Investigating the central engine of ultraluminous infrared galaxies: near-infrared imaging     

R.I. Davies  A. Burston  M.J. Ward 《Monthly notices of the Royal Astronomical Society》2002,329(2):367-376

We present 0.5-arcsec-resolution near-infrared images of six ultraluminous infrared galaxies with known redshifts. Six of the eight bright nuclei are resolved on kiloparsec scales, suggesting that there is significant circumnuclear star formation or close progenitor nuclei. At this spatial resolution, the nuclei have very red colours that cannot in general be reproduced by reddening stellar light, but require an additional component of hot dust emission. In five of the six primary nuclei more than 20 per cent of the K -band continuum originates in hot dust, but the temperature cannot be determined by JHK broad-band colours alone. Comparison with the spectral shapes, however, does allow the temperature to be constrained, and we find in every case that it is at the upper end of the permissible range, ≳1000 K. This does not necessarily imply that there is an active galactic nucleus present, since there is evidence that stellar processes can also generate dust this hot via stochastic heating of small grains. The quantities of hot dust we have found here can make up to 0.5-mag difference to the K -band magnitude even at     , with implications for observations and population synthesis models of higher-redshift objects. Observations in the L or M bands, where hot dust is most important at     , could help to discriminate between models of dusty starbursts and ellipticals.  相似文献   

Solar nebula dispersal and the stability of the planetary system: I. Scanning secular resonance theory     

William R. Ward 《Icarus》1981,47(2):234-264

Secular resonances in the early solar system are studied in an effort to establish constraints on the time scale and/or method of solar nebula dispersal. Simplified nebula models and dispersal routines are employed to approximate changes in an assumed axisymmetric nebula potential. These changes, in turn, drive an evolutionary sequence of Laplace-Lagrange solutions for the secular variations of the solar system. A general feature of these sequences is a sweep of one or more giant planet resonances through the inner solar system. Their effect is rate dependent; in the linearized models considered, characteristic dispersal times ≤O(10^4?5 years) are required to avoid the generation of terrestrial eccentricities and inclinations in excess of observed values. These times are short compared to typical estimates of the accretion time scales [i.e., ～O(10^7?9 years)] and may provide an important boundary condition for developing models of nebula dispersal and solar system formation in general.  相似文献   

Speculation on martian north polar wind circulation and the resultant orientations of polar sand dunes     

A.W. Ward  K.B. Doyle 《Icarus》1983,55(3):420-431

Dunes in the Martian north polar erg show two dominant orientations. When seen at frost cap minimum, dunes north of 80°N record east winds, dunes south of 80°N record west winds. Many of the transverse dunes are considered to be reversing dunes. Dunes in two fields may have reversed at least once during the lifetime of the Viking Orbiters. Poor agreement exists among published predictive models of north polar winds and the interpretations derived from the major published map of the north polar dunes. We propose that the average polar winds are: (1) strong, off-pole northwest winds in fall; (2) moderate west winds in winter; (3) latitude-dependent weak to strong off-pole northeast winds in spring; and (4) weak west winds in summer. Viking images of near-polar clouds confirm much of the hypothesis. Images discussed in other studies can be given alternative interpretations that support this hypothesis also. Over millenia, the combination of reversing west and east winds could produce the binodal distributions of dune orientations observed at the north pole.  相似文献   

On Detecting Changes in the Earth's Radius     

M. A. Ward 《Geophysical Journal International》1963,8(2):217-225

  相似文献   

Orbital inclination of Iapetus and the rotation of the Laplacian plane     

William R. Ward 《Icarus》1981,46(1):97-107

This paper explores the possibility that the orbit of Iapetus, with its relatively large inclination but small eccentricity, was generated by a rapid dispersal of a gaseous circumplanetary disk, assumed to be the progenitor of the satellite system. The orientation of the local Laplacian plane is shown to be a sensitive function of the disk's structure. Modification of the disk on a time scale comparable to the precesion of the orbit's nodal line, [i.e., $\text{O}$(10²?10³ years)], can produce a large inclination from one that is initially zero, while leaving the eccentricity unchanged. This time is of the same order of magnitude as the viscous evolution time scale for a fully turbulent disk. Hence Iapetus need not be a captured satellite to account for its curious orbital signature.  相似文献   

Experiments on δS mixing between organic and inorganic sulfur species during thermal maturation     

Alon Amrani  Ward Said-Ahamed  Zeev Aizenshtat 《Geochimica et cosmochimica acta》2006,70(20):5146-5161

Reduced sulfur species were studied to constrain isotopic exchange-mixing with synthetic polysulfide cross-linked macromolecules (PCLM), model sulfur containing molecules and natural sulfur-rich kerogen, asphalt and oil of the Dead Sea area. PCLM represents protokerogens that are rich in sulfur and thermally unstable. Mixing rates of PCLM with (added as (NH₄)₂S_(aq)) at low to moderate temperatures (50-200 °C) are rapid. Elemental sulfur and H₂S_(gas) fully mix isotopes with PCLM during pyrolysis conditions at 200 °C. During these reactions significant structural changes of the PCLM occur to form polysulfide dimers, thiolanes and thiophenes. As pyrolysis temperatures or reaction times increase, the PCLM thermal products are transformed to more aromatic sulfur compounds. Isotopic mixing rates increase with increasing pyrolysis temperature and time. Polysulfide bonds (S-S) in the PCLM are responsible for most of these structural and isotopic changes because of their low stability. Conversely, sulfur isotope mixing does not occur between dibenzothiophene (aromatic S) or hexadecanthiol (C-SH) and at 200 °C after 48 h. This shows that rates of sulfur isotope mixing are strongly dependent on the functionality of the sulfur in the organic matter. The order of isotopic mixing rates for organic matter is kerogen > asphalt > oil, which is inverse to their sulfur thermal stability. Asphalt and oil with more refractory sulfur show significantly lower isotopes mixing rates than the kerogen with more labile sulfur. Based on the findings of the present study we suggest that sulfur isotopes mixing can occur from early diagenesis into catagenesis and result in isotopic homogenization of the inorganic and organic reduced sulfur pools.  相似文献   

