1,032 research outputs found
Kinetic Turbulence
The weak collisionality typical of turbulence in many diffuse astrophysical
plasmas invalidates an MHD description of the turbulent dynamics, motivating
the development of a more comprehensive theory of kinetic turbulence. In
particular, a kinetic approach is essential for the investigation of the
physical mechanisms responsible for the dissipation of astrophysical turbulence
and the resulting heating of the plasma. This chapter reviews the limitations
of MHD turbulence theory and explains how kinetic considerations may be
incorporated to obtain a kinetic theory for astrophysical plasma turbulence.
Key questions about the nature of kinetic turbulence that drive current
research efforts are identified. A comprehensive model of the kinetic turbulent
cascade is presented, with a detailed discussion of each component of the model
and a review of supporting and conflicting theoretical, numerical, and
observational evidence.Comment: 31 pages, 3 figures, 99 references, Chapter 6 in A. Lazarian et al.
(eds.), Magnetic Fields in Diffuse Media, Astrophysics and Space Science
Library 407, Springer-Verlag Berlin Heidelberg (2015
Ultra-deep catalog of X-ray groups in the Extended Chandra Deep Field South
Ultra-deep observations of ECDF-S with Chandra and XMM-Newton enable a search
for extended X-ray emission down to an unprecedented flux of
ergs s cm. We present the search for the extended emission on
spatial scales of 32 in both Chandra and XMM data, covering
0.3 square degrees and model the extended emission on scales of arcminutes. We
present a catalog of 46 spectroscopically identified groups, reaching a
redshift of 1.6. We show that the statistical properties of ECDF-S, such as
logN-logS and X-ray luminosity function are broadly consistent with LCDM, with
the exception that dn/dz/d test reveals that a redshift range of
in ECDF-S is sparsely populated. The lack of nearby structure,
however, makes studies of high-redshift groups particularly easier both in
X-rays and lensing, due to a lower level of clustered foreground. We present
one and two point statistics of the galaxy groups as well as weak-lensing
analysis to show that the detected low-luminosity systems are indeed low-mass
systems. We verify the applicability of the scaling relations between the X-ray
luminosity and the total mass of the group, derived for the COSMOS survey to
lower masses and higher redshifts probed by ECDF-S by means of stacked weak
lensing and clustering analysis, constraining any possible departures to be
within 30% in mass. Abridged.Comment: 20 pages, 21 figures, 3 tables, to match the journal versio
A multinuclear solid state NMR, density functional theory and X-Ray diffraction study of hydrogen bonding in Group I hydrogen dibenzoates
An NMR crystallographic approach incorporating multinuclear solid state NMR (SSNMR), X-ray structure determinations and density functional theory (DFT) are used to characterise the H bonding arrangements in benzoic acid (BZA) and the corresponding Group I alkali metal hydrogen dibenzoates (HD) systems. Since the XRD data often cannot precisely confirm the proton position within the hydrogen bond, the relationship between the experimental SSNMR parameters and the ability of gauge included plane augmented wave (GIPAW) DFT to predict them becomes a powerful constraint that can assist with further structure refinement. Both the 1H and 13C MAS NMR methods provide primary descriptions of the H bonding via accurate measurements of the 1H and 13C isotropic chemical shifts, and the individual 13C chemical shift tensor elements; these are unequivocally corroborated by DFT calculations, which together accurately describe the trend of the H bonding strength as the size of the monovalent cation changes. In addition, 17O MAS and DOR NMR form a powerful combination to characterise the O environments, with the DOR technique providing highly resolved 17O NMR data which helps verify unequivocally the number of inequivalent O positions for the conventional 17O MAS NMR to process. Further multinuclear MAS and static NMR studies involving the quadrupolar 7Li, 39K, 87Rb and 133Cs nuclei, and the associated DFT calculations, provide trends and a corroboration of the H bond geometry which assist in the understanding of these arrangements. Even though the crystallographic H positions in each H bonding arrangement reported from the single crystal X-ray studies are prone to uncertainty, the good corroboration between the measured and DFT calculated chemical shift and quadrupole tensor parameters for the Group I alkali species suggest that these reported H positions are reliable
The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
We present the first results of the EMBLA survey (Extremely Metal-poor BuLge
stars with AAOmega), aimed at finding metal-poor stars in the Milky Way bulge,
where the oldest stars should now preferentially reside. EMBLA utilises
SkyMapper photometry to pre-select metal-poor candidates, which are
subsequently confirmed using AAOmega spectroscopy. We describe the discovery
and analysis of four bulge giants with -2.72<=[Fe/H]<=-2.48, the lowest
metallicity bulge stars studied with high-resolution spectroscopy to date.
