1,225 research outputs found
Star formation in metal-poor gas clouds
Observations of molecular clouds in metal-poor environments typically find
that they have much higher star formation rates than one would expect based on
their observed CO luminosities and the molecular gas masses that are inferred
from them. This finding can be understood if one assumes that the conversion
factor between CO luminosity and H2 mass is much larger in these low
metallicity systems than in nearby molecular clouds. However, it is unclear
whether this is the only factor at work, or whether the star formation rate of
the clouds is directly sensitive to the metallicity of the gas.
To investigate this, we have performed numerical simulations of the coupled
dynamical, chemical and thermal evolution of model clouds with metallicities
ranging from 0.01 Z_solar to Z_solar. We find that the star formation rate in
our model clouds has little sensitivity to the metallicity. Reducing the
metallicity of the gas by two orders of magnitude delays the onset of star
formation in the clouds by no more than a cloud free-fall time and reduces the
time-averaged star formation rate by at most a factor of two. On the other
hand, the chemical state of the clouds is highly sensitive to the metallicity,
and at the lowest metallicities, the clouds are completely dominated by atomic
gas. Our results confirm that the CO-to-H2 conversion factor in these systems
depends strongly on the metallicity, but also show that the precise value is
highly time-dependent, as the integrated CO luminosity of the most metal-poor
clouds is dominated by emission from short-lived gravitationally collapsing
regions. Finally, we find evidence that the star formation rate per unit H2
mass increases with decreasing metallicity, owing to the much smaller H2
fractions present in our low metallicity clouds.Comment: 14 pages, 6 figures. Updated to match version accepted by MNRA
On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
The most usual tracer of molecular gas is line emission from CO. However, the
reliability of that tracer has long been questioned in environments different
from the Milky Way. We study the relationship between H2 and CO abundances
using a fully dynamical model of magnetized turbulence coupled to a chemical
network simplified to follow only the dominant pathways for H2 and CO formation
and destruction, and including photodissociation using a six-ray approximation.
We find that the abundance of H2 is primarily determined by the amount of time
available for its formation, which is proportional to the product of the
density and the metallicity, but insensitive to photodissociation.
Photodissociation only becomes important at extinctions under a few tenths of a
visual magnitude, in agreement with both observational and prior theoretical
work. On the other hand, CO forms quickly, within a dynamical time, but its
abundance depends primarily on photodissociation, with only a weak secondary
dependence on H2 abundance. As a result, there is a sharp cutoff in CO
abundance at mean visual extinctions A_V < 3. At lower values of A_V we find
that the ratio of H2 column density to CO emissivity X_CO is proportional to
A_V^(-3.5). This explains the discrepancy observed in low metallicity systems
between cloud masses derived from CO observations and other techniques such as
infrared emission. Our work predicts that CO-bright clouds in low metallicity
systems should be systematically larger or denser than Milky Way clouds, or
both. Our results further explain the narrow range of observed molecular cloud
column densities as a threshold effect, without requiring the assumption of
virial equilibrium.Comment: 16 pages, 11 figures. Updated to match version accepted by MNRA
Periodic solutions of second order Hamiltonian systems bifurcating from infinity
The goal of this article is to study closed connected sets of periodic
solutions, of autonomous second order Hamiltonian systems, emanating from
infinity. The main idea is to apply the degree for SO(2)-equivariant gradient
operators defined by the second author. Using the results due to Rabier we show
that we cannot apply the Leray-Schauder degree to prove the main results of
this article. It is worth pointing out that since we study connected sets of
solutions, we also cannot use the Conley index technique and the Morse theory.Comment: 24 page
Spiral shocks and the formation of molecular clouds in a two phase medium
We extend recent numerical results (Dobbs et. al. 2006) on molecular cloud
formation in spiral galaxies by including a multi-phase medium. The addition of
a hot phase of gas enhances the structure in the cold gas, and significantly
increases the fraction of molecular hydrogen that is formed when the cold gas
passes through a spiral shock. The difference in structure is reflected in the
mass power spectrum of the molecular clouds, which is steeper for the
multi-phase calculations. The increase in molecular gas occurs as the addition
of a hot phase leads to higher densities in the cold gas. In particular, cold
gas is confined in clumps between the spiral arms and retains a higher
molecular fraction. Unlike the single phase results, molecular clouds are
present in the inter-arm regions for the multi-phase medium. However the
density of the inter-arm molecular hydrogen is generally below that which can
be reliably determined from CO measurements. We therefore predict that for a
multi-phase medium, there will be low density clouds containing cold atomic and
molecular hydrogen, which are potentially entering the spiral arms.Comment: 11 pages, 14 figures, accepted for publication in MNRA
Photodissociation of H2 in Protogalaxies: Modeling Self-Shielding in 3D Simulations
The ability of primordial gas to cool in proto-galactic haloes exposed to
Lyman-Werner (LW) radiation is critically dependent on the self-shielding of
H_2. We perform radiative transfer calculations of LW line photons,
post-processing outputs from three-dimensional adaptive mesh refinement (AMR)
simulations of haloes with T_vir > 10^4 K at redshifts around z=10. We
calculate the optically thick photodissociation rate numerically, including the
effects of density, temperature, and velocity gradients in the gas, as well as
line overlap and shielding of H_2 by HI, over a large number of sight-lines. In
low-density regions (n<10^4 cm^-3) the dissociation rates exceed those obtained
using most previous approximations by more than an order of magnitude; the
correction is smaller at higher densities. We trace the origin of the
deviations primarily to inaccuracies of (i) the most common fitting formula
(Draine & Bertoldi 1996) for the suppression of the dissociation rate and (ii)
estimates for the effective shielding column density from local properties of
the gas. The combined effects of gas temperature and velocity gradients are
comparatively less important, typically altering the spherically averaged rate
only by a factor of less than two. We present a simple modification to the DB96
fitting formula for the optically thick rate which improves agreement with our
numerical results to within approx. 15 per cent, and can be adopted in future
simulations. We find that estimates for the effective shielding column can be
improved by using the local Sobolev length. Our correction to the H_2
self-shielding reduces the critical LW flux to suppress H_2-cooling in
T_vir>10^4 K haloes by an order of magnitude; this increases the number of such
haloes in which supermassive (approx. M=10^5 M_sun) black holes may have
formed.Comment: 17 pages, 11 figures. Submitted to MNRA
Maternal prenatal depression is associated with decreased placental expression of the imprinted gene PEG3.
