1,113 research outputs found
Superbubbles
Individual massive stars with M sub bol -6 have huge stellar winds that create interstellar bubbles. Stars with masses greater than 8 solar mass are considered supernova progenitors. These massive stars are numerous in OB associations where few supernova remnants are detected. Model calculations describing the evolution of an association show: that large, hot cavities are formed by pushing the ambient gas into neutral shells; that the shell radii change with galactocentric radius; that only thirty percent of the interstellar medium is in the form of supercavities; and that a consequence is that only a small fraction of supernovae form supernova remnants
Revisited abundance diagnostics in quasars: Fe II/Mg II ratios
Both the Fe II UV emission in the 2000- 3000 A region [Fe II (UV)] and
resonance emission line complex of Mg II at 2800 A are prominent features in
quasar spectra. The observed Fe II UV/ Mg II emission ratios have been proposed
as means to measure the buildup of the Fe abundance relative to that of the
alpha-elements C, N, O, Ne and Mg as a function of redshift. The current
observed ratios show large scatter and no obvious dependence on redshift. Thus,
it remains unresolved whether a dependence on redshift exists and whether the
observed Fe II UV/ Mg II ratios represent a real nucleosynthesis diagnostic. We
have used our new 830-level model atom for Fe+ in photoionization calculations,
reproducing the physical conditions in the broad line regions of quasars. This
modeling reveals that interpretations of high values of Fe II UV/ Mg II are
sensitive not only to Fe and Mg abundance, but also to other factors such as
microturbulence, density, and properties of the radiation field. We find that
the Fe II UV/ Mg II ratio combined with Fe II (UV)/ Fe II (Optical) emission
ratio, where Fe II (Optical) denotes Fe II emission in 4000 - 6000 A can be
used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios
for the physical conditions relevant to the broad-line regions (BLRs) of
quasars. This has extreme importance for quasar observations with the Hubble
Space Telescope and also with the future James Webb Space Telescope.Comment: kverner.gzip, 9 pages, f1-5.eps; aastex.cls; aastexug.sty, ApJL in
pres
Low temperature properties of the infinite-dimensional attractive Hubbard model
We investigate the attractive Hubbard model in infinite spatial dimensions by
combining dynamical mean-field theory with a strong-coupling continuous-time
quantum Monte Carlo method. By calculating the superfluid order parameter and
the density of states, we discuss the stability of the superfluid state. In the
intermediate coupling region above the critical temperature, the density of
states exhibits a heavy fermion behavior with a quasi-particle peak in the
dense system, while a dip structure appears in the dilute system. The formation
of the superfluid gap is also addressed.Comment: 8 pages, 9 figure
SIMPLE: Stable Increased-throughput Multi-hop Protocol for Link Efficiency in Wireless Body Area Networks
In this work, we propose a reliable, power efficient and high throughput
routing protocol for Wireless Body Area Networks (WBANs). We use multi-hop
topology to achieve minimum energy consumption and longer network lifetime. We
propose a cost function to select parent node or forwarder. Proposed cost
function selects a parent node which has high residual energy and minimum
distance to sink. Residual energy parameter balances the energy consumption
among the sensor nodes while distance parameter ensures successful packet
delivery to sink. Simulation results show that our proposed protocol maximize
the network stability period and nodes stay alive for longer period. Longer
stability period contributes high packet delivery to sink which is major
interest for continuous patient monitoring.Comment: IEEE 8th International Conference on Broadband and Wireless
Computing, Communication and Applications (BWCCA'13), Compiegne, Franc
Thermodynamics of the 3D Hubbard model on approach to the Neel transition
We study the thermodynamic properties of the 3D Hubbard model for
temperatures down to the Neel temperature using cluster dynamical mean-field
theory. In particular we calculate the energy, entropy, density, double
occupancy and nearest-neighbor spin correlations as a function of chemical
potential, temperature and repulsion strength. To make contact with cold-gas
experiments, we also compute properties of the system subject to an external
trap in the local density approximation. We find that an entropy per particle
