19,173 research outputs found
Calibrations, Monopoles and Fuzzy Funnels
We present new non-Abelian solitonic configurations in the low energy
effective theory describing a collection of N parallel D1--branes. These
configurations preserve 1/4, 1/8, 1/16 and 1/32 of the spacetime supersymmetry.
They are solutions to a set of generalised Nahm's equations which are related
to self-duality equations in eight dimensions. Our solutions represent
D1--branes which expand into fuzzy funnel configurations ending on collections
of intersecting D3--branes. Supersymmetry dictates that such intersecting
D3--branes must lie on a calibrated three-surface of spacetime and we argue
that the generalised Nahm's equations encode the data for the construction of
magnetic monopoles on the relevant three-surfaces.Comment: 19 pages. Latex. v2: added references and acknowledgmen
The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A
The isotope ratio, 85Rb/87Rb, places constraints on models of the
nucleosynthesis of heavy elements, but there is no precise determination of the
ratio for material beyond the Solar System. We report the first measurement of
the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A
for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)
that differs significantly from the meteoritic value of 2.59. The Rb/K
elemental abundance ratio for the cloud also is lower than that seen in
meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values
indicates that the interstellar 85Rb abundance in this direction is lower than
the Solar System abundance. We attribute the lower abundance to a reduced
contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are
consistent with much less r-process synthesis for the solar neighborhood
compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter
The effects of restraint on uptake of radioactive sulfate in the salivary and gastric secretions of rats with pyloric ligation
The effects of restraint on the amount of nondialysable radioactive sulfate in the gastric wall and the gastric juice and saliva were investigated. It was found that restraint provokes a significant decrease in salivary radioactive sulfate. This, in turn, is responsible for the decrease of sulfate in the gastric contents observed under these conditions in rats with pyloric ligation. Esophageal ligation associated with this prevents passage of saliva and lowers the amount of radioactive sulfate in the gastric juice. Restraint causes then an increase in the amount of sulfate in the gastric juice, the value observed being very much lower than that of rats with a free esophagus. At the level of the gastric wall, the change observed during restraint does not reach a significant threshold
Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets
We derive atmospheric parameters and lithium abundances for 671 stars and
include our measurements in a literature compilation of 1381 dwarf and subgiant
stars. First, a "lithium desert" in the effective temperature (Teff) versus
lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K
and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li
side of the desert have experienced a short-lived period of severe surface
lithium destruction as main-sequence or subgiant stars. Next, we search for
differences in the lithium content of thin-disk and thick-disk stars, but we
find that internal processes have erased from the stellar photospheres their
possibly different histories of lithium enrichment. Nevertheless, we note that
the maximum lithium abundance of thick-disk stars is nearly constant from
[Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very
metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance
distribution of known planet-host stars relative to otherwise ordinary stars
appear when restricting the samples to narrow ranges of Teff or mass, but they
are fully explained by age and metallicity biases. We confirm the lack of a
connection between low lithium abundance and planets. However, we find that no
low A_Li planet-hosts are found in the desert Teff window. Provided that subtle
sample biases are not responsible for this observation, this suggests that the
presence of gas giant planets inhibit the mechanism responsible for the lithium
desert.Comment: ApJ, in press. Complete Tables 1 and 3 are available upon reques
Nucleosynthesis and mixing on the Asymptotic Giant Branch. III. Predicted and observed s-process abundances
We present the results of s-process nucleosynthesis calculations for AGB
stars of different metallicities and initial masses. The computations were
based on previously published stellar evolutionary models that account for the
III dredge up phenomenon occurring late on the AGB. Neutron production is
driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny
layer in radiative equilibrium at the top of the He- and C-rich shell. The
s-enriched material is subsequently mixed with the envelope by the III dredge
up, and the envelope composition is computed after each thermal pulse. We
follow the changes in the photospheric abundance of the Ba-peak elements (heavy
s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic
ratio [hs/ls] has often been adopted as an indicator of the s-process