Using FLAMES/UVES spectra through the Gaia-ESO Survey we have derived
abundances of twelve elements. Given the uncertainties, we find a chemical
similarity between these bulge stars and halo stars of the same metallicity,
although the abundance scatter may be larger, with some of the stars showing
unusual [{\alpha}/Fe] ratios.Comment: 7 pages, 5 figures. Accepted for publication by MNRA
Big Physics At Small Places: The Mongol Horde Model Of Undergraduate Research
A model for engaging undergraduates in cutting-edge experimental nuclear physics research at a national user facility is discussed. Methods to involve students and examples of their success are presented
Spin operator matrix elements in the superintegrable chiral Potts quantum chain
We derive spin operator matrix elements between general eigenstates of the
superintegrable Z_N-symmetric chiral Potts quantum chain of finite length. Our
starting point is the extended Onsager algebra recently proposed by R.Baxter.
For each pair of spaces (Onsager sectors) of the irreducible representations of
the Onsager algebra, we calculate the spin matrix elements between the
eigenstates of the Hamiltonian of the quantum chain in factorized form, up to
an overall scalar factor. This factor is known for the ground state Onsager
sectors. For the matrix elements between the ground states of these sectors we
perform the thermodynamic limit and obtain the formula for the order
parameters. For the Ising quantum chain in a transverse field (N=2 case) the
factorized form for the matrix elements coincides with the corresponding
expressions obtained recently by the Separation of Variables Method.Comment: 24 pages, 1 figur
Hyperprolactinaemia in first episode psychosis - A longitudinal assessment
Little is known about hyperprolactinaemia (HPL) in first episode psychosis (FEP) patients. We investigated longitudinal changes in serum prolactin in FEP, and the relationship between HPL, and antipsychotic medication and stress.
Serum prolactin was recorded in FEP patients at recruitment and again, 3 and 12 months later. HPL was defined as a serum prolactin level > 410 mIU/L (~ 19.3 ng/ml) for males, and a serum prolactin level > 510 mIU/L (~ 24.1 ng/ml) for females.
From a total of 174 people with serum prolactin measurements at study recruitment, 43% (n = 74) had HPL, whilst 27% (n = 21/78) and 27% (n = 26/95) had HPL at 3 and 12 months respectively. We observed higher serum prolactin levels in females versus males (p < 0.001), and in antipsychotic treated (n = 68) versus antipsychotic naïve patients (p < 0.0001). Prolactin levels were consistently raised in FEP patients taking risperidone, amisulpride and FGAs compared to other antipsychotics. No significant relationship was observed between perceived stress scores (β = 7.13, t = 0.21, df = 11, p = 0.0.84 95% CI − 72.91–87.16), or objective life stressors (β = − 21.74, t = − 0.31, df = 8, p = 0.77 95% CI − 218.57–175.09) and serum prolactin.
Our study found elevated rates of HPL over the course of the first 12 months of illness. We found no evidence to support the notion that stress is related to elevated serum prolactin at the onset of psychosis
Bottom mixed layer oxygen dynamics in the Celtic Sea
The seasonally stratified continental shelf seas are highly productive, economically important environments which are under considerable pressure from human activity. Global dissolved oxygen concentrations have shown rapid reductions in response to anthropogenic forcing since at least the middle of the twentieth century. Oxygen consumption is at the same time linked to the cycling of atmospheric carbon, with oxygen being a proxy for carbon remineralisation and the release of CO2. In the seasonally stratified seas the bottom mixed layer (BML) is partially isolated from the atmosphere and is thus controlled by interplay between oxygen consumption processes, vertical and horizontal advection. Oxygen consumption rates can be both spatially and temporally dynamic, but these dynamics are often missed with incubation based techniques. Here we adopt a Bayesian approach to determining total BML oxygen consumption rates from a high resolution oxygen time-series. This incorporates both our knowledge and our uncertainty of the various processes which control the oxygen inventory. Total BML rates integrate both processes in the water column and at the sediment interface. These observations span the stratified period of the Celtic Sea and across both sandy and muddy sediment types. We show how horizontal advection, tidal forcing and vertical mixing together control the bottom mixed layer oxygen concentrations at various times over the stratified period. Our muddy-sand site shows cyclic spring-neap mediated changes in oxygen consumption driven by the frequent resuspension or ventilation of the seabed. We see evidence for prolonged periods of increased vertical mixing which provide the ventilation necessary to support the high rates of consumption observed
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