591 research outputs found
Sensitivity analyses of dense cloud chemical models
Because of new telescopes that will dramatically improve our knowledge of the
interstellar medium, chemical models will have to be used to simulate the
chemistry of many regions with diverse properties. To make these models more
robust, it is important to understand their sensitivity to a variety of
parameters. In this article, we report a study of the sensitivity of a chemical
model of a cold dense core, with homogeneous and time-independent physical
conditions, to variations in the following parameters: initial chemical
inventory, gas temperature and density, cosmic-ray ionization rate, chemical
reaction rate coefficients, and elemental abundances. From the results of the
parameter variations, we can quantify the sensitivity of the model to each
parameter as a function of time. Our results can be used in principle with
observations to constrain some parameters for different cold clouds. We also
attempted to use the Monte Carlo approach with all parameters varied
collectively. Within the parameter ranges studied, the most critical parameters
turn out to be the reaction rate coefficients at times up to 4e5 yr and
elemental abundances at later times. At typical times of best agreement with
observation, models are sensitive to both of these parameters. The models are
less sensitive to other parameters such as the gas density and temperature. The
improvement of models will require that the uncertainties in rate coefficients
of important reactions be reduced. As the chemistry becomes better understood
and more robust, it should be possible to use model sensitivities concerning
other parameters, such as the elemental abundances and the cosmic ray
ionization rate, to yield detailed information on cloud properties and history.
Nevertheless, at the current stage, we cannot determine the best values of all
the parameters simultaneously based on purely observational constraints.Comment: Accepted for publication in Astronomy & Astrophysic
Carbon budget and carbon chemistry in Photon Dominated Regions
We present a study of small carbon chains and rings in Photon Dominated
Regions (PDRs) performed at millimetre wavelengths. Our sample consists of the
Horsehead nebula (B33), the rho,Oph L1688 cloud interface, and the
cometary-shaped cloud IC63. Using the IRAM 30-m telescope, the SEST and the
Effelsberg 100-m teles cope at Effelsberg., we mapped the emission of \cch,
c-C3H2 and C4H, and searched for heavy hydrocarbons such as c-C3H, l-C3H,
l-C3H2, l-C4H2 and C6H. The large scale maps show that small hydrocarbons are
present until the edge of all PDRs, which is surprising as they are expected to
be easily destroyed by UV radiation. Their spatial distribution reasonably
agrees with the aromatic emission mapped in mid-IR wavelength bands. Their
abundances relative to H2 are relatively high and comparable to the ones
derived in dark clouds such as L134N or TMC-1, known as efficient carbon
factories. In particular, we report the first detection of C6H in a PDR. We
have run steady-state PDR models using several gas-phase chemical networks
(UMIST95 and the New Standard Model) and conclude that both networks fail in
reproducing the high abundances of some of these hydrocarbons by an order of
magnitude. The high abundance of hydrocarbons in the PDR may suggest that the
photo-erosion of UV-irradiated large carbonaceous compounds could efficiently
feed the ISM with small carbon clusters or molecules. This new production
mechanism of carbon chains and rings could overcome their destruction by the UV
radiation field. Dedicated theoretical and laboratory measurements are required
in order to understand and implement these additional chemical routes.Comment: 18 pages, 12 figure
A partitioned model order reduction approach to rationalise computational expenses in multiscale fracture mechanics
We propose in this paper an adaptive reduced order modelling technique based
on domain partitioning for parametric problems of fracture. We show that
coupling domain decomposition and projection-based model order reduction
permits to focus the numerical effort where it is most needed: around the zones
where damage propagates. No \textit{a priori} knowledge of the damage pattern
is required, the extraction of the corresponding spatial regions being based
solely on algebra. The efficiency of the proposed approach is demonstrated
numerically with an example relevant to engineering fracture.Comment: Submitted for publication in CMAM
The effect of uncertainties on chemical models of dark clouds
The gas-phase chemistry of dark clouds has been studied with a treatment of
uncertainties caused both by errors in individual rate coefficients and
uncertainties in physical conditions. Moreover, a sensitivity analysis has been
employed to attempt to determine which reactions are most important in the
chemistry of individual species. The degree of overlap between calculated
errors in abundances and estimated observational errors has been used as an
initial criterion for the goodness of the model and the determination of a best
'chemical' age of the source. For the well-studied sources L134N and TMC-1CP,
best agreement is achieved at so-called "early times" ~10 yr, in
agreement with previous calculations but here put on a firmer statistical
foundation. A more detailed criterion for agreement, which takes into account
the degree of disagreement, is also proposed. Poorly understood but critical
classes of reactions are delineated, especially reactions between ions and
