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171.
Michaël De Becker 《Astronomy and Astrophysics Review》2007,14(3-4):171-216
In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of
non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio
domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical
aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively
the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons,
the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars.
Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be
combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected
in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related
to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet
binary—will be considered in details. Finally, strategies for future developments in this field will be discussed. 相似文献
172.
We reexamine the energetics of nonthermal-electron acceleration in solar noise storms. A new result is obtained for the minimum
nonthermal-electron number density required to produce a Langmuir-wave population of sufficient intensity to power the noise-storm
emission. We combine this constraint with the stochastic electron acceleration formalism developed by Subramanian and Becker
(2005) to derive a rigorous estimate for the efficiency of the overall noise-storm emission process, beginning with nonthermal-electron
acceleration and culminating in the observed radiation. We also calculate separate efficiencies for the electron acceleration–Langmuir
wave generation stage and the Langmuir wave–noise-storm production stage. In addition, we obtain a new theoretical estimate
for the energy density of the Langmuir waves in noise-storm continuum sources. 相似文献
173.
Tracy N. Tingle Christopher H. Becker Ripudaman Malhotra 《Meteoritics & planetary science》1991,26(2):117-127
Abstract— Organic compounds in the Murchison (C2M) and Allende (CV3) carbonaceous chondrites were analyzed by photoionization time-of-flight mass spectrometry; thermal (25–850 °C) and stimulated (7 keV Ar+) desorption were combined with either nonresonant single-photon ionization using 118 nm light or resonantly enhanced multiphoton ionization (selective for aromatic compounds) using 266 nm light. Samples weighing only 1–10 mg were sufficient for sensitive quantitative analysis of aromatic compounds using thermal desorption. The detection limits for phenanthrene and pyrene using 118 nm light were determined to be 0.8 and 1.4 picomoles, respectively, and the concentrations of these compounds (including their isomers anthracene and fluoranthene) in the Murchison meteorite were determined to be 9 and 12 μg/g, respectively, in good agreement with previously published values. Thermal-desorption (–75–500 °C) field-ionization mass spectra (activated foil-type ionizing source and magnetic sector mass analyzer) of 20–40 mg of the same meteorite material were obtained to verify that the 118 nm photoionization mass spectra were not affected by photofragmentation or photodecomposition and were representative of the organic material extracted by thermal desorption. Photoionization mass spectrometry is a useful technique for studying small quantities (< 1 nanomole) of organic matter in terrestrial and extraterrestrial samples. The present study aims to provide the background and analytical methods necessary for application to new and unsolved cosmochemical problems. Some potential applications are discussed. 相似文献
174.
C. Alcock R. A. Allsman D. R. Alves T. S. Axelrod A. C. Becker D. P. Bennett P. A. Charles K. H. Cook A. J. Drake K. C. Freeman M. Geha K. Griest M. J. Lehner S. L. Marshall K. E. McGowan † D. Minniti C. A. Nelson B. A. Peterson P. Popowski M. R. Pratt P. J. Quinn C. W. Stubbs W. Sutherland A.B. Tomaney T. Vandehei D. L. Welch 《Monthly notices of the Royal Astronomical Society》2001,321(4):678-684
We present a ∼5-yr optical light curve of the recurrent Be/X-ray transient A0538–66 obtained as a by-product of the MACHO Project. These data reveal both a long-term modulation at P =420.8±0.8 d and a short-term modulation at 16.6510±0.0022 d which, within errors, confirms the previously found orbital period. Furthermore, the orbital activity is only seen at certain phases of the 421-d cycle, suggesting that the long-term modulation is related to variations in the Be star envelope. 相似文献
175.
Gerald Herrling Marius Becker Alice Lefebvre Anna Zorndt Knut Krämer Christian Winter 《地球表面变化过程与地形》2021,46(11):2211-2228
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments. 相似文献
176.
T. Müller R. Dohmen H. W. Becker Jan H. ter Heege S. Chakraborty 《Contributions to Mineralogy and Petrology》2013,166(6):1563-1576
Chemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (X Di = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10?11 to 10?17 bar. Diffusion couples were prepared by ablating an olivine (X Fo = 0.3) target to deposit a thin film (20–100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of D Fe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di93–Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, D Fe–Mg, can be described by a single Arrhenius relation with $$D^{{{\text{Fe}} - {\text{Mg}}}} = 2. 7 7\pm 4. 2 7\times 10^{ - 7} {\text{exp(}}-3 20. 7\pm 1 6.0{\text{ kJ}}/{\text{mol}}/{\text{RT)m}}^{ 2} /{\text{s}}.$$ D Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D Ca–Mg) while it is slower than D Fe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks). 相似文献
177.
Wolfango Plastino Romano Plenteda George Azzari Andreas Becker Paul R. J. Saey Gerhard Wotawa 《Pure and Applied Geophysics》2010,167(4-5):559-573
Understanding radioxenon time series and being able to distinguish anthropogenic from nuclear explosion signals are fundamental issues for the technical verification of the Comprehensive Nuclear-Test-Ban Treaty. Every radioxenon event categorisation methodology must take into account the background at each monitoring site to uncover anomalies that may be related to nuclear explosions. Feedback induced by local meteorological patterns on the equipment and on the sampling procedures has been included in the analysis to improve a possible event categorisation scheme. The occurrence probability of radioxenon outliers has been estimated with a time series approach characterising and avoiding the influence of local meteorological patterns. A power spectrum estimator for radioxenon and meteorological time series was selected; the randomness of the radioxenon residual time series has been tested for white noise by Kolmogorov–Smirnov and Ljung–Box tests. This methodological approach was applied to radioxenon data collected at two monitoring sites located at St. John’s, Canada and Charlottesville, USA, equipped with two different noble gas systems. It shows different feedback with local meteorological patterns and randomness for the radioxenon data recorded at the selected sites of St. John’s and Charlottesville as well as a different occurrence probability of the outliers in the normalized radioxenon original and residual time series. 相似文献
178.
179.
180.
Stefan Becker M. L. Büker C. J. Matyas R. V. Rohli 《Theoretical and Applied Climatology》2010,102(3-4):379-392
Links between hurricane track changes and upper atmospheric potential vorticity (PV) anomaly patterns were identified qualitatively and analytically between 1990 and 2005 in the Western Atlantic. Strong track changes of hurricanes, particularly the constellations that triggered northward acceleration of the storm systems, were associated with upper-air PV patterns characterized by strongly positive anomalies to the northeast in combination with weak PV to the north of the system center. Constellations that triggered eventual eastward acceleration were associated with strongly positive PV anomalies to the northwest in combination with weak PV to the northeast of the system center. These results may assist hurricane forecasters and modelers in identifying possible signatures of future tropical cyclone tracks. 相似文献