15,615 research outputs found
Model of the W3(OH) environment based on data for both maser and 'quasi-thermal' methanol lines
In studies of the environment of massive young stellar objects, recent
progress in both observations and theory allows a unified treatment of data for
maser and 'quasi-thermal' lines. Interferometric maser images provide
information on the distribution and kinematics of masing gas on small spatial
scales. Observations of multiple masing transitions provide constraints on the
physical parameters.
Interferometric data on 'quasi-thermal' molecular lines permits an
investigation of the overall distribution and kinematics of the molecular gas
in the vicinity of young stellar objects, including those which are deeply
embedded. Using multiple transitions of different molecules, one can obtain
good constraints on the physical and chemical parameters.
Combining these data enables the construction of unified models, which take
into account spatial scales differing by orders of magnitude.
Here we present such a combined analysis of the environment around the
ultracompact HII region in W3(OH). This includes the structure of the methanol
masing region, physical structure of the near vicinity of W3(OH), detection of
new masers in the large-scale shock front and embedded sources in the vicinity
of the TW young stellar object.Comment: To appear in the Proceedings of the 2004 European Workshop: "Dense
Molecular Gas around Protostars and in Galactic Nuclei", Eds. Y.Hagiwara,
W.A.Baan, H.J. van Langevelde, 2004, a special issue of ApSS, Kluwe
A Model for Phase Transition based on Statistical Disassembly of Nuclei at Intermediate Energies
Consider a model of particles (nucleons) which has a two-body interaction
which leads to bound composites with saturation properties. These properties
are : all composites have the same density and the ground state energies of
composites with k nucleons are given by -kW+\sigma k^{2/3} where W and \sigma
are positive constants. W represents a volume term and \sigma a surface tension
term. These values are taken from nuclear physics. We show that in the large N
limit where N is the number of particles such an assembly in a large enclosure
at finite temperature shows properties of liquid-gas phase transition. We do
not use the two-body interaction but the gross properties of the composites
only. We show that (a) the p-\rho isotherms show a region where pressure does
not change as changes just as in Maxwell construction of a Van der Waals
gas, (b) in this region the chemical potential does not change and (c) the
model obeys the celebrated Clausius-Clapeyron relations. A scaling law for the
yields of composites emerges. For a finite number of particles N (upto some
thousands) the problem can be easily solved on a computer. This allows us to
study finite particle number effects which modify phase transition effects. The
model is calculationally simple. Monte-Carlo simulations are not needed.Comment: RevTex file, 21 pages, 5 figure
Virtual Meson Cloud of the Nucleon and Intrinsic Strangeness and Charm
We have applied the Meson Cloud Model (MCM) to calculate the charm and
strange antiquark distribution in the nucleon. The resulting distribution, in
the case of charm, is very similar to the intrinsic charm momentum distribution
in the nucleon. This seems to corroborate the hypothesis that the intrinsic
charm is in the cloud and, at the same time, explains why other calculations
with the MCM involving strange quark distributions fail in reproducing the low
x region data. From the intrinsic strange distribution in the nucleon we have
extracted the strangeness radius of the nucleon, which is in agreement with
other meson cloud calculations.Comment: 9 pages RevTex, 4 figure
The silicon stable isotope distribution along the GEOVIDE section (GEOTRACES GA-01) of the North Atlantic Ocean
The stable isotope composition of dissolved silicon in seawater (δ30SiDSi) was examined at 10 stations along the GEOVIDE section (GEOTRACES GA-01), spanning the North Atlantic Ocean (40–60∘ N) and Labrador Sea. Variations in δ30SiDSi below 500 m were closely tied to the distribution of water masses. Higher δ30SiDSi values are associated with intermediate and deep water masses of northern Atlantic or Arctic Ocean origin, whilst lower δ30SiDSi values are associated with DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly, the lowest δ30SiDSi values were observed in the deep and abyssal eastern North Atlantic, where dense southern-sourced waters dominate. The extent to which the spreading of water masses influences the δ30SiDSi distribution is marked clearly by Labrador Sea Water (LSW), whose high δ30SiDSi signature is visible not only within its region of formation within the Labrador and Irminger seas, but also throughout the mid-depth western and eastern North Atlantic Ocean. Both δ30SiDSi and hydrographic parameters document the circulation of LSW into the eastern North Atlantic, where it overlies southern-sourced Lower Deep Water. The GEOVIDE δ30SiDSi distribution thus provides a clear view of the direct interaction between subpolar/polar water masses of northern and southern origin, and allow examination of the extent to which these far-field signals influence the local δ30SiDSi distribution
Pion parton distribution functions from lattice QCD
We report on recent results for the pion matrix element of the twist-2
operator corresponding to the average momentum of non-singlet quark densities.
