Mass segregation in young stellar clusters     

Ian A. Bonnell  & Melvyn B. Davies 《Monthly notices of the Royal Astronomical Society》1998,295(3):691-698

We investigate the evolutionary effect of dynamical mass segregation in young stellar clusters. Dynamical mass segregation acts on a time-scale of order the relaxation time of a cluster. Although some degree of mass segregation occurs earlier, the position of massive stars in rich young clusters generally reflects the cluster's initial conditions. In particular, the positions of the massive stars in the Trapezium cluster in Orion cannot be due to dynamical mass segregation, but indicate that they formed in, or near, the centre of the cluster. Implications of this for cluster formation and for the formation of high-mass stars are discussed.  相似文献   

The expulsion and retention of dust grains by galactic discs     

J. I. Davies  P. Alton  S. Bianchi  & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014

We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 10⁹ yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium.  相似文献   

Crustal strain in central Greece from repeated GPS measurements in the interval 1989–1997     

P. J. Clarke  R. R. Davies  P. C. England  B. Parsons  H. Billiris  D. Paradissis  G. Veis  P. A. Cross  P. H. Denys  V. Ashkenazi  R. Bingley  H.-G. Kahle  M.-V. Muller  & P. Briole 《Geophysical Journal International》1998,135(1):195-214

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Optical, infrared and radio observations of the Seyfert galaxy Markarian 315     

M. Nonino  J. P. Henry  C. Fanti  R. Fanti  & J. Davies 《Monthly notices of the Royal Astronomical Society》1998,299(2):332-340

We report on new optical, infrared and radio observations of the Seyfert galaxy Markarian 315. We confirm the detection at all wavelengths of a secondary peak of emission, which lies ≈ 2 arcsec east of the Seyfert nucleus. Moreover, we detect a chain-like structure which surrounds the active nucleus, with peculiar behaviour westward of the nucleus. We consider different interpretations for the origin of the secondary peak emission.  相似文献   

The effect of uncertainty in aerodynamic resistance on evaporation estimates from the combination model   总被引：1，自引：0，他引：1  

W. G. Bailey  J. A. Davies 《Boundary-Layer Meteorology》1981,20(2):187-199

Evaporation estimates from a soybean crop calculated from the combination model are insensitive to aerodynamic resistance. The insensitivity arises from a strong link between evaporation and the vapour pressure deficit of the air and bulk stomatal resistance. The sensitivity of aerodynamic resistance to errors in surface roughness and zero-plane displacement is considered. The resistance is found to be more sensitive to errors in surface roughness than to errors in zero-plane displacement. However, large errors in these have little effect on calculated evaporation. Both surface roughness and zero-plane displacement are empirically related to crop height and leaf area index. Errors incurred by ignoring bluff-body effects and atmospheric stability are small in estimating both resistance and evaporation. Evaporation can be calculated adequately from empirical estimates of surface roughness and zero-plane displacement and single-level measurements of windspeed.  相似文献   

The mussel Mytilus edulis as a bio-assay organism for mercury in seawater     

I.M. Davies  J.M. Pirie 《Marine pollution bulletin》1978,9(5):128-132

Surveys of natural populations of mussels have identified areas of mercury contamination in the Firth of Forth, Scotland. A field bio-assay technique has been devised which accurately reflects the mean total mercury concentration in the surrounding seawater. The detection limit of the technique is estimated at 5–20 ng Hg l.^?1, and consequently the method can detect comparatively small enhancements over background mercury concentrations in estuarine and sea water.  相似文献   

Estimating the surface wind speed over drifting pack ice from surface weather charts     

Uri Feldman  Philip J. Howarth  John A. Davies 《Boundary-Layer Meteorology》1979,16(4):421-429

Routine surface wind speed data, vital for the study of drifting pack ice, are not available for the polar oceans. Over sea, it has been demonstrated by Hasse that estimates of surface wind speed may be obtained from the geostrophic wind speed using linear formulae. Comparing estimated with observed data for six sites in Canada, it is found that the formulae may also be applied to obtain estimates of surface wind speed over pack ice.  相似文献   

Recent changes in the level of Lake Naivasha,Kenya, as an indicator of equatorial westerlies over East Africa   总被引：1，自引：0，他引：1  

C. E. Vincent  T. D. Davies  A. K. C. Beresford 《Climatic change》1979,2(2):175-189

A previously unpublished record of lake levels from Lake Naivasha, Kenya from 1880 to 1976 has been analysed and shows little similarity to the level record from nearby Lake Victoria. Level changes from year to year of the two lakes show no significant correlation (at 5%) and spectral analysis of the two records shows no common significant peaks. Both lakes show significant correlations between their level changes and the strength of the North Atlantic winter circulation, with the correlation coefficients in opposing directions indicating important, but different, large scale climatic links.Lake Naivasha's major level increases occur during May and September. Lake Victoria's level increases mainly in May with a small December increase. East African rainfall is generally during April and November, corresponding with Lake Victoria's changes. Rainfall records from Kenyan highland areas, however, show an August rainfall peak and little rainfall in November. Rainfall amounts from Equator, a highland meteorological station, for July, August and September are highly correlated (at 1% significance level) with the change in Naivasha's level during September. Winds at the highland stations are westerly during August while the lower level stations experience the drier S.E. Trades. The level changes of Lake Naivasha indicate changes in the extent of the penetration of moist air from West Africa between the Trade winds and the 200 mb easterly jet.  相似文献   

Loss of mercury from sea water     

G. Topping  I.M. Davies 《Marine pollution bulletin》1980,11(3):83-84

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Synoptic Climatology of Air Pollution in Moscow   总被引：8，自引：0，他引：8  

M. Shahgedanova  T. P. Burt  T. D. Davies 《Theoretical and Applied Climatology》1998,61(1-2):85-102

Summary Seasonal synoptic indices are developed employing principal components analysis and cluster analysis for Moscow and using surface and upper air meteorological data. Synoptic types are analysed in terms of circulation patterns and air mass characteristics. The indices are used to examine weather-induced variability in CO and NO₂ concentrations for different land use types and in the context of multiple pollution sources. Anticyclonic conditions in spring, summer and autumn are identified as producing high pollution concentrations. Local depressions developing in the low pressure gradient fields in summer are another synoptic type resulting in high levels of air pollution. Extremely cold anticyclonic conditions in winter result in high CO but low NO₂ levels. Weather patterns characterised by strong winds associated either with an established zonal flow or the passage of a cold front caused high NO₂ concentrations in the areas affected by elevated sources. Received October 28, 1997 Revised January 26, 1998  相似文献