1,675 research outputs found
Photoionization cross sections of O II, O III, O IV, and O V: benchmarking R-matrix theory and experiments
For crucial tests between theory and experiment, ab initio close coupling
calculations are carried out for photoionization of O II, O III, O IV, O V. The
relativistic fine structure and resonance effects are studied using the
R-matrix and its relativistic variant the Breit Pauli R-matrix (BPRM)
approximation. Detailed comparison is made with high resolution experimental
measurements carried out in three different set-ups: Advanced Light Source at
Berkeley, and synchrotron radiation experiments at University of Aarhus and
University of Paris-Sud. The comparisons illustrate physical effects in
photoionization such as (i) fine structure, (ii) resolution, and (iii)
metastable components. Photoionization cross sections sigma{PI} of the ground
and a few low lying excited states of these ions obtained in the experimental
spectrum include combined features of these states. Theoretically calculated
resonances need to be resolved with extremely fine energy mesh for precise
comparison. In addition, prominent resonant features are observed in the
measured spectra from transitions allowed with relativistic fine structure, but
not in LS coupling. The sigma_{PI} are obtained for ground and metastable (i)
2s^22p^3(^4S^o, ^2D^o, ^2P^o) states of O II, (ii) 2s^22p^2(^3P,^1D,^1S) and
2s2p^3(^5S^o) states of O III, (iii) 2s^22p(^2P^o_J) and 2s2p^2(^4P_J) levels
of O IV, and (iv) 2s^2(^1S) and 2s2p(^3P^o,^1P^o) states of O V. It is found
that resonances in ground and metastable cross sections can be a diagnostic of
experimental beam composition, with potential ap plications to astrophysical
and laboratory plasma environments.Comment: 27 pages, 7 figs., submitted to Phys. Rev. A., text with high
resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/Oions.p
The Unusual Object IC 2144/MWC 778
IC 2144 is a small reflection nebula located in the zone of avoidance near
the Galactic anticenter. It has been investigated here largely on the basis of
Keck/HIRES optical spectroscopy (R ~ 48,000) and a SpeX spectrogram in the
near-IR (R = 2000) obtained at the NASA IRTF. The only star in the nebula that
is obvious in the optical or near-IR is the peculiar emission-line object MWC
778 (V = 12.8), which resembles a T Tauri star in some respects. What appear to
be F- or G-type absorption features are detectable in its optical region under
the very complex emission line spectrum; their radial velocity agrees with the
CO velocity of the larger cloud in which IC 2144 is embedded. There are
significant differences between the spectrum of the brightest area of the
nebula and of MWC 778, the presumed illuminator, an issue discussed in some
detail. The distance of IC 2144 is inferred to be about 1.0 kpc by reference to
other star-forming regions in the vicinity. The extinction is large, as
demonstrated by [Fe II] emission line ratios in the near-IR and by the strength
of the diffuse interstellar band spectrum; a provisional value of A_V of 3.0
mag was assumed. The SED of MWC 778 rises steeply beyond about 1 m, with a
slope characteristic of a Class I source. Integration of the flux distribution
leads to an IR luminosity of about 510 L_solar. If MWC 778 is indeed a F- or
G-type pre--main-sequence star several magnitudes above the ZAMS, a population
of faint emission Halpha stars would be expected in the vicinity. Such a
search, like other investigations that are recommended in this paper, has yet
to be carried out.Comment: 36 pages, 13 figures, accepted by A
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements VI. Ni II
We present the first detailed ab initio quantum mechanical calculations for
total and state-specific recombination rate coefficients for e + Ni III --> Ni
II. These rates are obtained using a unified treatment for total electron-ion
recombination that treats the nonresonant radiative recombination and the
resonant dielectronic recombination in a self-consistent unified manner in the
close coupling approximation. Large-scale calculations are carried out using a
49-state wavefunction expansion from core configurations 3d^8, 3d^74s, and
3d^64p that permits the inclusion of prominent dipole allowed core transitions.
