The colour–magnitude relation of early-type galaxies in the Hubble Deep Field     

Tadayuki Kodama  Richard G. Bower  Eric F. Bell 《Monthly notices of the Royal Astronomical Society》1999,306(3):561-566

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Hydrodynamic simulations of propagating warps and bending waves in accretion discs     

Richard P. Nelson  John C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》1999,309(4):929-940

We present the results of a study of propagating warp or bending waves in accretion discs. Three-dimensional hydrodynamic simulations were performed using smoothed particle hydrodynamics (SPH), and the results are compared with calculations based on the linear theory of warped discs.
We examine the response of a gaseous disc to an initially imposed warping disturbance under a variety of physical conditions. We consider primarily the physical regime in which the dimensionless viscosity parameter α < H r , where H r is the disc aspect ratio, so that bending waves are expected to propagate. We also performed calculations for disc models in which α > H r , where the warps are expected to evolve diffusively. Small-amplitude (linear) perturbations are studied in both Keplerian and slightly non-Keplerian discs, and we find that the results of the SPH calculations can be reasonably well fitted by those of the linear theory. The main results of these calculations are: (i) the warp in Keplerian discs when α < H r propagates with little dispersion, and damps at a rate expected from estimates of the code viscosity; (ii) warps evolve diffusively when α > H r ; (iii) the slightly non-Keplerian discs lead to a substantially more dispersive behaviour of the warps, which damp at a similar rate to the Keplerian case, when α < H r .
Initially imposed higher amplitude, non-linear warping disturbances were studied in Keplerian discs. The results indicate that non-linear warps can lead to the formation of shocks, and that the evolution of the warp becomes less wave-like and more diffusive in character.
This work is relevant to the study of the warped accretion discs that may occur around Kerr black holes or in misaligned binary systems, and is mainly concerned with discs in which α < H r . The results indicate that SPH can model the hydrodynamics of warped discs, even when using rather modest numbers of particles.  相似文献   

Space weathering on the Moon: Patina on Apollo 17 samples 75075 and 76015     

Susan J. WENTWORTH  Lindsay P. KELLER  David S. McKAY  Richard V. MORRIS 《Meteoritics & planetary science》1999,34(4):593-603

Abstract— We studied patinas on lunar rocks 75075 and 76015 from the Apollo collection using a multi-disciplinary approach, including scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDS), transmission electron microscopy (TEM), wavelength-dispersive x-ray (WDS) mapping, Mössbauer spectroscopy, spectral reflectance, and microspectrophotometry. Based on SEM petrography, we have defined three textural types of patina: glazed, fragmental, and classic (cratered). The presence of classic patina is diagnostic of lunar samples that have been exposed directly to the space weathering environment. It is characterized by the presence of microcraters and glass pancakes and is the patina type studied by earlier workers. Classic patina is found on 76015 but not on 75075. Glazed patina is found on both 76015 and 75075, whereas fragmental patina is found only on 75075. The glazed and fragmental patinas on 75075 were probably formed as a result of relatively large nearby impacts; and although these two types of patina are not strictly the result of direct exposure to the space weathering environment, they are important because they affect the optical properties of the rocks. Field emission gun SEM (FE-SEM) of classic patina on 76015 shows evidence of possible solar wind sputtering erosion. Transmission electron microscope studies of 76015 reveal the presence of impact-generated deposits and solar flare particle tracks which, like microcraters and pancakes, are diagnostic of direct exposure to space weathering processes. The outermost surface of the 76015 patina consists of an amorphous rim very much like the rims found on individual lunar soil grains; this amorphous patina rim probably formed by similar processes of impact-generated vapor condensation and possible sputter deposition. Wavelength-dispersive x-ray element maps of polished thin sections of 75075 and 76015 indicate that patina compositions are poor indicators of the compositions and mineralogies of the rocks underlying them. On average, the reflectance spectra of patinas on both samples are slightly darker than those of their unweathered equivalents. Microreflectance measurements show that a thick patina can dramatically alter the optical properties of the rock on which it forms. The backscatter Mössbauer (BaMS) spectrum of a patina-covered surface of 76015 is very similar to that of an unweathered surface, indicating that the Mössbauer signal is generated from beneath the patina. Because BaMS “sees” through surface space-weathering effects to the underlying rock, this technique has great potential for use in robotic missions to other planetary bodies.  相似文献   

Statistical tools for estimating and combining finite rotations and their uncertainties     

Bessie H. Kirkwood  Jean-Yves Royer  Theodore C. Chang  & Richard G. Gordon 《Geophysical Journal International》1999,137(2):408-428

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On the age calibration of the geomagnetic polarity timescale     

Pierre Agrinier  Yves Gallet  & Eric Lewin 《Geophysical Journal International》1999,137(1):81-90

