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991.
R. de Kok P.G.J. Irwin E. Lellouch B. Bézard S. Vinatier C.A. Nixon C. Howett N.E. Bowles F.W. Taylor 《Icarus》2007,186(2):354-363
We have investigated the abundances of Titan's stratospheric oxygen compounds using 0.5 cm−1 resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75° S-35° N) and emission angles (0°-60°), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47±8 ppm if CO is assumed to be uniform with latitude. H2O could not be detected and an upper limit of 0.9 ppb was determined. CO2 abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16±2 ppb. Mid-infrared limb spectra at 55° S were used to constrain the vertical profile of CO2 for the first time. A vertical CO2 profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of CO2 in Titan's stratosphere. Above 200 km the CO2 volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. 相似文献
992.
993.
M. J. Hardcastle C. C. Cheung I. J. Feain . Stawarz 《Monthly notices of the Royal Astronomical Society》2009,393(3):1041-1053
The nearby radio galaxy Centaurus A is poorly studied at high frequencies with conventional radio telescopes because of its very large angular size, but is one of a very few extragalactic objects to be detected and resolved by the Wilkinson Microwave Anisotropy Probe ( WMAP ). We have used the five-year WMAP data for Cen A to constrain the high-frequency radio spectra of the 10° giant lobes and to search for spectral changes as a function of position along the lobes. We show that the high-frequency radio spectra of the northern and southern giant lobes are significantly different: the spectrum of the southern lobe steepens monotonically (and is steeper further from the active nucleus) whereas the spectrum of the northern lobe remains consistent with a power law. The inferred differences in the northern and southern giant lobes may be the result of real differences in their high-energy particle acceleration histories, perhaps due to the influence of the northern middle lobe, an intermediate-scale feature which has no detectable southern counterpart. In light of these results, we discuss the prospects for Fermi Gamma-ray Space Telescope detections of inverse-Compton emission from the giant lobes and the lobes' possible role in the production of the ultra-high-energy cosmic rays (UHECR) detected by the Pierre Auger Observatory. We show that the possibility of a Fermi detection depends sensitively on the physical conditions in the giant lobes, with the northern lobe more likely to be detected, and that any emission observed by Fermi is likely to be dominated by photons at the soft end of the Fermi energy band. On the other hand, we argue that the estimated conditions in the giant lobes imply that UHECRs can be accelerated there, with a potentially detectable γ-ray signature at TeV energies. 相似文献
994.
The NASA Stardust mission has provided for laboratory study an extensive data set of cometary dust of known provenance (from comet 81P/Wild 2) yielding detailed insights into the composition of the comet. Combined with the results of data from other missions to short-period Jupiter family comets (JFC), this has greatly deepened the understanding of such objects. If depressions on the surface of comet 81P/Wild 2 are all taken as evidence of impact cratering, their number suggests a long occupancy in the outer region of the Solar System. The dust from comet 81P/Wild 2 has been shown to be heavily deficient in pre-Solar grains and rich in materials formed at high temperatures in the inner Solar System. Although it is too early to know if this is typical of JFC, it does argue for rapid and thorough mixing of materials in the disk on timescales related to comet formation, and may also suggest outward migration of small icy bodies after their formation. Thus, instead of providing mainly new knowledge of the pre-Solar materials expected to be rich in comets, Stardust and comet 81P/Wild 2 have instead focussed attention on large-scale transport processes during the critical period when cometary parent bodies were forming in the early Solar System. 相似文献
995.
996.
F. Kneer G. Scharmer W. Mattig A. Wyller G. Artzner P. Lemaire J. C. Vial 《Solar physics》1981,69(2):289-300
Observations with the French (L.P.S.P.) experiment on board OSO-8 of a sunspot and nearby plage region are described. The behaviour of the emission cores of the Ca II H and K and Mg II h and k resonance lines is very similar and the correspondence in intensity between the four lines persists in all observed features. In contrast, the Lyman lines show little correlation with the other lines. Their emission regions appear broader in the spectroheliograms than the underlying sunspot structure and must not necessarily possess a counterpart in lower layers. From the central intensity of L above the umbra an electron density of 4.3 × 1010 cm-3 n
e
* 2.3 × 1011 cm-3 at 20 000 K is estimated.Mitteilungen aus dem Kiepenheuer-Institut Nr. 186.Stockholm Observatorium, S-13300 Saltsjöbaden, Sweden.Laboratoire de Physique Stellaire et Planétaire, CNRS, P.O. Box 10, F-91370 Verrières-le-Buisson, France. 相似文献
997.
Edward C. D. Pope 《Monthly notices of the Royal Astronomical Society》2009,395(4):2317-2325
We have constructed an analytical model of active galactic nuclei (AGN) feedback and studied its implications for elliptical galaxies and galaxy clusters. The results show that momentum injection above a critical value will eject material from low-mass elliptical galaxies, and leads to an X-ray luminosity, L X , that is ∝σ8−10 , depending on the AGN fuelling mechanism, where σ is the velocity dispersion of the hot gas. This result agrees well with both observations and semi-analytic models. In more massive ellipticals and clusters, AGN outflows quickly become buoyancy dominated. This necessarily means that heating by a central cluster AGN redistributes the intracluster medium (ICM) such that the mass of hot gas, within the cooling radius, should be ∝ L X (< r cool )/[ g ( r cool )σ] , where g ( r cool ) is the gravitational acceleration at the cooling radius. This prediction is confirmed using observations of seven clusters. The same mechanism also defines a critical ICM cooling time of ∼0.5 Gyr , which is in reasonable agreement with recent observations showing that star formation and AGN activity are triggered below a universal cooling time threshold. 相似文献
998.
在乔治王岛中部的阿德默勒尔蒂湾 ,凯勒半岛、于尔曼山嘴和埃内坎角是第三纪火山岩集中分布区。野外地质调查和同位素年代学研究表明 ,凯勒半岛发生了三期火山喷发 (三个火山喷发旋回 ) ,同时伴随着火山活动中心的不断北移 ;在第二期火山喷发之后 ,火口塌陷 ,形成破火山 ,之后在半岛北部又发生火山喷发 ,并向东迁移到于尔曼山嘴。因此凯勒半岛是一复活破火山 ,火山活动具有随时间迁移的特性。埃内坎角火山活动虽与上述两地区大致同时 ,但化学成分上不存在明显演化关系 ,应属另一相对独立的火山活动中心 相似文献
999.
1000.