178 research outputs found
Dust and Gas as Seeds for Metal-Poor Star Formation
I address the issue of dust and gas as seeds for metal-poor star formation by
reviewing what we know about star formation in nearby dwarf galaxies and its
relationship to the gas and dust. I (try to) speculate on the extent to which
processes in nearby galaxies mimic star formation in the early universe.Comment: To be published in "Low Metallicity Star Formation: From the First
Stars to Dwarf Galaxies," Proceedings of IAU Symposium No. 255, eds. L.K.
Hunt, S. Madden, & R. Schneider (Cambridge: Cambridge Univ Press
A candidate protostellar object in the L1457 / MBM12 cloud
The association of young T Tauri stars, MBM12A, indicates that L1457 was
forming stars not too long ago. With our study we want to find out whether or
not there are still signs for ongoing star formation in that cloud. Using the
Max-Planck-Millimeter-Bolometer MAMBO at the IRAM 30m telescope we obtained a
map of about 8' by 8' centered on L1457 in the dust continuum emission at 230
GHz. Towards the most intense regions in our bolometer map we obtained spectra
at high angular resolution in the CS (2-1) and the N2H+(1-0) lines using the
IRAM 30m telescope. We find that the cold dust in L1457 is concentrated in
several small cores with high H2 column densities and solar masses. The density
profiles of the cores are inconsistent with a sphere with constant density.
These cores are closer to virial equilibrium than the cloud as a whole. Data
from the VLA and Spitzer archives reveal two point sources in the direction of
one dust core. One of the sources is probably a distant quasar, whereas the
other source is projected right on a local maximum of our dust map and shows
characteristics of a protostellar object.Comment: 4 pages, 4 figures, accepted by Astronomy & Astrophysic
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A semiconductor laser system for the production of antihydrogen
Laser-controlled charge exchange is a promising method for producing cold antihydrogen. Caesium atoms in Rydberg states collide with positrons and create positronium. These positronium atoms then interact with antiprotons, forming antihydrogen. Laser excitation of the caesium atoms is essential to increase the cross section of the charge-exchange collisions. This method was demonstrated in 2004 by the ATRAP collaboration by using an available copper vapour laser. For a second generation of charge-exchange experiments we have designed a new semiconductor laser system that features several improvements compared to the copper vapour laser. We describe this new laser system and show the results from the excitation of caesium atoms to Rydberg states within the strong magnetic fields in the ATRAP apparatus
Black hole solutions in Euler-Heisenberg theory
We construct static and spherically symmetric black hole solutions in the
Einstein-Euler-Heisenberg (EEH) system which is considered as an effective
action of a superstring theory. We considered electrically charged,
magnetically charged and dyon solutions. We can solve analytically for the
magnetically charged case. We find that they have some remarkable properties
about causality and black hole thermodynamics depending on the coupling
constant of the EH theory and , though they have central singularity as
in the Schwarzschild black hole.Comment: 8 pages, 13 figures, figures corrected and some comments adde
Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies
Highlights
âą Gas release from wells may counteract efforts to mitigate greenhouse gas emissions.
âą An approach for assessing methane release from marine decommissioned wells.
âą This gas release largely depends on the presence of shallow gas accumulations.
âą Methane release from hydrocarbon wells represents a major source in the North Sea.
