Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp)     

Owens  Alan  Oosterbroek  T.  Orr  A.  Parmar  A. N.  Schulz  R.  Tozzi  G.P. 《Earth, Moon, and Planets》1997,77(3):293-298

We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 10⁵ km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 10¹⁶ erg s⁻¹ which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 10¹⁵ and 7.8 x 10¹⁵ erg s⁻¹ to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers     

P.A. Carling  H. Orr  A. Kelsey   《Ore Geology Reviews》2006,28(4):402

Economic concentrations of heavy-minerals are often associated with fluvial point-bars but prospecting models identifying the heaviest concentrations are poorly developed. Consequently, the dispersal and storage of a heavy-mineral bedload tracer–magnetite–across a rapidly evolving point-bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration. The bar-head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface. In contrast, when viewed in plan, the bar-platform and bar-tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar-top rapidly aggraded. However, in section, false-bottom placers developed along bedding planes were evident. The latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets, consisting of a mix of shale and magnetite, moving repeatedly from the bar-head to accrete over the bar-tail. Differential density-sorting of magnetite and shale occurred during transport and deposition, such that the heavier magnetite tended to accumulate as a visible concentrated bed-layer, later to be over-run by layers of shale-sediment in which magnetite was present diffusely. However, the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below. The placer thickness, the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body. Finally, a simple mathematical model, described in the literature as reproducing the plan-view of flow and topographic patterns in river bends, was found to reproduce patterns of depth, velocity, shear stress and competence in the point-bar environment which were in accordance with the interpretation of the field data. It was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point-bar environment.  相似文献