586 research outputs found
The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation
We combine ground and space-based photometry of the Galactic globular cluster
47 Tuc to measure four independent lines of evidence for a helium gradient in
the cluster, whereby stars in the cluster outskirts would have a lower initial
helium abundance than stars in and near the cluster core. First and second, we
show that the red giant branch bump (RGBB) stars exhibit gradients in their
number counts and brightness. With increased separation from the cluster
center, they become more numerous relative to the other red giant (RG) stars.
They also become fainter. For our third and fourth lines of evidence, we show
that the horizontal branch (HB) of the cluster becomes both fainter and redder
for sightlines farther from the cluster center. These four results are
respectively detected at the 2.3, 3.6, 7.7 and
4.1 levels. Each of these independent lines of evidence is found to be
significant in the cluster-outskirts; closer in, the data are more compatible
with uniform mixing. Our radial profile is qualitatively consistent with but
quantitatively tighter than previous results based on CN absorption. These
observations are qualitatively consistent with a scenario wherein a second
generation of stars with modestly enhanced helium and CNO abundance formed deep
within the gravitational potential of a cluster of previous generation stars
having more canonical abundances.Comment: 20 pages, 6 figures, 1 table, submitted to The Astrophysical Journa
Anisotropy and shear-velocity heterogeneities in the upper mantle
Long-period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle. The dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure. The data are corrected for the effect of surface layers and both Love and Rayleigh waves are used. Shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km. The shear wave velocity distribution to 200 km depth correlates with surface tectonics, except in a few anomalous regions. Below that depth the correlation vanishes. Cold subducted material shows up weakly at 350 km as fast S-wave anomalies. In the transition region a large scale pattern appears with fast mantle in the South-Atlantic. S-anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones. The Pacific shows a NW-SE fabric
Reassessment of a reported S-delay under Trindade
We present a correction to a paper by Okal and Anderson (1975) about multiple ScS travel-time anomalies. We have reanalyzed data for ScS_2 surface bounces in the South Atlantic Ocean. From these data an ScS_2-S residual of 23.6 seconds was found by Okal and Anderson (1975). This corresponded to an ScS_2 surface bounce point under Trindade island and was inferred to be due to very slow upper mantle associated with the Trindade hot spot. The analysis we present here invalidates this conclusion. The nature of the upper mantle under Trindade is an open issue
Protected quantum computation with multiple resonators in ultrastrong coupling circuit QED
We investigate theoretically the dynamical behavior of a qubit obtained with
the two ground eigenstates of an ultrastrong coupling circuit-QED system
consisting of a finite number of Josephson fluxonium atoms inductively coupled
to a transmission line resonator. We show an universal set of quantum gates by
using multiple transmission line resonators (each resonator represents a single
qubit). We discuss the intrinsic 'anisotropic' nature of noise sources for
fluxonium artificial atoms. Through a master equation treatment with colored
noise and manylevel dynamics, we prove that, for a general class of anisotropic
noise sources, the coherence time of the qubit and the fidelity of the quantum
operations can be dramatically improved in an optimal regime of ultrastrong
coupling, where the ground state is an entangled photonic 'cat' state.Comment: Added results with N = 3,4,5 Josephson atoms and different anisotropy
ratios for the decoherence channels in the new figures 2 and
Experimental evidence of accelerated seismic release without critical failure in acoustic emissions of compressed nanoporous materials
The total energy of acoustic emission (AE) events in externally stressed
materials diverges when approaching macroscopic failure. Numerical and
conceptual models explain this accelerated seismic release (ASR) as the
approach to a critical point that coincides with ultimate failure. Here, we
report ASR during soft uniaxial compression of three silica-based (SiO)
nanoporous materials. Instead of a singular critical point, the distribution of
AE energies is stationary and variations in the activity rate are sufficient to
explain the presence of multiple periods of ASR leading to distinct brittle
failure events. We propose that critical failure is suppressed in the AE
statistics by dissipation and transient hardening. Some of the critical
exponents estimated from the experiments are compatible with mean field models,
while others are still open to interpretation in terms of the solution of
frictional and fracture avalanche models.Comment: preprint, Main article: 7 pages, 3 figures. Supplementary material
included in \anc folder: 6 pages, 3 figure
Red Giant Branch Bump Brightness and Number Counts in 72 Galactic Globular Clusters Observed with the Hubble Space Telescope
We present the broadest and most precise empirical investigation of red giant
branch bump (RGBB) brightness and number counts ever conducted. We implement a
new method and use data from two \textit{Hubble Space Telescope (HST)} globular
cluster (GC) surveys to measure the brightness and star counts of the RGBB in
72 GCs. The brightness is measured to a precision better than 0.01 mag while
the precision in number counts reaches 10%. The position of the main-sequence
turnoff (MSTO) and the number of horizontal branch (HB) stars are used as
comparisons where appropriate. Several independent scientific conclusions are
newly possible with our parametrization of the RGBB. Both brightness and number
counts are shown to have second parameters in addition to their strong
dependence on metallicity. The RGBBs are found to be anomalous in the GCs NGC
2808, 5286, 6388 and 6441, likely due to the presence of multiple populations.
Finally, we use our empirical calibration to predict the properties of the
Galactic bulge RGBB if the assumption of similar stellar physics for the bulge
and Galactic GC system holds. The RGBB properties for the bulge are shown to
differ from those of the Galactic GC system, with the former having lower
number counts, a lower brightness dispersion and a brighter peak luminosity
than would be expected from the latter. This discrepancy is well explained by
the Galactic bulge having a higher helium abundance than expected from GCs,
Y0.06 at the median metallicity.Comment: 66 pages, 21 figures, 7 tables, Accepted for publication in The
Astrophysical Journal, modified following referee repor
Strong-field dynamo action in rapidly rotating convection with no inertia
The earth's magnetic field is generated by dynamo action driven by convection in the outer core. For numerical reasons, inertial and viscous forces play an important role in geodynamo models; however, the primary dynamical balance in the earth's core is believed to be between buoyancy, Coriolis, and magnetic forces. The hope has been that by setting the Ekman number to be as small as computationally feasible, an asymptotic regime would be reached in which the correct force balance is achieved. However, recent analyses of geodynamo models suggest that the desired balance has still not yet been attained. Here we adopt a complementary approach consisting of a model of rapidly rotating convection in which inertial forces are neglected from the outset. Within this framework we are able to construct a branch of solutions in which the dynamo generates a strong magnetic field that satisfies the expected force balance. The resulting strongly magnetized convection is dramatically different from the corresponding solutions in which the field is weak
Avalanches from charged domain wall motion in BaTiO3 during ferroelectric switching
We report two methods for direct observations of avalanches in ferroelectric materials during the motion of domain walls. In the first method, we use optical imaging techniques to derive changes in domain structures under an electric field. All changes occur through small jumps (jerks) that obey avalanche statistics. In the second method, we analyze jerks by their displacement current. Both methods reveal a power law distribution with an energy exponent of 1.6, in agreement with previous acoustic emission measurements, and integrated mean field theory. This new combination of methods allows us to probe both polarization and strain variations during the motion of domain walls and can be used for a much wider class of ferroelectrics, including ceramic samples, than acoustic emission
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