Chemical Evolution of Protostars     

Michiel R. Hogerheijde 《Astrophysics and Space Science》2005,295(1-2):179-187

This review discusses the evolution of the chemical composition of the molecular gas throughout the process of star formation. It covers the initial stages of cloud-core formation, gravitational collapse and the formation of the star, the effects of stellar radiation and outflows on the surrounding cloud, and the composition of the circumstellar disk. A brief overview of relevant chemical processes sets the stage. The ensuing evolution of the chemistry is governed by several competing timescales and by energy input by the young star(s). Special attention is given to the role of molecular freeze-out on dust grains and to deuterium fractionation in cold gas.  相似文献   

Present-day and future Antarctic ice sheet climate and surface mass balance in the Community Earth System Model     

Jan T. M. Lenaerts  Miren Vizcaino  Jeremy Fyke  Leo van Kampenhout  Michiel R. van den Broeke 《Climate Dynamics》2016,47(5-6):1367-1381

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Normal-branch quasi-periodic oscillations during the high-intensity state of Cygnus X-2     

Rudy Wijnands †  Michiel van der Klis 《Monthly notices of the Royal Astronomical Society》2001,321(3):537-543

Using data obtained with the Rossi X-ray Timing Explorer , we report the detection of a 5-Hz quasi-periodic oscillation (QPO) in the bright low-mass X-ray binary and Z source Cygnus X-2 during high overall intensities (the high-intensity state). This QPO was detected on the so-called normal-branch and can be identified with the normal-branch QPO or NBO. Our detection of the NBO is the first one during times when Cygnus X-2 was in the high-intensity state. The rms amplitude of this QPO decreased from 2.8 per cent between 2 and 3.1 keV to <1.9 per cent between 5.0 and 6.5 keV. Above 6.5 keV, its amplitude rapidly increased to ∼12 per cent rms above 16 keV. The time lags of the QPO were consistent with being zero below 5 keV (compared with the 2–3.1 keV band), but they rapidly increased to ∼70 ms (140°) around 10 keV, above which the time lags remained approximately constant near 70 ms. The photon energy dependences of the rms amplitude and the time lags are very similar to those observed for the NBO with other satellites ( Ginga , EXOSAT ) at different (i.e. lower) intensity states.  相似文献   

Momentum and scalar transfer coefficients over aerodynamically smooth antarctic surfaces     

Richard Bintanja  Michiel R. Van Den Broeke 《Boundary-Layer Meteorology》1995,74(1-2):89-111

Vertical profiles of wind speed, temperature and humidity were used to estimate the roughness lengths for momentum (z ₀), heat (z _H ) and moisture (z _Q) over smooth ice and snow surfaces. The profile-measurements were performed in the vicinity of a blue ice field in Queen Maud Land, East Antarctica. The values ofz ₀ over ice (3·10^–6 m) seem to be the smallest ever obtained over permanent, natural surfaces. The settling of snow on the ice and the loss of momentum at saltating snow particles serve as momentum dissipating processes during snow-drift events, expressed as a strong dependence ofz ₀ on u_#.The scalar roughness lengths and surface temperature can be evaluated from the temperature and humidity profile measurements if the ratioz _H /z _Q is specified. This new method circumvents the difficult measurement of surface temperature. The scalar roughness lengths seem to be approximately equal toz0 for a large range of low roughness Reynolds numbers, despite the frequent occurrence of drifting snow. Possible reasons for this agreement with theory of non-saltating flow are discussed.  相似文献   

An introduction to the application and future use of Th in aquatic systems     

James T. Waples  Claudia Benitez-Nelson  Nicolas Savoye  Michiel Rutgers van der Loeff  Mark Baskaran   rjan Gustafsson 《Marine Chemistry》2006,100(3-4):166

²³⁴Th is an extremely useful radiotracer of particle dynamics in aquatic systems. Its utility, however, has yet to be realized by many within the aquatic science community. The reasons for this may in part be due to a lack of knowledge of how this nuclide has been used in the past as well as how and where ²³⁴Th might be profitably applied in future research. The purpose of this paper, then, is to examine the variety of ²³⁴Th applications that have been used to understand natural aquatic processes in four major areas: vertical transport, particle cycling, horizontal transport, and sediment dynamics. We provide a general overview of the possible applications of ²³⁴Th in the hopes of provoking an increased interest in the inherent potential and future application of ²³⁴Th in these systems. We end this paper with a discussion of future research avenues in the context of three specific regimes: (i) the upper 1000 m of the open ocean, (ii) coastal sediment/water processes and (iii) large freshwater lakes.  相似文献   

