From boulders to planetary systems     

Hubert Klahr   《New Astronomy Reviews》2008,52(2-5):78

In this chapter, we will give a brief overview on our current theoretical understanding how planets form from the solid material in circumstellar disks in the core accretion-gas capture model. This chapter will not be as concise and complete as a review on this matter, yet will serve as an introductory text to generate interest in the subject. Students are referred to comprehensive text books and some important reviews.This chapter will discuss “dusty storms”, e.g. the dust transport in turbulent protoplanetary disks, followed by the latest model of planetesimal formation, e.g. gravoturbulent planetesimal formation, which deals with particle concentration in turbulence and N-body simulations thereof. We also briefly describe the core accretion-gas capture process and talk about nascent planets, e.g. the observability of planet–disk interaction concluding with the migration of young planets and the final arrangement of planetary systems.  相似文献   

High-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries   总被引：1，自引：1，他引：0  

Cécile Agosta  Vincent Favier  Gerhard Krinner  Hubert Gallée  Xavier Fettweis  Christophe Genthon 《Climate Dynamics》2013,41(11-12):3247-3260

About 75 % of the Antarctic surface mass gain occurs over areas below 2,000 m asl, which cover 40 % of the grounded ice-sheet. As the topography is complex in many of these regions, surface mass balance modelling is highly dependent on horizontal resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a computationally efficient, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess Antarctic SMB variability in the twenty first and the twenty second centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the twentieth century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL reproduce the observed values equally well. Nevertheless, field data below 2,000 m asl are too scarce to efficiently show the added value of SMHiL and measuring the SMB in these undocumented areas should be a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15 km) may give a future increase in SMB in Antarctica that is about 30 % higher than by using its standard resolution (60 km) due to the higher increase in precipitation in coastal areas at 15 km. However, a part (～15 %) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the competition between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas.  相似文献   

Status of the eLISA on table (LOT) electro-optical simulator for space based,long arms interferometers     

Pierre Grüning  Hubert Halloin  Pierre Prat  Sylvain Baron  Julien Brossard  Christelle Buy  Antoine Petiteau  Gerhard Heinzel  Iouri Bykov 《Experimental Astronomy》2015,39(2):281-302

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The segmentation procedure as a tool for discrete modeling of hydrometeorological regimes     

P. Hubert 《Stochastic Environmental Research and Risk Assessment (SERRA)》2000,14(4):297-304

A possible cause of nonstationarity in time series is the existence of some abrupt modification of their statistical parameters, and especially of a sudden change of the mean. Series with such a change exhibit a strong temporal persistence, with high values of the Hurst coefficient, but with poor possibilities to fit any autoregressive model. Some classical tests (Pettitt, 1979; Buishand, 1982) enable to find a possible change point of the mean and then to split the original nonstationary series into two stationary sub-series. The Bayesian procedure defined by Lee and Heghinian (1977) supposes the “a-priori” existence of a change of the mean somewhere in the series and yields at each time step an “a-posteriori” probability of mean change. But these classical tests and procedures consider only one change point in the original series. To go further and to explore the theoretical multiple singularity models defined by Klemeš (1974) and Potter (1976), a segmentation procedure of time series has been designed. This procedure yields an optimal partition (from a least squares point of view) of the original series into as many subseries as possible, all differences between two contiguous means remaining simultaneously significant. This last requirement is ensured using the Scheffe test of contrasts. The main problem has been to master the combinatory explosion while exploring the tree of all possible segmentations of a series. Some applications of the procedure to hydrometeorological time series are reviewed and some possible improvements are presented.  相似文献   

Fe Mössbauer spectroscopy as a tool in astrobiology     

Christian Schrder  Brad Bailey  Gstar Klingelhfer  Hubert Staudigel 《Planetary and Space Science》2006,54(15):1622-1634