polar neutrals. Such reactions will have to be understood better before the
chemistry can be made more secure. Nevertheless, the level of agreement is low
enough to indicate that a static picture of physical conditions without
consideration of interactions with grain surfaces is inappropriate for a
complete understanding of the chemistry.Comment: Accepted for publication in A&A. Astronomy and Astrophysics in press
(2006) in pres
Stable Isotopic Evidence for Methane Seeps in Neoproterozoic Postglacial Cap Carbonates
The Earth's most severe glaciations are thought to have occurred about 600 million years ago, in the late Neoproterozoic era. A puzzling feature of glacial deposits from this interval is that they are overlain by 1â5-m-thick 'cap carbonates' (particulate deep-water marine carbonate rocks) associated with a prominent negative carbon isotope excursion. Cap carbonates have been controversially ascribed to the aftermath of almost complete shutdown of the ocean ecosystems for millions of years during such ice agesâthe 'snowball Earth' hypothesis. Conversely, it has also been suggested that these carbonate rocks were the result of destabilization of methane hydrates during deglaciation and concomitant flooding of continental shelves and interior basins. The most compelling criticism of the latter 'methane hydrate' hypothesis has been the apparent lack of extreme isotopic variation in cap carbonates inferred locally to be associated with methane seeps. Here we report carbon isotopic and petrographic data from a Neoproterozoic postglacial cap carbonate in south China that provide direct evidence for methane-influenced processes during deglaciation. This evidence lends strong support to the hypothesis that methane hydrate destabilization contributed to the enigmatic cap carbonate deposition and strongly negative carbon isotopic anomalies following Neoproterozoic ice ages. This explanation requires less extreme environmental disturbance than that implied by the snowball Earth hypothesis
Trade unions and work-life balance: changing times in France and the UK?
The mixed empirical findings to date have indicated that some, but not all, unions in industrialized countries are actively involved in campaigning and bargaining around workâlife balance (WLB) issues, as part of a modernization
agenda linked to feminization and to âpositive flexibilityâ. This article seeks to identify factors that might encourage or inhibit trade unions from involvement
in WLB issues, within a cross-national comparative perspective focusing on two countries (France and the UK) that have contrasting working time regimes and approaches to WLB. It draws on original research carried out in two sectors â insurance and social work â in these two countries. The article links the emergence of union WLB programmes and bargaining agendas to genderequality concerns within the union and to the gender composition of the sector, as well as to the working time regime, including the mode of action, partnership being a significant corollary of WLB campaigning in the UK. We find support for the modernization thesis in the UK, particularly in the public sector, but within severe constraints defined by employer initiative
Molecular Inventories and Chemical Evolution of Low-mass Protostellar Envelopes
This paper presents the first substantial study of the chemistry of the
envelopes around a sample of 18 low-mass pre- and protostellar objects for
which physical properties have previously been derived from radiative transfer
modeling of their dust continuum emission. Single-dish line observations of 24
transitions of 9 molecular species (not counting isotopes) including HCO+,
N2H+, CS, SO, SO2, HCN, HNC, HC3N and CN are reported. The line intensities are
used to constrain the molecular abundances by comparison to Monte Carlo
radiative transfer modeling of the line strengths. An empirical chemical
network is constructed on the basis of correlations between the abundances of
various species. For example, it is seen that the HCO+ and CO abundances are
linearly correlated, both increasing with decreasing envelope mass. Species
such as CS, SO and HCN show no trend with envelope mass. In particular no trend
is seen between ``evolutionary stage'' of the objects and the abundances of the
main sulfur- or nitrogen-containing species. Among the nitrogen-bearing species
abundances of CN, HNC and HC3N are found to be closely correlated, which can be
understood from considerations of the chemical network. The CS/SO abundance
ratio is found to correlate with the abundances of CN and HC3N, which may
reflect a dependence on the atomic carbon abundance. An anti-correlation is
found between the deuteration of HCO+ and HCN, reflecting different temperature
dependences for gas-phase deuteration mechanisms. The abundances are compared
to other protostellar environments. In particular it is found that the
abundances in the cold outer envelope of the previously studied class 0
protostar IRAS16293-2422 are in good agreement with the average abundances for
the presented sample of class 0 objects.Comment: Accepted for publication in A&A. 29 pages, 23 figures. Abstract
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The influence of treatment modality on illness perception and secondary prevention outcomes among patients with acute myocardial infarction
Objectives: This study aims to determine if patients with acute myocardial infarction differ in illness perception and secondary prevention outcomes depending on the treatment they received.