BACKGROUND: Maternal prenatal stress during pregnancy is associated with fetal growth restriction and adverse neurodevelopmental outcomes, which may be mediated by impaired placental function. Imprinted genes control fetal growth, placental development, adult behaviour (including maternal behaviour) and placental lactogen production. This study examined whether maternal prenatal depression was associated with aberrant placental expression of the imprinted genes paternally expressed gene 3 (PEG3), paternally expressed gene 10 (PEG10), pleckstrin homology-like domain family a member 2 (PHLDA2) and cyclin-dependent kinase inhibitor 1C (CDKN1C), and resulting impaired placental human placental lactogen (hPL) expression. METHOD: A diagnosis of depression during pregnancy was recorded from Manchester cohort participants' medical notes (n = 75). Queen Charlotte's (n = 40) and My Baby and Me study (MBAM) (n = 81) cohort participants completed the Edinburgh Postnatal Depression Scale self-rating psychometric questionnaire. Villous trophoblast tissue samples were analysed for gene expression. RESULTS: In a pilot study, diagnosed depression during pregnancy was associated with a significant reduction in placental PEG3 expression (41%, p = 0.02). In two further independent cohorts, the Queen Charlotte's and MBAM cohorts, placental PEG3 expression was also inversely associated with maternal depression scores, an association that was significant in male but not female placentas. Finally, hPL expression was significantly decreased in women with clinically diagnosed depression (44%, p < 0.05) and in those with high depression scores (31% and 21%, respectively). CONCLUSIONS: This study provides the first evidence that maternal prenatal depression is associated with changes in the placental expression of PEG3, co-incident with decreased expression of hPL. This aberrant placental gene expression could provide a possible mechanistic explanation for the co-occurrence of maternal depression, fetal growth restriction, impaired maternal behaviour and poorer offspring outcomes.The Manchester cohort
was supported by Manchester National Institute
for Health Research (NIHR) Biomedical Research.
The Queen Charlotte’s cohort was supported by the
Medical Research Council (MRC) (Eurostress),
National Institutes of Health (R01MH073842) and the
Genesis Research Trust. The MBAM cohort was supported
by the Genesis Research Trust. A.B.J. was supported
by a Biotechnology and Biological Sciences
Research Council (BBSRC) Doctoral Training Grants
(DTG) studentship and subsequently MRC project
grant MR/M013960/1. S.J.T. was supported by BBSRC
project grant BB/J015156/1. L.E.C. was supported by
an Imperial College London Ph.D. studentship and
both L.E.C. and P.G.R were supported by the NIHR
Imperial Biomedical Research Centre
Chemistry of heavy elements in the Dark Ages
Primordial molecules were formed during the Dark Ages, i.e. the time between
recombination and reionization in the early Universe. The purpose of this
article is to analyze the formation of primordial molecules based on heavy
elements during the Dark Ages, with elemental abundances taken from different
nucleosynthesis models. We present calculations of the full non-linear equation
set governing the primordial chemistry. We consider the evolution of 45
chemical species and use an implicit multistep method of variable order of
precision with an adaptive stepsize control. We find that the most abundant
Dark Ages molecules based on heavy elements are CH and OH. Non-standard
nucleosynthesis can lead to higher heavy element abundances while still
satisfying the observed primordial light abundances. In that case, we show that
the abundances of molecular species based on C, N, O and F can be enhanced by
two orders of magnitude compared to the standard case, leading to a CH relative
abundance higher than that of HD+ or H2D+.Comment: 14 pages, accepted by Astronomy and Astrophysic
The JCMT 12CO(3-2) Survey of the Cygnus X Region: I. A Pathfinder
Cygnus X is one of the most complex areas in the sky. This complicates
interpretation, but also creates the opportunity to investigate accretion into
molecular clouds and many subsequent stages of star formation, all within one
small field of view. Understanding large complexes like Cygnus X is the key to
understanding the dominant role that massive star complexes play in galaxies
across the Universe.