The element Fe and Fe-bearing minerals occur ubiquitously throughout the field of astrobiology. Cycling between the various oxidation states of Fe provides a source of energy available for life. Banded iron formations may record the rise of oxygenic photosynthesis. The distribution of Fe between Fe-bearing minerals and its oxidation states can help to characterize and understand ancient environments with respect to the suitability for life by constraining the primary rock type and the redox conditions under which it crystallized, the extent of alteration and weathering, the type of alteration and weathering products, and the processes and environmental conditions for alteration and weathering. Fe Mössbauer spectroscopy is a powerful tool to investigate Fe-bearing compounds. It can identify Fe-bearing minerals, determine Fe oxidation states with high accuracy, quantify the distribution of Fe between mineralogical phases, and provide clues about crystallinity and particle sizes. Two miniaturized Mössbauer spectrometers are on board of the NASA Mars Exploration Rovers Spirit and Opportunity. The Fe-bearing minerals goethite, an iron oxide-hydroxide, and jarosite, an iron hydroxide sulfate, were identified by Mössbauer spectroscopy in Gusev Crater and at Meridiani Planum, respectively, providing in situ proof of an aqueous history of the two landing sites and constraints on their habitability. Hematite identified by Mössbauer spectroscopy at both landing sites adds further evidence for an aqueous history. On Earth, Mössbauer spectroscopy was used to monitor possibly microbially-induced changes of Fe-oxidation states in basaltic glass samples exposed at the Loihi Seamount, a deep sea hydrothermal vent system, which might be analogous to possible extraterrestrial habitats on ancient Mars or the Jovian moon Europa today.  相似文献   

CLAIRE: First light for a gamma-ray lens     

Peter von Ballmoos  Hubert Halloin  Jean Evrard  Gerry Skinner  Nikolai Abrosimov  Jose Alvarez  Pierre Bastie  Bernard Hamelin  Margarida Hernanz  Pierre Jean  Jürgen Knödlseder  Bob Smither 《Experimental Astronomy》2005,20(1-3):253-267

The objective of the R&D project CLAIRE was to prove the principle of a gamma-ray lens for nuclear astrophysics. CLAIRE's Laue diffraction lens has a diameter of 45 cm and a focal length of 277 cm; 556 germanium-silicon crystals are tuned to focus 170 keV photons onto a 1.5 cm diameter focal spot. Laboratory measurements of the individual crystals and the entire lens have been used to validate a numerical model that we use to estimate the lens performance for a source at infinity. During a stratospheric balloon flight on 2001 June 14, CLAIRE was directed at the Crab nebula by a pointing system able to stabilize the lens to within a few arcseconds of the target. In 72 min of valid pointing time, 33 photons from the Crab were detected in the 3 keV bandpass of the lens: CLAIRE's first light! The performance of CLAIRE's gamma-ray lens, namely the peak reflectivity for a polychromatic source (9±1%), has been confirmed by ground data obtained on a 205 meter long test range. CLAIRE's measured performance validates the principle of a Laue lens for nuclear astrophysics, opening the way for a space-borne gamma-ray lens telescope that will achieve one to two orders of magnitude improvement in sensitivity over present technologies.  相似文献   

A Fokker-Planck Approach for the Expansion of the Fast Solar Wind: Why and How?     

F. Leblanc  D. Hubert 《Astrophysics and Space Science》2001,277(1-2):201-204

We present recent observations of the plasma parameters in coronal holes at the origin of the fast solar wind and in the interplanetary medium. A model based on the heat conductivity law in a dilute plasma shows the coherency of the electron and proton temperature observations from coronal holes to the interplanetary medium. These new observations are severe constraints for any model of the expansion of the fast solar wind. We discuss why and how non-equilibrium multispecies Fokker-Planck approach must be developed and present a generalized Grad's solution. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The systeme agraire concept in francophone peasant studies     