Methods: A repeated measures design was used to compare patients with acute myocardial infarction receiving three different treatment modalities: ST-elevation myocardial infarction treated by primary percutaneous coronary intervention, ST-elevation myocardial infarction treated by thrombolytic therapy, and non ST-elevation myocardial infarction treated by medication. A convenient sampling technique was used to recruit 206 patients with acute myocardial infarction who agreed to participate in the current study. Patients' illness perception, physical activity, and demographical and clinical data were collected during hospital admission and again at 6 months.
Results: A total of 186 patients completed the study. Results showed that the primary percutaneous coronary intervention group perceived their illness as acute rather than chronic (PÂ =Â 0.034) and has lower personal control (PÂ =Â 0.032), higher treatment control (PÂ =Â 0.025), and higher perception of illness coherence (PÂ =Â 0.022) compared with patients receiving thrombolytic therapy and treated after non-ST segment infarction. Moreover, they report low control of their blood pressure (PÂ =Â 0.013) and less physical activity (PÂ =Â 0.001).
Conclusion: The results of this study revealed that patients' treated with primary percutaneous coronary intervention had negative illness perception and limited behavioral changes 6 months after hospitalization in comparison with other treatment modalities such as percutaneous coronary intervention and thrombolytic treatment. Further research is recommended to confirm this association with longer follow-up study and among different cultures
Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 25 (2011): GB2002, doi:10.1029/2010GB003845.The magnitude and feedbacks of future methane release from the Arctic region are unknown. Despite limited documentation of potential future releases associated with thawing permafrost and degassing methane hydrates, the large potential for future methane releases calls for improved understanding of the interaction of a changing climate with processes in the Arctic and chemical feedbacks in the atmosphere. Here we apply a âstate of the artâ atmospheric chemistry transport model to show that large emissions of CH 4 would likely have an unexpectedly large impact on the chemical composition of the atmosphere and on radiative forcing (RF). The indirect contribution to RF of additional methane emission is particularly important. It is shown that if global methane emissions were to increase by factors of 2.5 and 5.2 above current emissions, the indirect contributions to RF would be about 250% and 400%, respectively, of the RF that can be attributed to directly emitted methane alone. Assuming several hypothetical scenarios of CH 4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry. In particular, the impact of CH 4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO 2 as a result of atmospheric chemical processes. Despite uncertainties in emission scenarios, our results provide a better understanding of the feedbacks in the atmospheric chemistry that would amplify climate warming
Assessment of an in vitro whole cigarette smoke exposure system: The Borgwaldt RM20S 8-syringe smoking machine
<p>Abstract</p> <p>Background</p> <p>There have been many recent developments of <it>in vitro </it>cigarette smoke systems closely replicating <it>in vivo </it>exposures. The Borgwaldt RM20S smoking machine (RM20S) enables the serial dilution and delivery of cigarette smoke to exposure chambers for <it>in vitro </it>analyses. In this study we have demonstrated reliability and robustness testing of the RM20S in delivering smoke to <it>in vitro </it>cultures using an in-house designed whole smoke exposure chamber.</p> <p>Results</p> <p>The syringe precision and accuracy of smoke dose generated by the RM20S was assessed using a methane gas standard and resulted in a repeatability error of â€9%. Differential electrical mobility particle spectrometry (DMS) measured smoke particles generated from reference 3R4F cigarettes at points along the RM20S. 53% ± 5.9% of particles by mass reached the chamber, the remainder deposited in the syringe or connecting tubing and ~16% deposited in the chamber. Spectrofluorometric quantification of particle deposition within chambers indicated a positive correlation between smoke concentration and particle deposition. <it>In vitro </it>air-liquid interface (ALI) cultures (H292 lung epithelial cells), exposed to whole smoke (1:60 dilution (smoke:air, equivalent to ~5 ÎŒg/cm<sup>2</sup>)) demonstrated uniform smoke delivery within the chamber.</p> <p>Conclusions</p> <p>These results suggest this smoke exposure system is a reliable and repeatable method of generating and exposing ALI <it>in vitro </it>cultures to cigarette smoke. This system will enable the evaluation of future tobacco products and individual components of cigarette smoke and may be used as an alternative <it>in vitro </it>tool for evaluating other aerosols and gaseous mixtures such as air pollutants, inhaled pharmaceuticals and cosmetics.</p
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