17,089 research outputs found
Ultraviolet variability of quasars: dependence on the accretion rate
We compiled a catalogue of about 4000 SDSS quasars including individual
estimators V for the variability strength, virial black hole masses M, and mass
accretion rates dM/dt from the Davis-Laor scaling relation. We confirm
significant anti-correlations between V and dM/dt, the Eddington ratio, and the
bolometric luminosity L, respectively. A weak, statistically not significant
positive trend is indicated for the dependence of V on M. As a side product, we
find a strong correlation of the radiative efficiency with M and show that this
trend is most likely produced by selection effects in combination with the mass
errors and the use of the scaling relation for dM/dt. The anti-correlations
found for V cannot be explained in such a way. The strongest anti-correlation
is found with dM/dt. However, it is difficult to decide which of the quantities
(L, Eddington ratio, dM/dt) is intrinsically correlated with V and which of the
observed correlations are produced by the relations between these quantities. A
V-dM/dt anti-correlation is qualitatively expected for the strongly
inhomogeneous accretion disks. We argue that several observed variability
properties are not adequately explained by the simple multi-temperature
black-body model of a standard disk and suggest to check whether the strongly
inhomogeneous disk model is capable of reproducing these observations better.Comment: 16 pages, 10 figures; accepted for publication in Astronomy and
Astrophysics; the full catalogue is only available in electronic form at CD
Mercury in the environs of the north slope of Alaska
The analysis of Greenland ice suggests that the flux of mercury from the continents
to the atmosphere has increased in recent times, perhaps partly as a result of the many of
man’s activities that effect an alteration of terrestrial surfaces. Upon the exposure of fresh
crustal matter, the natural outgassing of mercury vapor from the earth’s surface could be
enhanced.
Accordingly, mercury was measured in a variety of environmental materials gathered
from the North Slope of Alaska to provide background data prior to the anticipated increase
of activity in this environment. The materials were collected during the U. S. Coast Guard
WEBSEC 72-73 cruises as well as through the facilities provided by Naval Arctic Research
Laboratory in the spring of 1973.
The method of measurement depended upon radioactivation of mercury with neutrons
and the subsequent quantification of characteristic gamma radiations after radiochemical
purification.
Mercury concentrations in seawater at several locations in the vicinity of 151°W,
71°N averaged 20 parts per trillion. The waters from all stations east of this location showed
a significantly smaller concentration. This difference may relate to penetration o f Bering-
Chukchi Sea water into the southern Beaufort Sea to 151°W. Marine sediments on the shelf
and slope between 143°W and 153°W contained about 100 parts per billion mercury, except
for those on the continental shelf between Barter Island and the Canning River, where the
concentration was less than half this value. These results are consistent with sediment input
from the respective rivers when their mercury content and mineralogy are considered. The
mercury content of river waters was 18 ppt and in reasonable agreement with the average of
snow samples (13 ppt). The burden of mercury in plankton was 37 ppb.This work was supported by the office of Naval Research under grant N R 083-290
CLONING OF THE 1.4-kb mRNA SPECIES OF HUMAN COMPLEMENT FACTOR H REVEALS A NOVEL MEMBER OF THE SHORT CONSENSUS REPEAT FAMILY RELATED TO THE CARBOXY TERMINAL OF THE CLASSICAL 150-kDa MOLECULE
Three factor H mRNA species of 4.3 kb, 1.8 kb,
and 1.4 kb are constitutively expressed in human
liver. Having previously characterized full-length
cDNA clones derived from the 4.3-kb and 1.8-kb
factor mRNA, we report here the isolation and eucaryotic
expression of full-length cDNA clones coding
for the 1.4-kbm RNA species. The 1266-bp cDNA
codes for a polypeptide of 330 amino acids and
contains two polyadenylation signals and a short
poly(A)+tailT. he protein is composed of a leader
peptide followed by five short consensus repeat domains.
It shows a hybrid structure with the last
three domains being almost identical to the carboxy-
terminal of thcel assical 1 BO-kDa factor H molecule
and the two first domains representing unique
short consensus repeat structures. Eucaryotic
expression in COS7 cells revealed two polypeptides
derived from one cDNA clone that area lso found in
human serum. Differences between the cDcNloAn es
within the last three domains indicate two distinct,
possibly allelic sequences that, in addition, differ
from the authentic 150-kDa factor H sequence.
Southern blot results support the notion that the
4.3-kb factor H and the 1.4-kb factor H-related
mRNA are transcribed from two separate but highly
homologous genes.
Factor H, a glycoprotein of 150,00
Genetic and serological heterogeneity of the supertypic HLA-B locus specificities Bw4 and Bw6
Gene cloning and sequencing of the HLA-B
locus split antigens B38 (B16.1) and B39 (B16.2) allowed
localization of their subtypic as well as their public
specificities HLA-Bw4 or -Bw6 to the c~-helical region of
the c~ 1 domain flanked by the amino acid positions 74-83.
Comparison of their amino acid sequences with those of
other HLA-B-locus alleles established HLA-Bw6 to be
distinguished by Ser at residue 77 and Asn at residue 80.