Abstract
Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000âŻm below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence intervalâŻ=âŻ0.7-4.2] kt yrâ1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets
Chemical and physical small-scale structure in a pre-stellar core
We present a comparative study of several molecular lines and of the dust
contiunuum at 1.2mm in a pre-stellar core that is embedded in the Galactic
cirrus cloud MCLD123.5+24.9. Previous studies found that the core is
gravitationally stable and shows signs of inward motion. Using the Owens Valley
(OVRO) and Plateau de Bure (PdB) interferometers we obtained high-angular
resolution maps of the core in the carbon monosulfide CS 2-1 and the
cyanoacetylene HC3N 10-9 transitions. Together with CS 5-4, C34S 3-2, and
bolometer data obtained with the IRAM 30m telescope, we analyse the excitation
conditions and the structural properties of the cloud. On the one hand, the new
CS 2-1 observations reveal significant substructure on a scale of about 7",
i.e., the beam size, corresponding to about 1050 AU at an adopted distance of
150pc. On the other hand, the interferometric observations in the HC3N 10-9
transition shows just one single well resolved clump in the inner part of the
core. This core is well described by an intensity profile following from a
centrally peaked volume density distribution. We find no evidence for depletion
of CS onto dust grains. The inward motion seen in the CS 2-1 occurs one-sided
from the middle of the filamentary cloud towards the HC3N core.Comment: 8 pages, 8 figures, accepted by A&
A semiconductor laser system for the production of antihydrogen
Laser-controlled charge exchange is a promising method for producing cold antihydrogen. Caesium atoms in Rydberg states collide with positrons and create positronium. These positronium atoms then interact with antiprotons, forming antihydrogen. Las er excitation of the caesium atoms is essential to increase the cross section of the charge-exchange collisions. This method was demonstrated in 2004 by the ATRAP collaboration by using an available copper vapour laser. For a second generation of charge-e xchange experiments we have designed a new semiconductor laser system that features several improvements compared to the copper vapour laser. We describe this new laser system and show the results from the excitation of caesium atoms to Rydberg states wit hin the strong magnetic fields in the ATRAP apparatus
Seismic chimney characterisation in the North Sea â Implications for pockmark formation and shallow gas migration
Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 metres below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2
High resolution monitoring of valvular interstitial cell driven pathomechanisms in procalcific environment using label-free impedance spectroscopy
IntroductionFibro-calcific aortic valve disease has high prevalence and is associated with significant mortality. Fibrotic extracellular matrix (ECM) remodeling and calcific mineral deposition change the valvular microarchitecture and deteriorate valvular function. Valvular interstitial cells (VICs) in profibrotic or procalcifying environment are frequently used in vitro models. However, remodeling processes take several days to weeks to develop, even in vitro. Continuous monitoring by real-time impedance spectroscopy (EIS) may reveal new insights into this process.MethodsVIC-driven ECM remodeling stimulated by procalcifying (PM) or profibrotic medium (FM) was monitored by label-free EIS. Collagen secretion, matrix mineralization, viability, mitochondrial damage, myofibroblastic gene expression and cytoskeletal alterations were analyzed.Results and DiscussionEIS profiles of VICs in control medium (CM) and FM were comparable. PM reproducibly induced a specific, biphasic EIS profile. Phase 1 showed an initial impedance drop, which moderately correlated with decreasing collagen secretion (râ=â0.67, pâ=â0.22), accompanied by mitochondrial membrane hyperpolarization and cell death. Phase 2 EIS signal increase was positively correlated with augmented ECM mineralization (râ=â0.97, pâ=â0.008). VICs in PM decreased myofibroblastic gene expression (pâ<â0.001) and stress fiber assembly compared to CM. EIS revealed sex-specific differences. Male VICs showed higher proliferation and in PM EIS decrease in phase 1 was significantly pronounced compared to female VICs (male minimum: 7.4â±â4.2%, female minimum: 26.5â±â4.4%, pâ<â0.01). VICs in PM reproduced disease characteristics in vitro remarkably fast with significant impact of donor sex. PM suppressed myofibroblastogenesis and favored ECM mineralization. In summary, EIS represents an efficient, easy-to-use, high-content screening tool enabling patient-specific, subgroup- and temporal resolution
The influence of anesthetics, neurotransmitters and antibiotics on the relaxation processes in lipid membranes
In the proximity of melting transitions of artificial and biological
membranes fluctuations in enthalpy, area, volume and concentration are
enhanced. This results in domain formation, changes of the elastic constants,
changes in permeability and slowing down of relaxation processes. In this study
we used pressure perturbation calorimetry to investigate the relaxation time
scale after a jump into the melting transition regime of artificial lipid
membranes. This time corresponds to the characteristic rate of domain growth.
The studies were performed on single-component large unilamellar and
multilamellar vesicle systems with and without the addition of small molecules
such as general anesthetics, neurotransmitters and antibiotics. These drugs
interact with membranes and affect melting points and profiles. In all systems
we found that heat capacity and relaxation times are related to each other in a
simple manner. The maximum relaxation time depends on the cooperativity of the
heat capacity profile and decreases with a broadening of the transition. For
this reason the influence of a drug on the time scale of domain formation
processes can be understood on the basis of their influence on the heat
capacity profile. This allows estimations of the time scale of domain formation
processes in biological membranes.Comment: 12 pages, 6 figure
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