The main goal of this study is to establish feasibility of a high-resolution
CO survey of the entire Cygnus X region by observing part of it as a
Pathfinder, and to evaluate the survey as a tool for investigating the
star-formation process.
A 2x4 degree area of the Cygnus X region has been mapped in the 12CO(3-2)
line at an angular resolution of 15" and a velocity resolution of ~0.4km/s
using HARP-B and ACSIS on the James Clerk Maxwell Telescope. The star formation
process is heavily connected to the life-cycle of the molecular material in the
interstellar medium. The high critical density of the 12CO(3-2) transition
reveals clouds in key stages of molecule formation, and shows processes that
turn a molecular cloud into a star.
We observed ~15% of Cygnus X, and demonstrated that a full survey would be
feasible and rewarding. We detected three distinct layers of 12CO(3-2)
emission, related to the Cygnus Rift (500-800 pc), to W75N (1-1.8 kpc), and to
DR21 (1.5-2.5 kpc). Within the Cygnus Rift, HI self-absorption features are
tightly correlated with faint diffuse CO emission, while HISA features in the
DR21 layer are mostly unrelated to any CO emission. 47 molecular outflows were
detected in the Pathfinder, 27 of them previously unknown. Sequentially
triggered star formation is a widespread phenomenon.Comment: 18 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
The physiological bases of hidden noise-induced hearing loss: protocol for a functional neuroimaging study
Background: Rodent studies indicate that noise exposure can cause permanent damage to synapses between inner hair cells and high-threshold auditory nerve fibers, without permanently altering threshold sensitivity. These demonstrations of what is commonly known as “hidden hearing loss” have been confirmed in several rodent species, but the implications for human hearing are unclear.
Objective: Our Medical Research Council (MRC) funded programme aims to address this unanswered question, by investigating functional consequences of the damage to the human peripheral and central auditory nervous system that results from cumulative lifetime noise exposure. Behavioral and neuroimaging techniques are being used in a series of parallel studies aimed at detecting hidden hearing loss in humans. The planned neuroimaging study aims to (1) identify central auditory biomarkers associated with hidden hearing loss, (2) investigate if there are any additive contributions from tinnitus or diminished sound tolerance, which are often comorbid with hearing problems, and (3) explore the relation between subcortical functional Magnetic Resonance Imaging (fMRI) measures and the auditory brainstem response (ABR).
Methods: Individuals aged 25 to 40 years with pure tone hearing thresholds ≤ 20 dB HL over the range 500 Hz to 8 kHz and no contraindications for MRI or signs of ear disease will be recruited into the study. Lifetime noise exposure will be estimated using an in-depth structured interview. Auditory responses throughout the central auditory system will be recorded using ABR and fMRI. Analyses will focus predominantly on correlations between lifetime noise exposure and auditory response characteristics.
Results: This article reports the study protocol. The programme grant was awarded in July 2013. Enrollment for the study described in this protocol commenced in February 2017 and was completed in December 2017. Results are expected in 2018.
Conclusions: This challenging and comprehensive study will have the potential to impact diagnostic procedures for hidden hearing loss, enabling early identification of noise-induced auditory damage via the detection of changes in central auditory processing. Consequently, this will generate the opportunity to give personalized advice regarding provision of ear defense and monitoring of further damage, thus reducing the incidence of noise-induced hearing loss
UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code
Our main purpose is to estimate the effect of assuming uniform density on the
line-of-sight in PDR chemistry models, compared to a more realistic
distribution for which total gas densities may well vary by several orders of
magnitude. A secondary goal of this paper is to estimate the amount of
molecular hydrogen which is not properly traced by the CO (J = 1 -> 0) line,
the so-called "dark molecular gas". We use results from a magnetohydrodynamical
(MHD) simulation as a model for the density structures found in a turbulent
diffuse ISM with no star-formation activity. The Meudon PDR code is then
applied to a number of lines of sight through this model, to derive their
chemical structures. It is found that, compared to the uniform density
assumption, maximal chemical abundances for H2, CO, CH and CN are increased by
a factor 2 to 4 when taking into account density fluctuations on the line of
sight. The correlations between column densities of CO, CH and CN with respect
to those of H2 are also found to be in better overall agreement with
observations. For instance, at N(H2) > 2.10^{20} cm-2, while observations
suggest that d[log N(CO)]=d[log N(H2)] = 3.07 +/- 0.73, we find d[log
N(CO)]=d[log N(H2)] =14 when assuming uniform density, and d[log N(CO)]=d[log
N(H2)] = 5.2 when including density fluctuations.Comment: 14 pages, 16 figures, accepted for publication in Astronomy &
Astrophysic
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