9,912 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
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
Does the CO-to-H2 conversion factor depend on the star formation rate?
We present a series of numerical simulations that explore how the `X-factor',
-- the conversion factor between the observed integrated CO emission
and the column density of molecular hydrogen -- varies with the environmental
conditions in which a molecular cloud is placed. Our investigation is centred
around two environmental conditions in particular: the cosmic ray ionisation
rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since
both these properties of the interstellar medium have their origins in massive
stars, we make the assumption in this paper that both the strength of the ISRF
and the CRIR scale linearly with the local star formation rate (SFR). The cloud
modelling in this study first involves running numerical simulations that
capture the cloud dynamics, as well as the time-dependent chemistry, and ISM
heating and cooling. These simulations are then post-processed with a line
radiative transfer code to create synthetic 12CO (1-0) emission maps from which
can be calculated. We find that for 1e4 solar mass virialised clouds
with mean density 100 cm, is only weakly dependent on the local
SFR, varying by a factor of a few over two orders of magnitude in SFR. In
contrast, we find that for similar clouds but with masses of 1e5 solar masses,
the X-factor will vary by an order of magnitude over the same range in SFR,
implying that extra-galactic star formation laws should be viewed with caution.
However, for denser ( cm), super-virial clouds such as those found
at the centre of the Milky Way, the X-factor is once again independent of the
local SFR.Comment: 16 pages, 5 figures. Accepted by MNRA
Is atomic carbon a good tracer of molecular gas in metal-poor galaxies?
Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H2) in
metal-rich galaxies, but is known to become ineffective in low metallicity
dwarf galaxies. Atomic carbon has been suggested as a superior tracer of H2 in
these metal-poor systems, but its suitability remains unproven. To help us to
assess how well atomic carbon traces H2 at low metallicity, we have performed a
series of numerical simulations of turbulent molecular clouds that cover a wide
range of different metallicities. Our simulations demonstrate that in
star-forming clouds, the conversion factor between [CI] emission and H2 mass,
, scales approximately as . We recover a
similar scaling for the CO-to-H2 conversion factor, , but find that
at this point in the evolution of the clouds, is consistently
smaller than , by a factor of a few or more. We have also examined
how and evolve with time. We find that
does not vary strongly with time, demonstrating that atomic carbon remains a
good tracer of H2 in metal-poor systems even at times significantly before the
onset of star formation. On the other hand, varies very strongly
with time in metal-poor clouds, showing that CO does not trace H2 well in
starless clouds at low metallicity.Comment: 16 pages, 9 figures. Updated to match the version accepted by MNRAS.
The main change from the previous version is a new sub-section (3.6)
discussing the possible impact of freeze-out and other processes not included
in our numerical simulation
On column density thresholds and the star formation rate
We present the results of a numerical study designed to address the question
of whether there is a column density threshold for star formation within
molecular clouds. We have simulated a large number of different clouds, with
volume and column densities spanning a wide range of different values, using a
state-of-the-art model for the coupled chemical, thermal and dynamical
evolution of the gas. We show that star formation is only possible in regions
where the mean (area-averaged) column density exceeds . Within the clouds, we also show that there is a good correlation
between the mass of gas above a K-band extinction and the
star formation rate (SFR), in agreement with recent observational work.
Previously, this relationship has been explained in terms of a correlation
between the SFR and the mass in dense gas. However, we find that this
correlation is weaker and more time-dependent than that between the SFR and the
column density. In support of previous studies, we argue that dust shielding is
the key process: the true correlation is one between the SFR and the mass in
cold, well-shielded gas, and the latter correlates better with the column
density than the volume density.Comment: 21 pages and 12 figures. Accepted for publication in 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
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
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