1,342 research outputs found
Cosmological hydrogen recombination: The effect of extremely high-n states
Calculations of cosmological hydrogen recombination are vital for the
extraction of cosmological parameters from cosmic microwave background (CMB)
observations, and for imposing constraints to inflation and re-ionization. The
Planck} mission and future experiments will make high precision measurements of
CMB anisotropies at angular scales as small as l~2500, necessitating a
calculation of recombination with fractional accuracy of ~10^{-3}. Recent work
on recombination includes two-photon transitions from high excitation states
and many radiative transfer effects. Modern recombination calculations
separately follow angular momentum sublevels of the hydrogen atom to accurately
treat non-equilibrium effects at late times (z<900). The inclusion of extremely
high-n (n>100) states of hydrogen is then computationally challenging,
preventing until now a determination of the maximum n needed to predict CMB
anisotropy spectra with sufficient accuracy for Planck. Here, results from a
new multi-level-atom code (RecSparse) are presented. For the first time,
`forbidden' quadrupole transitions of hydrogen are included, but shown to be
negligible. RecSparse is designed to quickly calculate recombination histories
including extremely high-n states in hydrogen. Histories for a sequence of
values as high as n_max=250 are computed, keeping track of all angular momentum
sublevels and energy shells of the hydrogen atom separately. Use of an
insufficiently high n_max value (e.g., n_max=64) leads to errors (e.g., 1.8
sigma for Planck) in the predicted CMB power spectrum. Extrapolating errors,
the resulting CMB anisotropy spectra are converged to 0.5 sigma at
Fisher-matrix level for n_max=128, in the purely radiative case.Comment: 19 pages, 12 figures, replaced with version published in Physical
Review D (added discussion of collisions)
Structural evaluation of candidate designs for the large space telescope primary mirror
Structural performance analyses were conducted on two candidate designs (Itek and Perkin-Elmer designs) for the large space telescope three-meter mirror. The mirror designs and the finite-element models used in the analyses evaluation are described. The results of the structural analyses for several different types of loading are presented in tabular and graphic forms. Several additional analyses are also reported: the evaluation of a mirror design concept proposed by the Boeing Co., a study of the global effects of local cell plate deflections, and an investigation of the fracture mechanics problems likely to occur with Cervit and ULE. Flexibility matrices were obtained for the Itek and Perkin-Elmer mirrors to be used in active figure control studies. Summary, conclusions, and recommendations are included
Cosmological Implications Of Ultralight Axionlike Fields
Cosmological observations are used to test for imprints of an ultralight axionlike field (ULA), with a range of potentials V(ϕ)∝[1−cos(ϕ/f)]ⁿ set by the axion-field value ϕ and decay constant f. Scalar field dynamics dictate that the field is initially frozen and then begins to oscillate around its minimum when the Hubble parameter drops below some critical value. For n=1, once dynamical, the axion energy density dilutes as matter; for n=2 it dilutes as radiation and for n=3 it dilutes faster than radiation. Both the homogeneous evolution of the ULA and the dynamics of its linear perturbations are included, using an effective fluid approximation generalized from the usual n=1 case. ULA models are parametrized by the redshift z(c) when the field becomes dynamical, the fractional energy density f(z(c))≡Ωₐ(z(c))/Ωₜₒₜ(z(c)) in the axion field at zc, and the effective sound speed c²ₛ. Using Planck, BAO and JLA data, constraints on fzc are obtained. ULAs are degenerate with dark energy for all three potentials if 1+z(c)≲10. When 3×10⁴≳1+z(c)≳10, f(z(c)) is constrained to be ≲0.004 for n=1 and f(z(c))≲0.02 for the other two potentials. The constraints then relax with increasing zc. These results have implications for ULAs as a resolution to cosmological tensions, such as discrepant measurements of the Hubble constant, or the EDGES measurement of the global 21 cm signal
Subsonic structure and optically thick winds from Wolf--Rayet stars
Wolf-Rayet star's winds can be so dense and so optically thick that the
photosphere appears in the highly supersonic part of the outflow, veiling the
underlying subsonic part of the star, and leaving the initial acceleration of
the wind inaccessible to observations. We investigate the conditions and the
structure of the subsonic part of the outflow of Galactic WR stars, in
particular of the WNE subclass; our focus is on the conditions at the sonic
point. We compute 1D hydrodynamic stellar structure models for massive helium
stars adopting outer boundaries at the sonic point. We find that the outflows
of our models are accelerated to supersonic velocities by the radiative force
from opacity bumps either at temperatures of the order of 200kK by the Fe
opacity bump or of the order of 50kK by the HeII opacity bump. For a given
mass-loss rate, the conditions in the subsonic part of the outflow are
independent from the detailed physical conditions in the supersonic part. The
close proximity to the Eddington limit at the sonic point allows us to
construct a Sonic HR diagram, relating the sonic point temperature to the L/M
ratio and the stellar mass-loss rate, thereby constraining the sonic point
conditions, the subsonic structure, and the stellar wind mass-loss rates from
observations. The minimum mass-loss rate necessary to have the flow accelerated
to supersonic velocities by the Fe opacity bump is derived. A comparison of the
observed parameters of Galactic WNE stars to this minimum mass-loss rate
indicates that their winds are launched to supersonic velocities by the
radiation pressure arising from the Fe-bump. Conversely, models which do not
show transonic flows from the Fe opacity bump form inflated envelopes. We
derive an analytic criterion for the appearance of envelope inflation in the
subphotospheric layers.Comment: A&A, Forthcoming article. 13 pages+
Unconventional magnetism in multivalent charge-ordered YbPtGe probed by Pt- and Yb-NMR
Detailed Pt- and Yb nuclear magnetic resonance (NMR) studies
on the heterogeneous mixed valence system YbPtGe are reported. The
temperature dependence of the Pt-NMR shift indicates the
opening of an unusual magnetic gap below 200\,K. was analyzed by a
thermal activation model which yields an isotropic gap \,K. In contrast, the spin-lattice relaxation rate () does
not provide evidence for the gap. Therefore, an intermediate-valence picture is
proposed while a Kondo-insulator scenario can be excluded. Moreover,
() follows a simple metallic behavior, similar to the reference
compound YPtGe. A well resolved NMR line with small shift is assigned to
divalent Yb. This finding supports the proposed model with two sub-sets
of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the
YbPtGe lattice.Comment: Submitted in Physical Review B (Rapid Communication
Intermetallic compounds in heterogeneous catalysis - a quickly developing field
The application of intermetallic compounds for understanding in heterogeneous catalysis developed in an excellent way during the last decade. This review provides an overview of concepts and developments revealing the potential of intermetallic compounds in fundamental as well as applied catalysis research. Intermetallic compounds may be considered as platform materials to address current and future catalytic challenges, e.g. in respect to the energy transition
Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure
New germanium-platinum compounds with the filled-skutterudite crystal
structure were synthesized. The structure and composition were investigated by
X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific
heat, and electrical resistivity measurements evidence superconductivity in
LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and
superconducting states were established. Strong coupling and a crystal electric
field singlet groundstate is found for the Pr compound. Electronic structure
calculations show a large density of states at the Fermi level. Similar
behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 200
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