29,647 research outputs found
Molecular cooling in the diffuse interstellar medium
We use a simple one-zone model of the thermal and chemical evolution of
interstellar gas to study whether molecular hydrogen (H2) is ever an important
coolant of the warm, diffuse interstellar medium (ISM). We demonstrate that at
solar metallicity, H2 cooling is unimportant and the thermal evolution of the
ISM is dominated by metal line cooling. At metallicities below 0.1 Z_solar,
however, metal line cooling of low density gas quickly becomes unimportant and
H2 can become the dominant coolant, even though its abundance in the gas
remains small. We investigate the conditions required in order for H2 to
dominate, and show that it provides significant cooling only when the ratio of
the interstellar radiation field strength to the gas density is small. Finally,
we demonstrate that our results are insensitive to changes in the initial
fractional ionization of the gas or to uncertainties in the nature of the dust
present in the low-metallicity ISM.Comment: 13 pages, 6 figures. Minor changes to match version accepted by MNRA
Approximations for modelling CO chemistry in GMCs: a comparison of approaches
We examine several different simplified approaches for modelling the
chemistry of CO in three-dimensional numerical simulations of turbulent
molecular clouds. We compare the different models both by looking at the
behaviour of integrated quantities such as the mean CO fraction or the
cloud-averaged CO-to-H2 conversion factor, and also by studying the detailed
distribution of CO as a function of gas density and visual extinction. In
addition, we examine the extent to which the density and temperature
distributions depend on our choice of chemical model.
We find that all of the models predict the same density PDF and also agree
very well on the form of the temperature PDF for temperatures T > 30 K,
although at lower temperatures, some differences become apparent. All of the
models also predict the same CO-to-H2 conversion factor, to within a factor of
a few. However, when we look more closely at the details of the CO
distribution, we find larger differences. The more complex models tend to
produce less CO and more atomic carbon than the simpler models, suggesting that
the C/CO ratio may be a useful observational tool for determining which model
best fits the observational data. Nevertheless, the fact that these chemical
differences do not appear to have a strong effect on the density or temperature
distributions of the gas suggests that the dynamical behaviour of the molecular
clouds on large scales is not particularly sensitive to how accurately the
small-scale chemistry is modelled.Comment: 18 pages, 10 figures. Minor revisions, including the addition of a
comparison of simulated and observed C/CO ratios. Accepted by MNRA
Investigation of the dc-excited xenon laser final report, 24 mar. 1964 - 24 mar. 1965
Electron energy spectra in gas laser discharges and investigation of new discharge configuration
Quantitative multielement analysis using high energy particle bombardment
Charged particles ranging in energy from 0.8 to 4.0 MeV are used to induce resonant nuclear reactions, Coulomb excitation (gamma X-rays), and X-ray emission in both thick and thin targets. Quantitative analysis is possible for elements from Li to Pb in complex environmental samples, although the matrix can severely reduce the sensitivity. It is necessary to use a comparator technique for the gamma-rays, while for X-rays an internal standard can be used. A USGS standard rock is analyzed for a total of 28 elements. Water samples can be analyzed either by nebulizing the sample doped with Cs or Y onto a thin formvar film or by extracting the sample (with or without an internal standard) onto ion exchange resin which is pressed into a pellet
Dust emission from high latitude cirrus clouds
In order to study dust emission from grains in the interstellar medium, the infrared properties were analyzed in a number of isolated high latitude dust clouds which contain no dominant internal heating sources. The clouds are spatially resolved, have a simple geometry, and are mapped in the IRAS bands at 12, 25, 60, and 100 microns. For a number of these clouds, extinction data (A sub B) were obtained from starcounts. A large part (30 to 50 percent) of the infrared radiation of the clouds in the IRAS wavelength range of 8 to 130 micron is emitted in the short wavelength bands at 12 and 25 micron. The 60/100 micron ratios for the integrated fluxes of the clouds have a typical value of 0.19 + or - 0.05
Infrared properties of dust grains derived from IRAS observations
The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species
Getting to know you: Accuracy and error in judgments of character
Character judgments play an important role in our everyday lives. However, decades of empirical research on trait attribution suggest that the cognitive processes that generate these judgments are prone to a number of biases and cognitive distortions. This gives rise to a skeptical worry about the epistemic foundations of everyday characterological beliefs that has deeply disturbing and alienating consequences. In this paper, I argue that this skeptical worry is misplaced: under the appropriate informational conditions, our everyday character-trait judgments are in fact quite trustworthy. I then propose a mindreading-based model of the socio-cognitive processes underlying trait attribution that explains both why these judgments are initially unreliable, and how they eventually become more accurate
Self-organization, scaling and collapse in a coupled automaton model of foragers and vegetation resources with seed dispersal
We introduce a model of traveling agents ({\it e.g.} frugivorous animals) who
feed on randomly located vegetation patches and disperse their seeds, thus
modifying the spatial distribution of resources in the long term. It is assumed
that the survival probability of a seed increases with the distance to the
parent patch and decreases with the size of the colonized patch. In turn, the
foraging agents use a deterministic strategy with memory, that makes them visit
the largest possible patches accessible within minimal travelling distances.
The combination of these interactions produce complex spatio-temporal patterns.
If the patches have a small initial size, the vegetation total mass (biomass)
increases with time and reaches a maximum corresponding to a self-organized
critical state with power-law distributed patch sizes and L\'evy-like movement
patterns for the foragers. However, this state collapses as the biomass sharply
decreases to reach a noisy stationary regime characterized by corrections to
scaling. In systems with low plant competition, the efficiency of the foraging
rules leads to the formation of heterogeneous vegetation patterns with
frequency spectra, and contributes, rather counter-intuitively,
to lower the biomass levels.Comment: 11 pages, 5 figure
Oral contraceptives, hormone replacement therapy, thrombophilias and risk of venous thromboembolism: a systematic review The Thrombosis: Risk and Economic Assessment of Thrombophilia Screening (TREATS) Study
Combined oral contraceptives,oral hormone replacement therapy and thrombophilias are recognised risk factors for venous thromboembolism in women.The objective of this study was to assess the risk of thromboembolism among women with thrombophilia who are taking oral contraceptives or hormone replacement therapy, conducting a systematic review and metaanalysis. Of 201 studies identified, only nine met the inclusion criteria. Seven studies included pre-menopausal women on oral contraceptives and two studies included peri-menopausal women on hormone replacement therapy. For oral contraceptive use, significant associations of the risk of venous thromboembolism were found in women with factor V Leiden (OR 15.62; 95%CI 8.66 to 28.15); deficiencies of antithrombin (OR 12.60; 95%CI 1.37 to 115.79), protein C (OR 6.33; 95%CI 1.68 to 23.87), or protein S (OR 4.88; 95%CI 1.39 to 17.10), elevated levels of factor VIIIc (OR 8.80; 95%CI 4.13 to 18.75); and factor V Leiden and prothrombin G20210A (OR 7.85; 95%CI 1.65 to 37.41). For hormone replacement therapy, a significant association was found in women with factor V Leiden (OR 13.16; 95%CI 4.28 to 40.47).Although limited by the small number of studies, the findings of this study support the presence of interaction between thrombophilia and venous thromboembolism among women taking oral contraceptives. However, further studies are required to establish with greater confidence the associations of these, and other, thrombophilias with venous thromboembolism among hormone users
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