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791.
A. Frank 《Astrophysics and Space Science》2007,307(1-3):35-39
The expected lifetimes for molecular clouds has become a topic of considerable debate as numerical simulations have shown
that MHD turbulence, the nominal means of support for clouds against self-gravity, will decay on short timescales. Thus it
appears that either molecular clouds are transient features or they are resupplied with turbulent energy through some means.
Jets and molecular outflows are recognized as a ubiquitous phenomena associated with star formation. Stars however form not
isolation but in clusters of different density and composion. The ubiquity and high density of outflows from young stars in
clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we
present new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their abilityto
drive turbulent flows in molecular clouds. Our studies focus on scales associated with young star forming clusters. In particular
we first show that direct collisions between active outflows are not effective at stirring the ambient medium. We then show
that fossil cavities from “extinct” outflows may provide the missing link in terms of transferring momentum and energy to
the cloud. 相似文献
792.
Ulrike Herzschuh Chengjun Zhang Steffen Mischke Rainer Herzschuh Farhid Mohammadi Birgit Mingram Harald Kürschner Frank Riedel 《Global and Planetary Change》2005,46(1-4):361
The history (45–0 ka BP) of the aquatic vegetation composition of the shallow alpine Lake Luanhaizi from the NE Tibetan Plateau is inferred from aquatic plant macrofossil frequencies and aquatic pollen and algae concentrations in the sediments. C/N (range: 0.3–100), δ13C (range: −28 to −15‰), and n-alkane measurements yielded further information on the quantitative composition of sedimentary organic matter. The inferred primary production of the former lake ecosystem has been examined in respect of the alternative stable state theory of shallow lakes [Scheffer, M., 1989. Alternative stable states in eutrophic, shallow freshwater systems: a minimal model. Hydrobiological Bulletin 23, 73–83]. Switches between clear and turbid water conditions are explained by a colder climate and forest decline in the catchment area of Lake Luanhaizi. The macrofossil-based reconstruction of past water depth and salinity ranges, as well as other organic matter (OM) proxies allowed climatic inferences of the summer monsoon intensity during the late Quaternary. Around 45 ka BP, conditions similar to or even moister than present-day climate occurred. The Lake Luanhaizi record is further evidence against an extensive glaciation of the Tibetan Plateau and its bordering mountain ranges during the Last Glacial Maximum. Highest lake levels and consequently a strong summer monsoon are recorded for the early Holocene period, while gradually decreasing lake levels are reconstructed for the middle and late Holocene. 相似文献
793.
Andrew Cunningham Adam Frank Peggy Varnière Alexei Poludnenko Sorin Mitran Lee Hartmann 《Astrophysics and Space Science》2005,298(1-2):317-322
We present two dimensional cylindrically symmetric hydrodynamic simulations and synthetic emission maps of a stellar wind
propagating into an infalling, rotating environment. The resulting outflow morphology, collimation and stability observed
in these simulations have relevance to the study of young stellar objects, Herbig-Haro jets and molecular outflows. Our code
follows hydrogen gas with molecular, atomic and ionic components tracking the associated time dependent molecular chemistry
and ionization dynamics with radiative cooling appropriate for a dense molecular gas. We present tests of the code as well
as new simulations which indicate the presence of instabilities in the wind-blown bubble’s swept-up shell. 相似文献
794.
