10,836 research outputs found
Chemistry and cooling in metal-free and metal-poor gas
I summarize four of the most important areas of uncertainty in the study of
the chemistry and cooling of gas with zero or very low metallicity. These are:
i) the importance and effects of HD cooling in primordial gas; ii) the
importance of metal-line and dust cooling in low metallicity gas; iii) the
impact of the large uncertainties that exist in the rate coefficients of
several key reactions involved in the formation of H2; and iv) the
effectiveness of grain surface chemistry at high redshifts.Comment: 5 pages, 2 figures. To appear in "First Stars III", eds. B. O'Shea,
A. Heger & T. Abel. This revised version fixes a minor error brought to my
attention by K. Omuka
Molluscs from a shallow-water whale-fall and their affinities with adjacent benthic communities on the Swedish west coast
We conducted a species-level study of molluscs associated with a 5-m long carcass of a minke whale at a depth of 125 m in the Kosterfjord (North Sea, Sweden). The whale-fall community was quantitatively compared with the community commonly living in the surrounding soft-bottom sediments. Five years after the deployment of the dead whale at the sea floor, the sediments around the carcass were dominated by the bivalve Thyasira sarsi, which is known to contain endosymbiotic sulphur-oxidizing bacteria, while background sediments were dominated by another thyasirid, T. equalis, less dependent on chemosynthesis for its nutrition. The Kosterfjord samples were further compared at the species level with mollusc abundance data derived from the literature, including samples from different marine settings of the west coast of Sweden (active methane seep, fjords, coastal and open marine environments). The results show high similarity between the Kosterfjord whale-fall community and the community that developed in one of the Swedish fjords (Gullmar Fjord) during hypoxic conditions. This study indicates that at shallow-water whale-falls, the sulphophilic stage of the ecological succession is characterized by generalist chemosynthetic bivalves commonly living in organic-rich, sulphidic environments. © 2014 Taylor & Francis
Subband coding for image data archiving
The use of subband coding on image data is discussed. An overview of subband coding is given. Advantages of subbanding for browsing and progressive resolution are presented. Implementations for lossless and lossy coding are discussed. Algorithm considerations and simple implementations of subband systems are given
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
Is H3+ cooling ever important in primordial gas?
Studies of the formation of metal-free Population III stars usually focus
primarily on the role played by H2 cooling, on account of its large chemical
abundance relative to other possible molecular or ionic coolants. However,
while H2 is generally the most important coolant at low gas densities, it is
not an effective coolant at high gas densities, owing to the low critical
density at which it reaches local thermodynamic equilibrium (LTE) and to the
large opacities that develop in its emission lines. It is therefore possible
that emission from other chemical species may play an important role in cooling
high density primordial gas. A particularly interesting candidate is the H3+
molecular ion. This ion has an LTE cooling rate that is roughly a billion times
larger than that of H2, and unlike other primordial molecular ions such as H2+
or HeH+, it is not easily removed from the gas by collisions with H or H2. It
is already known to be an important coolant in at least one astrophysical
context -- the upper atmospheres of gas giants -- but its role in the cooling
of primordial gas has received little previous study. In this paper, we
investigate the potential importance of H3+ cooling in primordial gas using a
newly-developed H3+ cooling function and the most detailed model of primordial
chemistry published to date. We show that although H3+ is, in most
circumstances, the third most important coolant in dense primordial gas (after
H2 and HD), it is nevertheless unimportant, as it contributes no more than a
few percent of the total cooling. We also show that in gas irradiated by a
sufficiently strong flux of cosmic rays or X-rays, H3+ can become the dominant
coolant in the gas, although the size of the flux required renders this
scenario unlikely to occur.Comment: 60 pages, 22 figures. Submitted to 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
Population health profile of the NSW Outback Division of General Practice: supplement
© Commonwealth of Australia To view the data presented in the profiles in Excel spreadsheets or via Interactive Mapping, please see the PHIDU website at: www.publichealth.gov.au
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