For the first time finite volume effects of this matrix element are
investigated and come out to be surprisingly large. We use standard Wilson and
non-perturbatively improved clover actions in order to control better the
extrapolation to the continuum limit. Moreover, we compute, fully
non-perturbatively, the renormalization group invariant matrix element, which
allows a comparison with experimental results in a broad range of energy
scales. Finally, we discuss the remaining uncertainties, the extrapolation to
the chiral limit and the quenched approximation.Comment: Lattice2003(matrix), 3 pages, 4 figure
Meso- and Bathypelagic Fish Interactions with Seamounts and Mid-Ocean Ridges
The World Ocean\u27s midwaters contain the vast majority of Earth\u27s vertebrates in the form of mesoand bathypelagic (\u27deep-pelagic,\u27 in the combined sense) fishes. Understanding the ecology and variability of deep-pelagic ecosystems has increased substantially in the past few decades due to advances in sampling/observation technology. Researchers have discovered that the deep sea hosts a complex assemblage of organisms adapted to a “harsh” environment by terrestrial standards (i.e., dark, cold, high pressure). We have learned that despite the lack of physical barriers, the deep-sea realm is not a homogeneous ecosystem, but is spatially and temporally variable on multiple scales. While there is a well-documented reduction of biomass as a function of depth (and thus distance from the sun, ergo primary production) in the open ocean, recent surveys have shown that pelagic fish abundance and biomass can \u27peak\u27 deep in the water column in association with abrupt topographic features such as seamounts and mid-ocean ridges. We review the current knowledge on deep-pelagic fish interactions with these features, as well as effects of these interactions on ecosystem functioning. We highlight the recent discoveries from the Mid-Atlantic Ridge (via the Census of Marine Life field project MAR-ECO) that were presented at the international symposium “Into the Unknown, Researching Mysterious Deep-Sea Animals,” hosted by the Okinawa Churaumi Aquarium, Okinawa, Japan, Feb 2007
Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at √s = 8TeV with the ATLAS detector
Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb−1 of proton-proton collision data at s√ = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model
Deep-Pelagic Fishes and the Mid-Atlantic Ridge: Interactions and Vectoring of Gelatinous Carbon to Higher Trophic Levels?
The assemblage structure and vertical distribution of deep-pelagic fishes relative to a mid-ocean ridge system is described from an acoustic and discrete-depth trawling survey conducted as part of the international Census of Marine Life field project MAR-ECO. A survey along the Mid-Atlantic Ridge (MAR), covering the full depth range (0 to \u3e3000 m) with a combination of gear types, was conducted to understand the role of the pelagic fauna in ecosystem dynamics. A total of 205 fish species were collected by midwater sampling. Depth was by far the primary assemblage composition determinant, with ridge section secondary. The dominant ichthyofaunal component was a widespread assemblage of fishes between 750-3000 m, from Iceland to the Azores. Some zonation was apparent in the northern and southern ends of this large depth stratum, with six smaller assemblages of fishes exhibiting limited distributions. Biomass per volume reached a water column maximum in the bathypelagic zone between 1500-2300 m. This stands in stark contrast to the general “open ocean” paradigm that biomass decreases exponentially from the surface downwards. As much of the summit of the MAR extends into this depth layer, a likely explanation for this midwater maximum is ridge association. Fish density within the benthic boundary layer (within 200 m of the ridge) was nearly double that of the water column and biomass was approximately 50% higher. Of the ‘ridge-associating’ species, two species known to consume gelata, Bathylagus euryops and Scopelogadus beanii, contributed over half of the fish biomass of this layer. These data suggest that a pelagic fish-gelata trophic linkage may be a key element of benthic-pelagic coupling over mid-ocean ridges, thus supporting enhanced nekton biomass over ridges in the absence of terrigenous nutrient input. Ongoing research to better understand this trophic linkage will be presented
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