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R. N.Ogley S.Chaty M.Crocker S. P. S.Eyres M. A.Kenworthy A. M. S.Richards L. F.Rodríguez A. M.Stirling 《Monthly notices of the Royal Astronomical Society》2002,330(4):772-777
We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam−1 , however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg−2 , comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems. 相似文献
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Richard Massey Barnaby Rowe Alexandre Refregier David J. Bacon Joel Bergé 《Monthly notices of the Royal Astronomical Society》2007,380(1):229-245
We derive expressions, in terms of 'polar shapelets', for the image distortion operations associated with weak gravitational lensing. Shear causes galaxy shapes to become elongated, and is sensitive to the second derivative of the projected gravitational potential along their line of sight; flexion bends galaxy shapes into arcs, and is sensitive to the third derivative. Polar shapelets provide a natural representation, in which both shear and flexion transformations are compact. Through this tool, we understand progress in several weak lensing methods. We then exploit various symmetries of shapelets to construct a range of shear estimators with useful properties. Through an analogous investigation, we also explore several flexion estimators. In particular, some of the estimators can be measured simultaneously and independently for every galaxy, and will provide unique checks for systematics in future weak lensing analyses. Using simulated images from the Shear TEsting Programme, we show that we can recover input shears with no significant bias. A complete software package to parametrize astronomical images in terms of polar shapelets, and to perform a full weak lensing analysis, is available on the Internet. 相似文献
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A. M. S. Richards R. J. Cohen M. Crocker E. E. Lekht E. Mendoza V. A. Samodurov 《Astrophysics and Space Science》2005,295(1-2):19-25
We have imaged H2O maser emission from the star-forming region S128 at milli-arcsec resolution using MERLIN, to complement 20 years of monitoring data from the Puschino radio telescope. The drift velocities of the masers and the velocity and location of a new maser region add depth to the model of two colliding CO clouds triggering collapse. Some H2O masers appear to originate directly from this shock front. The brightest maser appears typical of a YSO jet and remains unsaturated close to peak intensity. The distribution of maser clumps has a fractal dimension 0.4; combined with analysis of drift velocity variations this suggests that the masers trace the dissipation of supersonic turbulence. The spatial distribution of velocities shows that this is in parts more structured than the Kolmogorov cascade. 相似文献
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Douglas B. Yager Raymond H. Johnson Barnaby W. Rockwell Jonathan Saul Caine Kathleen S. Smith 《Environmental Earth Sciences》2013,70(3):1057-1082
Hydrothermally altered bedrock in the Silverton mining area, southwest Colorado, USA, contains sulfide minerals that weather to produce acidic and metal-rich leachate that is toxic to aquatic life. This study utilized a geographic information system (GIS) and statistical approach to identify watershed-scale geologic variables in the Silverton area that influence water quality. GIS analysis of mineral maps produced using remote sensing datasets including Landsat Thematic Mapper, advanced spaceborne thermal emission and reflection radiometer, and a hybrid airborne visible infrared imaging spectrometer and field-based product enabled areas of alteration to be quantified. Correlations between water quality signatures determined at watershed outlets, and alteration types intersecting both total watershed areas and GIS-buffered areas along streams were tested using linear regression analysis. Despite remote sensing datasets having varying watershed area coverage due to vegetation cover and differing mineral mapping capabilities, each dataset was useful for delineating acid-generating bedrock. Areas of quartz–sericite–pyrite mapped by AVIRIS have the highest correlations with acidic surface water and elevated iron and aluminum concentrations. Alkalinity was only correlated with area of acid neutralizing, propylitically altered bedrock containing calcite and chlorite mapped by AVIRIS. Total watershed area of acid-generating bedrock is more significantly correlated with acidic and metal-rich surface water when compared with acid-generating bedrock intersected by GIS-buffered areas along streams. This methodology could be useful in assessing the possible effects that alteration type area has in either generating or neutralizing acidity in unmined watersheds and in areas where new mining is planned. 相似文献
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Kimberley C. Crocker Michael J. DeNiro Peter D. Ward 《Geochimica et cosmochimica acta》1985,49(12):2527-2532
Eggwaters from the chambered cephalopod Nautilus are depleted in both 18O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the values of shell carbonates for some but not all extinct and extant chambered cephalopods.The values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested. 相似文献
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Abstract Wave‐induced ice motions measured during the Labrador Ice Margin Experiment (LIMEX ‘89) were interpreted to determine the cause and the frequency of collisions between floes. The LIMEX acceleration data were acquired with an optimal resolution near the predominant wave frequency and did not contain information above 0.5 Hz. It was therefore possible to establish the frequency of collisions, but not the magnitude of the events. Events were defined by any contact between floes in a wave cycle, and the distribution of times between events indicates that floes are more likely to collide in adjacent wave cycles than if the events were independent. Periods of continuous and intermittent collisions were related to the wave characteristics, and the frequency of events increased with a decrease in air temperature and an increase in local wind speed. Contrary to expectations, there was not a positive relation between collision frequency and wave amplitude. 相似文献