These extensive calculations for the recombination rates of Ni II required
hundreds of CPU hours on the Cray T90. The total recombination rate
coefficients are provided for a wide range of temperature. The state-specific
recombination rates for 532 bound states of doublet and quartet symmetries, and
the corresponding photoionization cross sections for leaving the core in the
ground state, are presented. Present total recombination rate coefficients
differ considerably from the currently used data in astrophysical models.Comment: ApJ Suppl. (submitted), 4 figure
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The Persistent Southern Disadvantage in Us Early Life Mortality, 1965‒2014
Background: Recent studies of US adult mortality demonstrate a growing disadvantage among southern states. Few studies have examined long-term trends and geographic patterns in US early life (ages 1 to 24) mortality, ages at which key risk factors and causes of death are quite different than among adults. Objective: This article examines trends and variations in early life mortality rates across US states and census divisions. We assess whether those variations have changed over a 50-year time period and which causes of death contribute to contemporary geographic disparities. Methods: We calculate all-cause and cause-specific death rates using death certificate data from the Multiple Cause of Death files, combining public-use files from 1965‒2004 and restricted data with state geographic identifiers from 2005‒2014. State population (denominator) data come from US decennial censuses or intercensal estimates. Results: Results demonstrate a persistent mortality disadvantage for young people (ages 1 to 24) living in southern states over the last 50 years, particularly those located in the East South Central and West South Central divisions. Motor vehicle accidents and homicide by firearm account for most of the contemporary southern disadvantage in US early life mortality. Contribution: Our results illustrate that US children and youth living in the southern United States have long suffered from higher levels of mortality than children and youth living in other parts of the country. Our findings also suggest the contemporary southern disadvantage in US early life mortality could potentially be reduced with state-level policies designed to prevent deaths involving motor vehicles and firearms
The 21cm Signature of the First Stars
We predict the 21-cm signature of the first metal-free stars. The soft X-rays
emitted by these stars penetrate the atomic medium around their host halos,
generating Lyman alpha photons that couple the spin and kinetic temperatures.
These creates a region we call the Lyman alpha sphere, visible in 21-cm against
the CMB, which is much larger than the HII region produced by the same star.
The spin and kinetic temperatures are strongly coupled before the X-rays can
substantially heat the medium, implying that a strong 21-cm absorption signal
from the adiabatically cooled gas in Hubble expansion around the star is
expected when the medium has not been heated previously. A central region of
emission from the gas heated by the soft X-rays is also present although with a
weaker signal than the absorption. The Lyman alpha sphere is a universal
signature that should be observed around any first star illuminating its
vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere
can be calculated as a function of the luminosity, spectrum and age of the
star. For a star of a few hundred solar masses and zero metallicity (as
expected for the first stars), the physical radius of the Lyman alpha sphere
can reach tens of kiloparsecs. The first metal-free stars should be strongly
clustered because of high cosmic biasing; this implies that the regions
producing a 21-cm absorption signal may contain more than one star and will
generally be irregular and not spherical, because of the complex distribution
of the gas. We discuss the feasiblity of detecting these Lyman alpha spheres,
which would be present at redshifts in the Cold Dark Matter model.
Their observation would represent a direct proof of the detection of a first
star.Comment: replaced with ApJ accepted version. Many minor revisions and
additional references, major results unchange
Kinetics and mechanism of proton transport across membrane nanopores
We use computer simulations to study the kinetics and mechanism of proton
passage through a narrow-pore carbon-nanotube membrane separating reservoirs of
liquid water. Free energy and rate constant calculations show that protons move
across the membrane diffusively in single-file chains of hydrogen-bonded water
molecules. Proton passage through the membrane is opposed by a high barrier
along the effective potential, reflecting the large electrostatic penalty for
desolvation and reminiscent of charge exclusion in biological water channels.