In contrast, HLA-Bw4 is characterized by at least seven
different patterns of amino acid exchanges at positions 77
and 80-83. Reactivity patterns of Bw4- or Bw6-specific
monoclonal antibodies reveal two alloantigenic epitopes
contributing to the HLA-Bw4 or -Bw6 specificity residing
next to the region of highest diversity of the cr domain
On the helium flash in low-mass Population III Red Giant stars
We investigate the evolution of initially metal-free, low-mass Red Giant
stars through the He core flash at the tip of the Red Giant Branch. The low
entropy barrier between the helium- and hydrogen-rich layers enables a
penetration of the helium flash driven convective zone into the inner tail of
the extinguishing H-burning shell. As a consequence, protons are mixed into
high-temperature regions triggering a H-burning runaway. The subsequent
dredge-up of matter processed by He and H burning enriches the stellar surface
with large amounts of helium, carbon and nitrogen. Extending previous results
by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the
H-burning runaway is an intrinsic property of extremely metal-poor low-mass
stars, we found that its occurrence depends on additional parameters like the
initial composition and the treatment of various physical processes.
We perform some comparisons between predicted surface chemical abundances and
observational measurements for extremely metal-deficient stars. As in previous
investigations, our results disclose that although the described scenario
provides a good qualitative agreement with observations, considerable
discrepancies still remain. They may be due to a more complex evolutionary path
of `real' stars, and/or some shortcomings in current evolutionary models.
In addition, we analyze the evolutionary properties after the He core flash,
during both the central and shell He-burning phases, allowing us to deduce some
interesting differences between models whose Red Giant Branch progenitor has
experienced the H-flash and canonical models. In particular, the Asymptotic
Giant Branch evolution of extremely metal-deficient stars and the occurrence of
thermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap
On apparent breaking the second law of thermodynamics in quantum transport studies
We consider a model for stationary electronic transport through a
one-dimensional chain of two leads attached to a perturbed central region
(quantum dot) in the regime where the theory proposed recently by Capek for a
similar model of phonon transport predicts the striking phenomenon of a
permanent current between the leads. This result based on a rigorous but
asymptotic Davies theory is at variance with the zero current yielded by direct
transport calculations which can be carried out in the present model. We find
the permanent current to be within the error of the asymptotic expansion for
finite couplings, and identify cancelling terms of the same order.Comment: 5 pages, 3 figure
Observation of directly interacting coherent two-level systems in a solid
Parasitic two-level tunneling systems originating from structural material
defects affect the functionality of various microfabricated devices by acting
as a source of noise. In particular, superconducting quantum bits may be
sensitive to even single defects when these reside in the tunnel barrier of the
qubit's Josephson junctions, and this can be exploited to observe and
manipulate the quantum states of individual tunneling systems.
Here, we detect and fully characterize a system of two strongly interacting
defects using a novel technique for high-resolution spectroscopy. Mutual defect
coupling has been conjectured to explain various anomalies of glasses, and was
recently suggested as the origin of low frequency noise in superconducting
devices. Our study provides conclusive evidence of defect interactions with
full access to the individual constituents, demonstrating the potential of
superconducting qubits for studying material defects. All our observations are
consistent with the assumption that defects are generated by atomic tunneling.Comment: 13 pages, 7 figures. Includes supplementary materia
A many-fermion generalization of the Caldeira-Leggett model
We analyze a model system of fermions in a harmonic oscillator potential
under the influence of a dissipative environment: The fermions are subject to a
fluctuating force deriving from a bath of harmonic oscillators. This represents
an extension of the well-known Caldeira-Leggett model to the case of many
fermions. Using the method of bosonization, we calculate one- and two-particle
Green's functions of the fermions. We discuss the relaxation of a single extra
particle added above the Fermi sea, considering also dephasing of a particle
added in a coherent superposition of states. The consequences of the separation
of center-of-mass and relative motion, the Pauli principle, and the
bath-induced effective interaction are discussed. Finally, we extend our
analysis to a more generic coupling between system and bath, that results in
complete thermalization of the system.Comment: v3: fixed pdf problem; v2: added exact formula (Eq. 42) for Green's
function and discussion of equilibrium density matrix (new Fig. 2); 10
figures, 21 pages, see quant-ph/0305098 for brief version of some of these
result
Influence of Phosphorus and Sulphur Segregation on Stress Relief Cracking
Three commercial weldable fine grained structural steels and four experimental melts with lowered contents of trace elements were subjected to a welding simulation treatment followed by stress relaxation (SR) tests. After testing times of up to 8 hours the samples were removed from the testing rig, broken in the fracture stage of an Auger instrument, and the fracture surface was examined in situ for segregation of elements. SEM investigations of the fracture surfaces and light microscope served to characterize fracture mode and microstructure.
The microfractural appearance of the grain boundaries exposed by the impact loading in the Auger instrument and of the ones separated by SR-testing was significantly different. While the fracture surfaces originating from SR-testing were flat, the samples subsequently broken in the Auger instrument showed a dimpled structure.
It could be shown that cracks always started at MnS - precipitates, and that the intergranular crack propagation was enhanced by the segregation of phosphorus. The segregation of elementary sulphur was initiated by the stress field of the cracks already formed and, contrary to the phosphorus enrichment, could be prevented by lowering the S-content in the melt. The other trace elements seemed to play no part in the stress relief cracking of the steels investigated
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