相似文献
984.
It is found that the surface bog temperature and bog water level position are major factors that determine CO2 emission from the bog massif. The emission intensity increases with increasing temperature and with a bog water level decrease. The dependence of CO2 emission is shown to be different for each bog microlandscape. Monthly mean CO2 emissions are given for the central bog massif in the years with different water content. Based on modeling, it is shown that in the case of a low bog water level the CO2 flux is directed to the atmosphere (the emission value exceeds the gas amount consumed for photosynthesis), while in the case of a high level, it is taken up by the growing plants. 相似文献
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Main stages in tectonic evolution of the Eastern Arctic region 总被引:1,自引:0,他引:1
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