A review of present techniques and methodological advances in analyzing Th in aquatic systems     

Michiel Rutgers van der Loeff  Manmohan M. Sarin  Mark Baskaran  Claudia Benitez-Nelson  Ken O. Buesseler  Matt Charette  Minhan Dai   rjan Gustafsson  Pere Masque  Paul J. Morris  Kent Orlandini  Alessia Rodriguez y Baena  Nicolas Savoye  Sabine Schmidt  Robert Turnewitsch  Ingrid Vge  James T. Waples 《Marine Chemistry》2006,100(3-4):190

The short-lived thorium isotope ²³⁴Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of ²³⁴Th in both marine and freshwater systems. The original ²³⁴Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of ²³⁴Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate ²³⁴Th analytical technique for a specific aquatic regime and known sampling constraints.  相似文献   

Scavenging and particle flux: seasonal and regional variations in the Southern Ocean (Atlantic sector)     

Michiel M. Rutgers van der Loeff  Gijs W. Berger 《Marine Chemistry》1991,35(1-4)

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Grain size dependency in the occurrence of sand waves     

Hennie Henriët van der Veen  Suzanne Jacqueline Marie Hélène Hulscher  Michiel Adrianus Frederik Knaapen 《Ocean Dynamics》2006,56(3-4):228-234

Sandy shallow seas, like the North Sea, are very dynamic. Several morphological features are present on the bed, from small ripples to sand waves and large tidal sandbanks. The larger patterns induce significant depth variations that have an impact on human activities taking place in this area. Therefore, it is important to know where these large-scale features occur, what their natural behaviour is and how they interact with human activities. Here, we extend earlier research that compares the results of an idealized model of large-scale seabed patterns with data of seabed patterns in the North Sea. The idealized model is extended with a grain size dependency. The adaptations lead to more accurate predictions of the occurrence of large-scale bed forms in the North Sea. Therefore, grain size dependency and, in particular, critical shear stress are important to explain the occurrence of sand waves and sandbanks in the North Sea. Responsible Editor: Alejandro Souza  相似文献   

Transportation of water,particulate and dissolved organic and inorganic matter between a salt marsh and the Ems-Dollard estuary,The Netherlands     

Norbert Dankers  Michel Binsbergen  Koos Zegers  Remi Laane  Michiel Rutgers van der Loeff 《Estuarine, Coastal and Shelf Science》1984,19(2):143-165

Transport processes were studied in a gully between a salt marsh and an estuary. After storm tides, ebb currents in the gully reached high values. It is concluded that particulate matter (both organic and inorganic) are imported into the marsh. Coarse organic debris is exported during storm tides, but this amount is low when compared with the primary production on the marsh. Exports are shown for dissolved organic carbon, ammonia, phosphate and silica, while nitrate and possibly nitrite are imported. Organic matter derived from in situ production and net import is buried and partly mineralized in the marsh.  相似文献   

Microbiological monitoring and classification of karst springs     

Michael Sinreich  Michiel Pronk  Ronald Kozel 《Environmental Earth Sciences》2014,71(2):563-572

The use of groundwater as a drinking water resource requires knowledge of its microbiological status and quality. In contrast to conventional microbiological monitoring of groundwater, the present study not only considers faecal indicator bacteria, but also covers a wide spectrum of microorganisms, including bacterial pathogens (verotoxin-producing E. coli, Campylobacter spp. and Salmonella spp., as well as Pseudomonas aeruginosa), human enteric viruses (norovirus, enterovirus, rotavirus and adenovirus) and parasitic protozoa (Cryptosporidium oocysts and Giardia cysts). Samples collected at karst sites of the Swiss National Groundwater Monitoring network revealed the presence of a large diversity of microorganisms of faecal origin, the occurrence of which could be linked to specific hydrogeological settings and situations. The findings represent a ‘snapshot’ of the microbiological status at the monitoring sites and provide a national overview of the types and presence of microorganisms in Swiss karst groundwater. In addition to microbiological parameters related to faecal contamination, the overall bacterial load in groundwater was assessed using cell density measurements (i.e. total cell count), which yielded typical ranges for this ecological parameter. The study highlights differential vulnerability of karst groundwater to microbiological contamination, as well as its relationship with the microbial biocenoses, i.e. the interplay of allochthonous and autochthonous microbial components. On the basis of this data set, a microbiological classification of karst aquifers is proposed and discussed with respect to spring dynamics and vulnerability.  相似文献