3,696 research outputs found
Constraints on neutrino and dark radiation interactions using cosmological observations
Observations of the cosmic microwave background (CMB) and large-scale
structure (LSS) provide a unique opportunity to explore the fundamental
properties of the constituents that compose the cosmic dark radiation
background (CDRB), of which the three standard neutrinos are thought to be the
dominant component. We report on the first constraint to the CDRB rest-frame
sound speed, ceff^2, using the most recent CMB and LSS data. Additionally, we
report improved constraints to the CDRB viscosity parameter, cvis^2. For a
non-interacting species, these parameters both equal 1/3. Using current data we
find that a standard CDRB, composed entirely of three non-interacting neutrino
species, is ruled out at the 99% confidence level (C.L.) with ceff^2 = 0.30
+0.027 -0.026 and cvis^2 = 0.44 +0.27 -0.21 (95% C.L.). We also discuss how
constraints to these parameters from current and future observations (such as
the Planck satellite) allow us to explore the fundamental properties of any
anomalous radiative energy density beyond the standard three neutrinos.Comment: 6 pages, 3 figures, comments welcome; v2: updated with SPT data,
corrected minor typos; v3: version accepted for publication in PR
Dynamical Tide in Solar-Type Binaries
Circularization of late-type main-sequence binaries is usually attributed to
turbulent convection, while that of early-type binaries is explained by
resonant excitation of g modes. We show that the latter mechanism operates in
solar-type stars also and is at least as effective as convection, despite
inefficient damping of g modes in the radiative core. The maximum period at
which this mechanism can circularize a binary composed of solar-type stars in
10 Gyr is as low as 3 days, if the modes are damped by radiative diffusion only
and g-mode resonances are fixed; or as high as 6 days, if one allows for
evolution of the resonances and for nonlinear damping near inner turning
points. Even the larger theoretical period falls short of the observed
transition period by a factor two.Comment: 17 pages, 2 postscript figures, uses aaspp4.sty. Submitted to Ap
Viscous dissipation by tidally forced inertial modes in a rotating spherical shell
We investigate the properties of forced inertial modes of a rotating fluid
inside a spherical shell. Our forcing is tidal like, but its main property is
that it is on the large scales. Our solutions first confirm some analytical
results obtained on a two-dimensional model by Ogilvie (2005). We also note
that as the frequency of the forcing varies, the dissipation varies drastically
if the Ekman number E is low (as is usually the case). We then investigate the
three-dimensional case and compare the results to the foregoing model. These
solutions show, like their 2D counterpart, a spiky dissipation curve when the
frequency of the forcing is varied; they also display small frequency intervals
where the viscous dissipation is independent of viscosity. However, we show
that the response of the fluid in these frequency intervals is crucially
dominated by the shear layer that is emitted at the critical latitude on the
inner sphere. The asymptotic regime is reached when an attractor has been
excited by this shear layer. This property is not shared by the two-dimensional
model. Finally, resonances of the three-dimensional model correspond to some
selected least-damped eigenmodes. Unlike their two-dimensional counter parts
these modes are not associated with simple attractors; instead, they show up in
frequency intervals with a weakly contracting web of characteristics. Besides,
we show that the inner core is negligible when its relative radius is less than
the critical value 0.4E^{1/5}. For these spherical shells, the full sphere
solutions give a good approximation of the flows (abridged abstract).Comment: 32 pages, 19 figs, accepted in J. Fluid Mec
A Robust Measure of Tidal Circularization in Coeval Binary Populations: The solar-type spectroscopic Binary Population in The Open Cluster M35
We present a new homogeneous sample of 32 spectroscopic binary orbits in the
young (~ 150 Myr) main-sequence open cluster M35. The distribution of orbital
eccentricity vs. orbital period (e-log(P)) displays a distinct transition from
eccentric to circular orbits at an orbital period of ~ 10 days. The transition
is due to tidal circularization of the closest binaries. The population of
binary orbits in M35 provide a significantly improved constraint on the rate of
tidal circularization at an age of 150 Myr. We propose a new and more robust
diagnostic of the degree of tidal circularization in a binary population based
on a functional fit to the e-log(P) distribution. We call this new measure the
tidal circularization period. The tidal circularization period of a binary
population represents the orbital period at which a binary orbit with the most
frequent initial orbital eccentricity circularizes (defined as e = 0.01) at the
age of the population. We determine the tidal circularizationperiod for M35 as
well as for 7 additional binary populations spanning ages from the pre
main-sequence (~ 3 Myr) to late main-sequence (~ 10 Gyr), and use Monte Carlo
error analysis to determine the uncertainties on the derived circularization
periods. We conclude that current theories of tidal circularization cannot
account for the distribution of tidal circularization periods with population
age.Comment: 37 pages, 9 figures, to be published in The Astrophysical Journal,
February 200
Synthesis of [(C5Me5)2Mo2XS3Co(CO)2] Clusters (X = P, As) and Structural Characterization of a μ2, η2-AsS Ligand
On calculating the Berry curvature of Bloch electrons using the KKR method
We propose and implement a particularly effective method for calculating the
Berry curvature arising from adiabatic evolution of Bloch states in wave vector
k space. The method exploits a unique feature of the Korringa-Kohn-Rostoker
(KKR) approach to solve the Schr\"odinger or Dirac equations. Namely, it is
based on the observation that in the KKR method k enters the calculation via
the structure constants which depend only on the geometry of the lattice but
not the crystal potential. For both the Abelian and non-Abelian Berry curvature
we derive an analytic formula whose evaluation does not require any numerical
differentiation with respect to k. We present explicit calculations for Al, Cu,
Au, and Pt bulk crystals.Comment: 13 pages, 5 figure
Structure and Evolution of Giant Cells in Global Models of Solar Convection
The global scales of solar convection are studied through three-dimensional
simulations of compressible convection carried out in spherical shells of
rotating fluid which extend from the base of the convection zone to within 15
Mm of the photosphere. Such modelling at the highest spatial resolution to date
allows study of distinctly turbulent convection, revealing that coherent
downflow structures associated with giant cells continue to play a significant
role in maintaining the strong differential rotation that is achieved. These
giant cells at lower latitudes exhibit prograde propagation relative to the
mean zonal flow, or differential rotation, that they establish, and retrograde
propagation of more isotropic structures with vortical character at mid and
high latitudes. The interstices of the downflow networks often possess strong
and compact cyclonic flows. The evolving giant-cell downflow systems can be
partly masked by the intense smaller scales of convection driven closer to the
surface, yet they are likely to be detectable with the helioseismic probing
that is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate out these
signals from those attributed to the faster horizontal flows of
supergranulation. To aid in such detection, we use our simulations to describe
how the properties of giant cells may be expected to vary with depth, how their
patterns evolve in time, and analyze the statistical features of correlations
within these complex flow fields.Comment: 22 pages, 16 figures (color figures are low res), uses emulateapj.cls
Latex class file, Results shown during a Press release at the AAS meeting in
June 2007. Submitted to Ap
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
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