7,962 research outputs found
Modeling the Black Hole Excision Problem
We analyze the excision strategy for simulating black holes. The problem is
modeled by the propagation of quasi-linear waves in a 1-dimensional spatial
region with timelike outer boundary, spacelike inner boundary and a horizon in
between. Proofs of well-posed evolution and boundary algorithms for a second
differential order treatment of the system are given for the separate pieces
underlying the finite difference problem. These are implemented in a numerical
code which gives accurate long term simulations of the quasi-linear excision
problem. Excitation of long wavelength exponential modes, which are latent in
the problem, are suppressed using conservation laws for the discretized system.
The techniques are designed to apply directly to recent codes for the Einstein
equations based upon the harmonic formulation.Comment: 21 pages, 14 postscript figures, minor contents updat
Instability and `Sausage-String' Appearance in Blood Vessels during High Blood Pressure
A new Rayleigh-type instability is proposed to explain the `sausage-string'
pattern of alternating constrictions and dilatations formed in blood vessels
under influence of a vasoconstricting agent. Our theory involves the nonlinear
elasticity characteristics of the vessel wall, and provides predictions for the
conditions under which the cylindrical form of a blood vessel becomes unstable.Comment: 4 pages, 4 figures submitted to Physical Review Letter
New Abundances for Old Stars - Atomic Diffusion at Work in NGC 6397
A homogeneous spectroscopic analysis of unevolved and evolved stars in the
metal-poor globular cluster NGC 6397 with FLAMES-UVES reveals systematic trends
of stellar surface abundances that are likely caused by atomic diffusion. This
finding helps to understand, among other issues, why the lithium abundances of
old halo stars are significantly lower than the abundance found to be produced
shortly after the Big Bang.Comment: 8 pages, 7 colour figures, 1 table; can also be downloaded via
http://www.eso.org/messenger
Observation of Parity Nonconservation in Møller Scattering
We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_(PV) = [-175 ± 30(stat)± 20(syst)] X 10^(-9). This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron’s weak charge at low energy Q^e_W = -0:053 ± 0:011. This is consistent with the standard model expectation at the current level of precision: sin^2θ_W = (M_Z)_(MS) = 0:2293 ± 0:0024(stat) ± 0:0016(syst) ± 0:0006(theory)
The r-Process Enriched Low Metallicity Giant HD 115444
New high resolution, very high signal-to-noise spectra of ultra-metal-poor
(UMP) giant stars HD 115444 and HD 122563 have been gathered with the
High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7m
Telescope. With these spectra, line identification and model atmosphere
analyses have been conducted, emphasizing the neutron-capture elements. Twenty
elements with Z > 30 have been identified in the spectrum of HD 115444. This
star is known to have overabundances of the neutron-capture elements, but it
has lacked a detailed analysis necessary to compare with nucleosynthesis
predictions. The new study features a line-by-line differential abundance
comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For
HD 115444, the overall metallicity is [Fe/H]~ -3.0. The abundances of the light
and iron-peak elements generally show the same pattern as other UMP stars (e.g.
overdeficiencies of manganese and chromium, overabundances of cobalt), but the
differential analysis indicates several nucleosynthesis signatures that are
unique to each star.Comment: To Appear in the Astrophysical Journa
C and N Abundances in Stars At the Base of the Red Giant Branch in M5
We present an analysis of a large sample of moderate resolution Keck LRIS
spectra of subgiant (V \sim 17.2) and fainter stars in the Galactic globular
cluster M5 (NGC 5904) with the goal of deriving C and N abundances.
Star-to-star stochastic variations with significant range in both [C/Fe] and
[N/Fe] are found at all luminosities extending to the bottom of the RGB at M_V
\sim +3. Similar variations in CH appear to be present in the main sequence
turnoff spectra. There is no sign of a change in the behavior of C and N with
evolutionary stage over the full range in luminosity of the RGB and SGB. The C
and N abundances appear strongly anti-correlated, as would be expected from the
CN-cycle processing of stellar material. Yet the present stars are considerably
fainter than the RGB bump, the point at which deep mixing is believed to set
in. On this basis, while the observed abundance pattern is consistent with
proton capture nucleosynthesis, we infer that the site of the reactions is
likely not within the present sample, but rather in a population of more
massive (2 -- 5 M(Sun)) now defunct stars. The range of variation of the N
abundances is very large and the sum of C+N increases as C decreases. To
reproduce this requires the incorporation not only of CN but also of
ON-processed material. Furthermore, the existence of this correlation is quite
difficult to reproduce with an external mechanism such as ``pollution'' with
material processed in a more massive AGB star, which mechanism is fundamentally
stochastic in nature. We therefore suggest that although the internal mixing
hypothesis has serious flaws,new theoretical insights are needed and it should
not be ruled out yet. (abridged)Comment: Slightly updated version to conform to that accepted by the A
Improving the Precision of Abstract Interpretation Based Cache Persistence Analysis
When designing hard real-time embedded systems, it is required to estimate the worst-case execution time (WCET) of each task for schedulability analysis. Precise cache persistence analysis can significantly tighten the WCET estimation, especially when the program has many loops. Methods for persistence analysis should safely and precisely classify memory references as persistent. Ex-isting safe approaches suffer from multiple sources of pessimism and may not provide precise results. In this paper, we first identify some sources of pessimism that two recent approaches based on younger set and may analysis may encounter. Then, we propose two methods to eliminate these sources of pessimism. The first method improves the update function of the may analysis-based approach; and the second method integrates the younger set-based and may analysis-based approaches together to further reduce pes-simism. We also prove the two proposed methods are still safe. We evaluate the approaches on a set of benchmarks and observe the number of memory references classified as persistent is increased by the proposed methods. Moreover, we empirically compare the storage space and analysis time used by different methods
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