at is sufficient to achieve a Neel state in the
center of the trap, substantially higher than the entropy required in a
homogeneous system. Precursors to antiferromagnetism can clearly be observed in
nearest-neighbor spin correlators.Comment: 4 pages, 6 figure
Time variations of the narrow FeII and HI spectral emission lines from the close vicinity of Eta Carinae during the spectral event of 2003
The spectrum of Eta Carinae and its ejecta shows slow variations over a
period of 5.5 years. However, the spectrum changes drastically on a time scale
of days once every period called the 'spectral event'. We report on variations
in the narrow emission line spectrum of gas condensations (the Weigelt blobs)
close to the central star during a spectral event. The rapid changes in the
stellar radiation field illuminating the blobs make the blobs a natural
astrophysical laboratory to study atomic photoprocesses. The different
responses of the HI Paschen lines, fluorescent lines and forbidden
[FeII] lines allow us to identify the processes and estimate physical
conditions in the blobs. This paper is based on observations from the Pico dos
Dias Observatory (LNA/Brazil) during the previous event in June 2003.Comment: Accepted for publication in A&
Ultraviolet spectroscopy of the brightest supergiants in M31 and M33
Ultraviolet spectroscopy from the IUE, in combination with groundbased visual and infrared photometry, are to determine the energy distributions of the luminous blue variables, the Hubble-Sandage variables, in M31 and M33. The observed energy distributions, especially in the ultraviolet, show that these stars are suffering interstellar reddening. When corrected for interstellar extinction, the integrated energy distributions yield the total luminosities and black body temperatures of the stars. The resulting bolometric magnitudes and temperatures confirm that these peculiar stars are indeed very luminous, hot stars. They occupy the same regions of the sub B01 vs. log T sub e diagram as do eta Car, P Cyg and S Dor in our galaxy and the LMC. Many of the Hubble-Sandage variables have excess infrared radiation which is attributed to free-free emission from their extended atmospheres. Rough mass loss estimates from the infrared excess yield rates of 0.00001 M sub annual/yr. The ultraviolet spectra of the H-S variables are also compared with similar spectra of eta Car, P Cyg and S For
Discovery of two distorted interstellar bubbles
During an extensive program of direct imagery of emission nebulae, arcuate structures were found around two stars. A well-defined shock-like structure was found about the T-Orionis variable LL Orionis, located to the side of the Orion Nebula. A less extensive shock-like structure was also found about the runaway star zeta Ophiuchus. These structures can best be described in terms of distorted interstellar bubbles. A direct consequence of this interpretation is an independent estimate of the rates of mass loss for these stars
Archaeal phylogenomics provides evidence in support of a methanogenic origin of the Archaea and a thaumarchaeal origin for the eukaryotes
We have developed a machine-learning approach to identify 3537 discrete orthologue protein sequence groups distributed across all available archaeal genomes. We show that treating these orthologue groups as binary detection/non-detection data is sufficient to capture the majority of archaeal phylogeny. We subsequently use the sequence data from these groups to infer a method and substitution-model-independent phylogeny. By holding this phylogeny constrained and interrogating the intersection of this large dataset with both the Eukarya and the Bacteria using Bayesian and maximum-likelihood approaches, we propose and provide evidence for a methanogenic origin of the Archaea. By the same criteria, we also provide evidence in support of an origin for Eukarya either within or as sisters to the Thaumarchaea
Diagrammatic Monte Carlo for Correlated Fermions
We show that Monte Carlo sampling of the Feynman diagrammatic series (DiagMC)
can be used for tackling hard fermionic quantum many-body problems in the
thermodynamic limit by presenting accurate results for the repulsive Hubbard
model in the correlated Fermi liquid regime. Sampling Feynman's diagrammatic
series for the single-particle self-energy we can study moderate values of the
on-site repulsion () and temperatures down to . We
compare our results with high temperature series expansion and with single-site
and cluster dynamical mean-field theory.Comment: 4 pages, 5 figures, stylistic change
- …