Jean d’Elbée Iker Castège Georges Hémery Yann Lalanne Claude Mouchès Françoise Pautrizel Frank D’Amico 《Continental Shelf Research》2009,29(8):1136-1144
From September 2000 to December 2006, surface plankton samples were collected on a monthly basis, from a station located in the southern Bay of Biscay (43°37N; 1°43W France), near the deep Capbreton canyon. In this paper, the results for the ichthyoplankton assemblage are presented. Among the 62 taxa recorded, only 35 were present in the larval stage whilst only 10 were represented by their eggs. Taxa represented by both stages (eggs+larvae; N=17) had the highest abundance. The presence in the surface plankton assemblage of species, at either or both stage, is interpreted within the context of the bathymetric distribution of species. The maxima in abundance and diversity occurred in February–March, for eggs, and May–June, for larvae. This 3-month time-lag between the stages is proposed to be related to the timing of egg spawning and larval recruitment to the pelagic environment. Mean egg abundances (82.4±29.8 eggs/10 m2) were 10-fold higher than the larval abundances (7.1±1.8 larvae/10 m2). Despite pronounced monthly variability, no statistically significant decrease in either egg or larvae abundance was observed during this 6-year study period. Compared with previous published studies, this study shows that the peak in ichthyoplankton diversity occurred two months earlier. In addition, the spawning period occurred over the whole year, even during autumn and winter. Using ordination techniques, the annual sequence appearance of taxa are described at the study site: Gadiforms, Ammodytidae and Pleuronectiforms were present during the winter whilst Sparidae, Blennidae, Labridae and Gobiidae, formed the summer group. Only three species, European anchovy Engraulis encrasicolus, European pilchard Sardina pilchardus and Atlantic horse mackerel Trachurus trachurus were recorded throughout the year. 相似文献
795.
796.
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798.
A library site for the storage of computer programs used in the analysis of variable stars has recently been set up. It is directly accessible across the computer networks, using standard procedures. Currently only a few programs for the analysis of the light curves of eclipsing binaries are stored at the site, although it is hoped that as the library becomes more well known, more investigators will deposit copies of their programs in it. This would result in a library where interested researchers could obtain the latest versions of analysis techniques from a comprehensive listing. Instructions for access to the site are also discussed. 相似文献
799.
800.
Dominik C. Hezel Herbert Palme Frank E. Brenker Lutz Nasdala 《Meteoritics & planetary science》2003,38(8):1199-1215
Abstract— We performed a detailed study of silica‐rich components (SRC) in the paired CH chondrites Acfer 182 and 207. These SRCs appear either as chondrules or fragments, and they contribute <0.1 vol% to the bulk meteorite. They usually contain a silica and a silicate portion. Both portions are, in most cases, cryptocrystalline and have bulk SiO2‐concentrations between 65 and 85 wt%. The silicate generally has a pyroxene normative composition. The silica often appears as blebs within the silicate matrix or vice versa. If there are no blebs, silica and silicate still form rounded interfaces. The SRCs are depleted in refractory elements like Ca, Al, and Ti relative to CI. A few SRC‐like objects are extremely rich in Mn and show no depletion in refractory elements. We conducted micro‐Raman studies on the silica portions of the SRCs to determine their structure, and we identified several silica phases: α‐quartz, cristobalite, glass, and a yet unidentified polymorph. The silicate portion is glass when the silica is glass and crystalline when the silica is crystalline. The low contents of Al and Ca make an igneous origin of the SRCs very unlikely, and the absence of metal excludes the formation by reduction of pyroxene. We suggest, instead, a fractional condensation origin of the SRCs from a Si‐enriched gas after removal of gaseous Mg by forsterite condensation. Additional evidence for fractional condensation is provided by a unique layered object with olivine in the core, pyroxene and metal at the rim, and silica at the outermost border; these layers record the condensation sequence. Two chondrules were found with several percent of Mn and high Cr, Na, and K contents, providing further evidence for condensation from a fractionated gas. The texture of the SRCs and the occurrence of cristobalite and silica glass, however, require formation by liquid immiscibility at high temperatures, above 1968 K, and subsequent fast cooling. Therefore, we propose a 2‐stage model for the formation of SRCs in CH chondrites: 1) fractional condensation of forsterite, enstatite, and SiO2‐rich phases; and 2) reheating of SiO2‐rich components to temperatures above 1968 K followed by rapid cooling. All other phases identified in CH chondrites can be understood within the framework of this model. Thus, the extremely unequilibrated CH chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula, in metals (Meibom et al. 1999), and in silicates. 相似文献