Knowledge of the age of undated events is not null if a time-order relationship can be found among these events. The knowledge of such a time-ordered sequence can be formalized by using non-informative (uniform) prior probability densities for the ages of undated events and Bayes' theorem to introduce the time-order relationship condition. We show that the conditional probability densities of the ages of events of unknown age are given by various forms of Euler's beta distribution. These distributions yield an estimate of the probability for an undated event to occur in a given age interval.
  We use this method to propose appropriate probabilistic representations of our actual knowledge of the dating of the magnetic polarity reversals during the Cenozoic. These representations take into account the uncertainties arising from irregularities in accretion process and from the quality of a few calibration points. Both types of uncertainties generate large ambiguities in the age of magnetic reversals, which should be taken into consideration when the geomagnetic polarity timescale is used for dating purposes. We propose to use the entropy function to quantify these ambiguities.  相似文献   

Modelling of local velocity anomalies: a cookbook   总被引：1，自引：0，他引：1  

Corinne Juliard  & Pierre D. Thore 《Geophysical Prospecting》1999,47(3):299-312

The determination of small-scale velocity anomalies (from tens to a few hundreds of metres) is a major problem in seismic exploration. The impact of such anomalies on a structural interpretation can be dramatic and conventional techniques such as tomographic inversion or migration velocity analysis are powerless to resolve the ambiguity between structural and velocity origins of anomalies. We propose an alternative approach based on stochastic modelling of numerous anomalies until a set of models is found which can explain the real data. This technique attempts to include as much a priori geological information as possible. It aims at providing the interpreter with a set of velocity anomalies which could possibly be responsible for the structural response. The interpreter can then choose one or several preferred models and pursue a more sophisticated analysis. The class of retained models are all equivalent in terms of data and therefore represent the uncertainty in the model space. The procedure emulates the real processing sequence using a simplified scheme. Essentially, the technique consists of five steps: 1 Interpretation of a structural anomaly in terms of a velocity anomaly with its possible variations in terms of position, size and amplitude. 2 Drawing a model by choosing the parameters of the anomaly within the acceptable range. 3 Modelling the traveltimes in this model and producing the imaging of the reflected interface. 4 Comparing the synthetic data with the real data and keeping the model if it lies within the data uncertainty range. 5 Iterate from step 2. In order to avoid the high computational cost inherent in using statistical determinations, simplistic assumptions have been made: ? The anomaly is embedded in a homogeneous medium: we assume that the refraction and the time shift due to the anomaly have a first-order effect compared with ray bending in the intermediate layers. ? We model only the zero-offset rays and therefore we restrict ourselves to structural problems. ? We simulate time migration and so address only models of limited structural complexity. These approximations are justified in a synthetic model which includes strong lateral velocity variations, by comparing the result of a full processing sequence (prestack modelling, stack and depth migration) with the simplified processing. This model is then used in a blind test on the inversion scheme.  相似文献   

On the Search for the Correct Economic Assessment Method     

Richard A. Adams 《Climatic change》1999,41(3-4):363-370

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Water Resources Implications of Global Warming: A U.S. Regional Perspective   总被引：8，自引：1，他引：7  

Dennis P. Lettenmaier  Andrew W. Wood  Richard N. Palmer  Eric F. Wood  Eugene Z. Stakhiv 《Climatic change》1999,43(3):537-579

The implications of global warming for the performance of six U.S. water resource systems are evaluated. The six case study sites represent a range of geographic and hydrologic, as well as institutional and social settings. Large, multi-reservoir systems (Columbia River, Missouri River, Apalachicola-Chatahoochee-Flint (ACF) Rivers), small, one or two reservoir systems (Tacoma and Boston) and medium size systems (Savannah River) are represented. The river basins range from mountainous to low relief and semi-humid to semi-arid, and the system operational purposes range from predominantly municipal to broadly multi-purpose. The studies inferred, using a chain of climate downscaling, hydrologic and water resources systems models, the sensitivity of six water resources systems to changes in precipitation, temperature and solar radiation. The climate change scenarios used in this study are based on results from transient climate change experiments performed with coupled ocean-atmosphere General Circulation Models (GCMs) for the 1995 Intergovernmental Panel on Climate Change (IPCC) assessment. An earlier doubled-CO₂ scenario from one of the GCMs was also used in the evaluation. The GCM scenarios were transferred to the local level using a simple downscaling approach that scales local weather variables by fixed monthly ratios (for precipitation) and fixed monthly shifts (for temperature). For those river basins where snow plays an important role in the current climate hydrology (Tacoma, Columbia, Missouri and, to a lesser extent, Boston) changes in temperature result in important changes in seasonal streamflow hydrographs. In these systems, spring snowmelt peaks are reduced and winter flows increase, on average. Changes in precipitation are generally reflected in the annual total runoff volumes more than in the seasonal shape of the hydrographs. In the Savannah and ACF systems, where snow plays a minor hydrological role, changes in hydrological response are linked more directly to temperature and precipitation changes. Effects on system performance varied from system to system, from GCM to GCM, and for each system operating objective (such as hydropower production, municipal and industrial supply, flood control, recreation, navigation and instream flow protection). Effects were generally smaller for the transient scenarios than for the doubled CO₂ scenario. In terms of streamflow, one of the transient scenarios tended to have increases at most sites, while another tended to have decreases at most sites. The third showed no general consistency over the six sites. Generally, the water resource system performance effects were determined by the hydrologic changes and the amount of buffering provided by the system's storage capacity. The effects of demand growth and other plausible future operational considerations were evaluated as well. For most sites, the effects of these non-climatic effects on future system performance would about equal or exceed the effects of climate change over system planning horizons.  相似文献   