At neutral pH, we estimate a translocation rate of about 1 proton per hour and
tube.Comment: 4 pages, 4 figure
Measuring the energy landscape roughness and the transition state location of biomolecules using single molecule mechanical unfolding experiments
Single molecule mechanical unfolding experiments are beginning to provide
profiles of the complex energy landscape of biomolecules. In order to obtain
reliable estimates of the energy landscape characteristics it is necessary to
combine the experimental measurements with sound theoretical models and
simulations. Here, we show how by using temperature as a variable in mechanical
unfolding of biomolecules in laser optical tweezer or AFM experiments the
roughness of the energy landscape can be measured without making any
assumptions about the underlying reaction oordinate. The efficacy of the
formalism is illustrated by reviewing experimental results that have directly
measured roughness in a protein-protein complex. The roughness model can also
be used to interpret experiments on forced-unfolding of proteins in which
temperature is varied. Estimates of other aspects of the energy landscape such
as free energy barriers or the transition state (TS) locations could depend on
the precise model used to analyze the experimental data. We illustrate the
inherent difficulties in obtaining the transition state location from loading
rate or force-dependent unfolding rates. Because the transition state moves as
the force or the loading rate is varied it is in general difficult to invert
the experimental data unless the curvature at the top of the one dimensional
free energy profile is large, i.e the barrier is sharp. The independence of the
TS location on force holds good only for brittle or hard biomolecules whereas
the TS location changes considerably if the molecule is soft or plastic. We
also comment on the usefulness of extension of the molecule as a surrogate
reaction coordinate especially in the context of force-quench refolding of
proteins and RNA.Comment: 44 pages, 7 figure
HID-1 controls formation of large dense core vesicles by influencing cargo sorting and trans-Golgi network acidification
Large dense core vesicles (LDCVs) mediate the regulated release of neuropeptides and peptide hormones. They form at the trans-Golgi network (TGN), where their soluble content aggregates to form a dense core, but the mechanisms controlling biogenesis are still not completely understood. Recent studies have implicated the peripheral membrane protein HID-1 in neuropeptide sorting and insulin secretion. Using CRISPR/Cas9, we generated HID-1 KO rat neuroendocrine cells, and we show that the absence of HID-1 results in specific defects in peptide hormone and monoamine storage and regulated secretion. Loss of HID-1 causes a reduction in the number of LDCVs and affects their morphology and biochemical properties, due to impaired cargo sorting and dense core formation. HID-1 KO cells also exhibit defects in TGN acidification together with mislocalization of the Golgi-enriched vacuolar H+-ATPase subunit isoform a2. We propose that HID-1 influences early steps in LDCV formation by controlling dense core formation at the TGN.</jats:p
Radiative transfer effects in primordial hydrogen recombination
The calculation of a highly accurate cosmological recombination history has
been the object of particular attention recently, as it constitutes the major
theoretical uncertainty when predicting the angular power spectrum of Cosmic
Microwave Background anisotropies. Lyman transitions, in particular the
Lyman-alpha line, have long been recognized as one of the bottlenecks of
recombination, due to their very low escape probabilities. The Sobolev
approximation does not describe radiative transfer in the vicinity of Lyman
lines to a sufficient degree of accuracy, and several corrections have already
been computed in other works. In this paper, the impact of some previously
ignored radiative transfer effects is calculated. First, the effect of Thomson
scattering in the vicinity of the Lyman-alpha line is evaluated, using a full
redistribution kernel incorporated into a radiative transfer code. The effect
of feedback of distortions generated by the optically thick deuterium
Lyman-alpha line blueward of the hydrogen line is investigated with an analytic
approximation. It is shown that both effects are negligible during cosmological
hydrogen recombination. Secondly, the importance of high-lying, non overlapping
Lyman transitions is assessed. It is shown that escape from lines above
Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected
without loss of accuracy. Thirdly, a formalism generalizing the Sobolev
approximation is developed to account for the overlap of the high-lying Lyman
lines, which is shown to lead to negligible changes to the recombination
history. Finally, the possibility of a cosmological hydrogen recombination
maser is investigated. It is shown that there is no such maser in the purely
radiative treatment presented here.Comment: 23 pages, 4 figures, to be submitted to PR
Formation of Primordial Protostars
The evolution of collapsing metal free protostellar clouds is investigated
for various masses and initial conditions.
We perform hydrodynamical calculations for spherically symmetric clouds
taking account of radiative transfer of the molecular hydrogen lines and the
continuum, as well as of chemistry of the molecular hydrogen.
The collapse is found to proceed almost self-similarly like Larson-Penston
similarity solution.
In the course of the collapse, efficient three-body processes transform
atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular
form.
However, hydrogen in the outer part remains totally atomic although there is
an intervening transitional layer of several solar masses, where hydrogen is in
partially molecular form.
No opaque transient core is formed although clouds become optically thick to
H collision-induced absorption continuum, since H dissociation
follows successively.
When the central part of the cloud reaches stellar densities (), a very small hydrostatic core (\sim
5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the
ambient gas accretes onto it.
The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun}
{\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where is instantaneous
mass of the central core, by using a similarity solution which reproduces the
evolution of the cloud before the core formation.Comment: 20 pages, 5 Postscript figures, uses AAS LaTe
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