Hubert Cochet 《Geoforum》2012,43(1):128-136

Among the many concepts used in farming systems research, the agrarian system—système agraire—stands out in the works of francophone agronomists, agricultural economists and geographers. It is an all-encompassing concept, capable of making sense of agricultural activities at a regional scale in a way that accounts for both ecological and socio-economic dimensions. The objective of this article is to trace the evolution of this original concept, reflect on the difficulties inherent to its use, and examine its relevance to agricultural research. After exploring its origins and development within the French school of Comparative Agriculture, the author analyzes its components in terms of constituting sub-systems. Next, the challenges associated with the agrarian system approach are addressed, particularly in terms of boundaries and scales of analysis. Part four discusses the similarities and differences between this approach and others, more commonly used by non-francophones. The article concludes with a discussion of the usefulness of the approach in terms of identifying obstacles to agricultural development.  相似文献   

Ceilometer observations of aerosol layer structure above the Petit Lubéron during ESCOMPTE's IOP 2     

Marcel Zphoris  Hubert Holin  Franck Lavie  Nadine Cenac  Michel Cluzeau  Olivier Delas  Franoise Eideliman  Jacqueline Gagneux  Alain Gander  Corinne Thibord 《Atmospheric Research》2005,74(1-4):581-595

A modified ceilometer has been used during the second Intensive Observation Period (IOP) of the “Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transport d'Émission” (ESCOMPTE) to perform continuous remote observations of aerosol accumulations in the first 3 km of the atmosphere. These observations encompassed an episode of intense particulate and photochemical pollution. The submicronic particles density, measured at an altitude of 600 m, went from a very low point of a few tens of particles per cubic centimeter (at the end of a Mistral episode in the free atmosphere) to a high point of more than 4500 particles per cubic centimeter (when pollutants were trapped by thermal inversions).The main result is that this instrument enables a fine documentation of the mixing layer height and of aerosol particles stratifications and circulation. Airborne aerosol measurements have been made above the mountainous region of Mérindol in order to validate in situ the remote sensing measurements. Ozone measurements near the summit of the mountains as well as in the valley were performed in order to correlate aerosol accumulation and ozone concentration. As a notable example, the two-layer aerosol stratification seen in the first 2 days of IOP 2b in that part of the ESCOMPTE domain confirms the results of another team which used backtrajectories. The low-altitude pollution for this timeframe had a local origin (the Fos industrial area), whereas above 500 m, the air masses had undergone regional-scale transport (from north-eastern Spain).The second major result is the highlighting of a pattern, in sea breeze conditions and in this part of the ESCOMPTE experiment zone, of nocturnal aerosol accumulation at an altitude of between 500 and 2000 m, followed by high ozone concentration the next day.  相似文献   

Proterozoic–Early Paleozoic evolution in western South America—a discussion     

Florencio G. Aceolaza  Hubert Miller  Alejandro J. Toselli 《Tectonophysics》2002,354(1-2)

The hypothesis of exotic terranes in Perú, Bolivia, Argentina and Chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern Andes during the Late Proterozoic–Early Paleozoic. Initially, a tectogenesis based on autochthonous mobile fold belts was discussed. Following ideas emphasised the fragmentation of the supercontinent Rodinia, Laurentia moving along the West Gondwana border and colliding with the Gondwana western margin. The most important effect of this Laurentia/Gondwana relationship was attributed to the Argentine Precordillera (or Cuyania) terrane splitting off from Laurentia and docking to Gondwana in the Early Paleozoic. In this study, the most cited arguments for this Laurentia/Precordillera relationship are discussed, emphasising paleontological considerations. It is shown that these arguments do not exclude a close original vicinity of the Precordillera terrane to Gondwana.The Precordillera terrane is suggested to be part of a hypothetical platform, which developed between South America, Africa and Antarctica (SAFRAN platform), and which was displaced to its actual position by transcurrent faults. The collisional events in the Sierras Pampeanas ensued from strike–slip movements and were responsible for the S and I type transpressional magmatism along the Pampean and Famatinian terranes. The final result of this continent-parallel movement of terrane slices is similar to that of a terrane split off from Laurentia, but the first-named way of formation easier explains the general continuity of plate convergence at the western border of Gondwana than the Laurentia/Precordillera connection does.  相似文献