Flood tide circulation near beaufort inlet, North Carolina: Implications for larval recruitment     

James H. Churchill  James L. Hench  Richard A. Luettich  Jackson O. Blanton  Francisco E. Werner 《Estuaries and Coasts》1999,22(4):1057-1070

Drifter tracks and shipboard CTD observations have revealed a number of distinct features of the flood tide circulation carrying water through Beaufort Inlet, North Carolina. One of the most noteworthy of these features is a nearshore jet in the flow carrying water to the inlet on a flood tide. Characterized by a shoreward increase in longshore flow, the jet produces a narrow coastal zone over which water is carried into the inlet. The jet appears to be principally a tidal phenomenon, as it is closely reproduced by a tidally-driven barotropic numerical model. The model results also indicate the jet may be a near-inlet feature. Model simulations of spring tide conditions show the jet confined to within 4 km of the inlet mouth. Another observed phenomenon, which is reproduced by the tidal model, is a distinct splitting of the flow entering the inlet, in which water passing through a particular inlet segment tends to move up-estuary along a well-defined path. An observed flow feature not reproduced by the tidal model is an eastward skew of the region over which water is drawn into the inlet on a flood tide. This asymmetry is unrelated to the local wind. Modeling results from a previous study suggest it may be due to convergent flow at the edge of the low salinity plume issuing from the inlet. Taken together, the results of this and other recent studies in the Beaufort Inlet region reveal the importance of nearshore currents on the eastern side of the inlet in delivering oceanic-spawned larvae to the estuarine system connected to the inlet.  相似文献   

The use of proxy chemical records in Coral skeletons to ascertain past environmental conditions in Florida Bay     

Peter Koenraad Swart  Genevieve Healy  Lisa Greer  Michael Lutz  Amel Saied  Daniel Anderegg  Richard E. Dodge  David Rudnick 《Estuaries and Coasts》1999,22(2):384-397

This paper will discuss the use of chemical proxies in coral skeletons to reconstruct the history of salinity (from the δ¹⁸O of the skeleton) and nutrients in the water (from the δ¹³C) in Florida Bay between 1824 and 1994. Monthly salinity and water temperature data collected since 1989 were used to establish a correlation between salinity, temperature, and the δ¹⁸O of the skeleton of the coralSolenastrea bournoni from Lignumvitae Basin in Florida Bay. This relationship explains over 50% of the variance in the δ¹⁸O of the skeleton. Assuming that interannual variations in the temperature of the water are small, we have applied this relationship to the δ¹⁸O measured in the coral skeleton collected from Lignumvitae Basin which has a record between 1824 and 1993. These data provide a revised estimate of salinity variation in Lignumvitae Basin for the period when historical records for salinity were not available, and show that the highest salinity events occurred in the past 30 yr. Using the relationships between the salinity in Lignumvitate Basin and other basins, obtained using a modern dataset, we are able to estimate ranges in salinity for other portions of Florida Bay. Skeletons of specimens of the coral speciesSiderastrea radians collected from other areas of Florida Bay show similar patterns in the δ¹⁸O over the past 10 yr, indicating that corals in most portions of Florida Bay are recording salinity variations in their skeletons and therefore support the idea that salinity variations in different portions of Florida Bay can be related. Fluorescence analysis of the coral from Lignumvitae Basin shows a large change in the magnitude of the 10-yr signal coincident with the construction of the railway, confirming that this event had a significant impact upon Florida Bay. The δ¹³C of the coral skeletons reveals a long-term history of the oxidation of organic material, fixation of carbon by photosynthesis (algal blooms), and the intrusion of marine water into the bay. Since the construction of the railway from Miami to Key West there has been a long-term decrease in the δ¹³C of the coral skeleton from Lignumvitae Basin, suggesting the increased oxidation of organic material in this area. This decrease in δ¹³C appears to have reached a minimum value around 1984 and has increased since this time in the western portions of Florida Bay. The increase may be related to the algal blooms prevalent in the area or alternatively could result from intrusion of more marine water. In the eastern areas, a small increase in the δ¹⁸C between 1984 and 1988 was followed by further decline suggesting more oxidation of organic material. We have also attempted to use the concentration of barium in the coral skeleton as a proxy indicator of the nutrient status in Florida Bay.  相似文献