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121.
Melvyn B. Davies rew King Hans Ritter 《Monthly notices of the Royal Astronomical Society》2002,333(2):463-468
We investigate whether the recently observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disc of the Galaxy. In particular, we consider binaries where the primary is sufficiently massive to explode as a Type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova, owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems that are broken up, a large number of the secondaries receive kick velocities ∼100–200 km s−1 and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birth rate of these stars over the entire Galaxy to those seen locally with high velocities relative to the local standard of rest (LSR) . We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disc may make a significant contribution to the observed population of high-velocity white dwarfs. 相似文献
122.
123.
Paleoecological investigation of recent lake acidification in the northern Great Lakes states 总被引:2,自引:0,他引:2
J. C. Kingston R. B. Cook R. G. Kreis Jr. K. E. Camburn S. A. Norton P. R. Sweets M. W. Binford M. J. Mitchell S. C. Schindler L. C. K. Shane G. A. King 《Journal of Paleolimnology》1990,4(2):153-201
Paleoecological analyses of sediments from nine northern Great Lakes states (NGLS) lakes reveal small pH changes in seven of these lakes since 1860, four of these being declines. The largest diatom-inferred (DI) pH declines of 0.5 pH units were found in Brown L. and Denton L., Wisconsin. Two other lakes with suspected total alkalinity declines (based on an acidification model and on historical water chemistry, respectively), McNearney L., Michigan, and Camp 12 L., Wisconsin, have not acidified recently according to diatom-inference techniques. Many of the observed trends of increasing pH are coincident with logging; floristic composition of diatom assemblages also changed coincident with fisheries manipulations in some lakes, but these floristic trends did not affect DI pH. Sediment core profiles of Pb, S, and polycyclic aromatic hydrocarbons provide a record of atmospheric deposition of fossil fuel combustion products beginning around the turn of the century; onset is later and accumulation rates are smaller than for other northeastern study regions of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) Project. The response of diatom species to lakewater pH in the NGLS region is very strong and similar to response in other regions. Overall, there is little paleoecological evidence that acidic deposition has caused significant acidification of lakes in the NGLS region.This is the twelfth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series. 相似文献
124.
125.
Matt J. Jarvis Ross J. McLure 《Monthly notices of the Royal Astronomical Society》2006,369(1):182-188
We have identified two new galaxies with gas counter-rotation (NGC 1596 and 3203) and have confirmed similar behaviour in another one (NGC 128), this using results from separate studies of the ionized-gas and stellar kinematics of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10 ± 5 per cent of our sample, but the fraction climbs to 21 ± 11 per cent when only lenticular (S0) galaxies are considered and to 27 ± 13 per cent for S0 galaxies with detected ionized gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0 galaxies in the literature yields fractions of 15 ± 4 and 23 ± 5 per cent, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed. 相似文献
126.
127.
THE REMANENT MAGNETISM OF ARTIFICIALLY DEPOSITED SEDIMENTS 总被引:1,自引:0,他引:1
R. F. King 《Geophysical Journal International》1955,7(S3):115-134
128.
Unit cell parameters of 12 selected lunar plagioclase specimens have been obtained using least-squares refinement of X-ray powder diffraction data. Compositions of splits of 11 of these plagioclase specimens were determined using the electron microprobe. Optical axial angles were measured on analyzed spots for nine of the probed specimens. Analyses cluster about An 96.5 ± 1.0 and unit cell parameters and 2V
x fit the values for this composition obtained in other studies. 相似文献
129.
130.
Summary. A reduced equation of motion is used to compute the residual velocity and the residual transport through the West Solent from the water levels recorded over an eight-month period at tide gauges at either end of the channel. A coefficient of bottom friction of 5·0 × 10−3 is assumed. There was a spring-neap variation and a significant correlation of fluctuations in the residual velocity with meteorological conditions. Westward residual velocities occurred at spring tides with low barometric pressure and south-westerly winds. Eastward residual velocities occurred at neap tides with high pressure and north-easterly winds. Because of the progressive nature of the tidal wave the long term residual transport appeared to be towards the west and the flushing time for the Solent system was long for considerable periods. The maximum velocities experienced during a tidal cycle half way along the channel are towards the west with a probability of values exceeding 160 cm s−1 for 10 min in 5·4 days. 相似文献