Consequences of change and variability in sea ice on marine ecosystem and biogeochemical processes during the 2007–2008 Canadian International Polar Year program     

David G. Barber  Matthew G. Asplin  Tim N. Papakyriakou  Lisa Miller  Brent G. T. Else  John Iacozza  C. J. Mundy  M. Gosslin  Natalie C. Asselin  Steve Ferguson  Jennifer V. Lukovich  Gary A. Stern  Ashley Gaden  Monika Pu?ko  N.-X. Geilfus  Fei Wang 《Climatic change》2012,115(1):135-159

Change and variability in the timing and magnitude of sea ice geophysical and thermodynamic state have consequences on many aspects of the arctic marine system. The changes in both the geophysical and thermodynamic state, and in particular the timing of the development of these states, have consequences throughout the marine system. In this paper we review the ??consequences?? of change in sea ice state on primary productivity, marine mammal habitats, and sea ice as a medium for storage and transport of contaminants and carbon exchange across the ocean-sea-ice-atmosphere interface based upon results from the International Polar Year. Pertinent results include: 1) conditions along ice edges can bring deep nutrient-rich ??pacific?? waters into nutrient-poor surface waters along the arctic coast, affecting local food webs; 2) both sea ice thermodynamic and dynamic processes ultimately affect ringed seal/polar bear habitats by controlling the timing, location and amount of surface deformation required for ringed seal and polar bear preferred habitat 3) the ice edges bordering open waters of flaw leads are areas of high biological production and are observed to be important beluga habitat. 4) exchange of climate-active gases, including CO₂, is extremely active in sea ice environments, and the overall question of whether the Arctic Ocean is (or will be) a source or sink for CO₂ will be dependent on the balance of competing climate-change feedbacks.  相似文献   

Global exposure to river and coastal flooding: Long term trends and changes   总被引：1，自引：0，他引：1  

Brenden Jongman  Philip J. Ward  Jeroen C.J.H. Aerts 《Global Environmental Change》2012,22(4):823-835

Flood damage modelling has traditionally been limited to the local, regional or national scale. Recent flood events, population growth and climate change concerns have increased the need for global methods with both spatial and temporal dynamics. This paper presents a first estimation of global economic exposure to both river and coastal flooding for the period 1970–2050, using two different methods for damage assessment. One method is based on population and the second is based on land-use within areas subject to 1/100 year flood events. On the basis of population density and GDP per capita, we estimate a total global exposure to river and coastal flooding of 46 trillion USD in 2010. By 2050, these numbers are projected to increase to 158 trillion USD. Using a land-use based assessment, we estimated a total flood exposure of 27 trillion USD in 2010. For 2050 we simulate a total exposure of 80 trillion USD. The largest absolute exposure changes between 1970 and 2050 are simulated in North America and Asia. In relative terms we project the largest increases in North Africa and Sub-Saharan Africa. The models also show systematically larger growth in the population living within hazard zones compared to total population growth. While the methods unveil similar overall trends in flood exposure, there are significant differences in the estimates and geographical distribution. These differences result from inherent model characteristics and the varying relationship between population density and the total urban area in the regions of analysis. We propose further research on the modelling of inundation characteristics and flood protection standards, which can complement the methodologies presented in this paper to enable the development of a global flood risk framework.  相似文献   

Atmospheric circulation and storminess derived from Royal Navy logbooks: 1685 to 1750     

D. Wheeler  R. Garcia-Herrera  C. W. Wilkinson  C. Ward 《Climatic change》2010,101(1-2):257-280

This paper is concerned with the use of Royal Navy ships’ logbooks for the period 1685 to 1750, thereby embracing the oldest members of this documentary series that extends to the present day. The geographic range of the undertaking is confined to the English Channel and its western approaches where the abundance of logbooks for this period allows for the abstraction of a daily series of wind force and direction data. These are verified and processed for inclusion in a database from which indices are derived for air circulation patterns based on the frequency of winds from north, east, south and west quarters, and of gale frequency, based on the contemporary terminology of wind force. The methods by which the data are abstracted and processed are described, as is the nature of the raw data and source material. The results provide a uniquely detailed insight into the changing patterns of air circulation over this critical period that marked the transition from the depths of the Little Ice Age in the late seventeenth century. Attention is also drawn to the changing nature of gale frequency, which revealed a notable decrease over the study period. Associations between changing circulation patterns and temperature regimes are also explored.  相似文献   

The optimal choice of residue management, crop rotations, and cost of carbon sequestration: empirical results in the Midwest US   总被引：1，自引：0，他引：1  

Suk-Won Choi  Brent Sohngen 《Climatic change》2010,99(1-2):279-294

This study investigates the cost of soil carbon sequestration in the Midwest US. The model addresses several missing components in earlier analyses: the link between the residue level choice and carbon payments, crop rotations, carbon loss when shifting from conservation to conventional tillage and the spatial pattern of carbon sequestration across different soil types. The results suggest that for $100 per metric ton of carbon, 1.5 million metric tons of carbon could be sequestered per year on the 19.9 million hectares of cropland in the study region. These estimates suggest less carbon potential than existing studies because the opportunity costs associated with conservation tillage are fairly high. Annual carbon rental payments are found to be more efficient, as expected, but for smaller programs, per hectare rental payments are not substantially more costly.  相似文献