efficiency. The theoretical predictions are compared with published abundances
of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB
supergiants, and for various classes of binary stars. The observations in
general confirm the complex dependence of n captures on metallicity. They
suggest that a moderate spread exists in the abundance of 13C that is burnt in
different stars. Although additional observations are needed, a good
understanding has been achieved of s-process operation in AGB. The detailed
abundance distribution including the light elements (CNO) of a few s-enriched
stars at different metallicity are examined.Comment: Accepted for ApJ, 59 pages, 19 figures, 5 table
Investigation of tidal displacements of the Earth's surface by laser ranging to GEOS-3
An analysis of laser ranging data from three stations was carried out in an attempt to measure the geometric Earth tide. Two different approaches to the problem were investigated. The dynamic method computes pass to pass apparent movements in stations height relative to short arcs fitted to several passes of data from the same station by the program GEODYNE. The quasi-geometric method reduces the dependence on unmodelled satellite dynamics to a knowledge of only the radial position of the satellite by considering two station simultaneous ranging at the precise time that the satellite passes through the plane defined by two stations and the center of mass of the Earth
Boron Abundances in Diffuse Interstellar Clouds
We present a comprehensive survey of B abundances in diffuse interstellar
clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is
the result of a complete search of archival STIS data for the B II resonance
line at 1362 angstroms, with each detection confirmed by the presence of
absorption from other dominant ions at the same velocity. The data probe a
range of astrophysical environments including both high-density regions of
massive star formation as well as low-density paths through the Galactic halo,
allowing us to clearly define the trend of B depletion onto interstellar grains
as a function of gas density. Many extended sight lines exhibit complex
absorption profiles that trace both local gas and gas associated with either
the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher
B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of
the Sun, which may suggest that B production in the current epoch is dominated
by a secondary process. The average gas-phase B abundance in the warm diffuse
ISM is consistent with the abundances determined for a variety of Galactic disk
stars, but is depleted by 60 percent relative to the solar system value. Our
survey also reveals sight lines with enhanced B abundances that potentially
trace recent production of B-11 either by cosmic-ray or neutrino-induced
spallation. Such sight lines will be key to discerning the relative importance
of the two production routes for B-11 synthesis.Comment: To be published in the proceedings of the IAU Symposium 268, Light
Elements in the Universe, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini,
ed
One step multiderivative methods for first order ordinary differential equations
A family of one-step multiderivative methods based on Padé approximants to the exponential function is developed.
The methods are extrapolated and analysed for use in PECE mode.
Error constants and stability intervals are calculated and the combinations compared with well known linear multi-step combinations and combinations using high accuracy Newton-Cotes quadrature formulas as correctors.
w926020
Rubidium and lead abundances in giant stars of the globular clusters M 13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb in five
giant stars of the globular cluster NGC 6752 and Pb measurements in four giants
of the globular cluster M 13. The abundances were derived by comparing
synthetic spectra with high resolution, high signal-to-noise ratio spectra
obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope.
The program stars span the range of the O-Al abundance variation. In NGC 6752,
the mean abundances are [Rb/Fe] = -0.17 +/- 0.06 (sigma = 0.14), [Rb/Zr] =
-0.12 +/- 0.06 (sigma = 0.13), and [Pb/Fe] = -0.17 +/- 0.04 (sigma = 0.08). In
M 13 the mean abundance is [Pb/Fe] = -0.28 +/- 0.03 (sigma = 0.06). Within the
measurement uncertainties, we find no evidence for a star-to-star variation for
either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe],
[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have
slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample
of field stars at the same metallicity. For M 13 and NGC 6752 the Pb abundances
are in accord with predictions from a Galactic chemical evolution model. If
metal-poor intermediate-mass asymptotic giant branch stars did produce the
globular cluster abundance anomalies, then such stars do not synthesize
significant quantities of Rb or Pb. Alternatively, if such stars do synthesize
large amounts of Rb or Pb, then they are not responsible for the abundance
anomalies seen in globular clusters.Comment: Accepted for publication in Ap
Solar cell research, phase 2 Semiannual report
Radiation effects on properties of lithium solar cell
- …