106 research outputs found
Ingestive behaviour and physiology of the medicinal leech
Ingestion lasts 25 min in Hirudo medicinalis and is characterized by pharyngeal peristalsis which fills the crop. This peristalsis has an initial rate of 2.4 Hz which decays smoothly to 1.2 Hz at termination of ingestion. During ingestion, the leech body wall undergoes peristalsis which appears to aid in filling the crop diverticula. Body peristalsis begins at a rate of 10 min^(-1) and decreases linearly to 2 min^(-1) at termination. The body also undergoes dorsoventral flexions when blood flow is occluded. Blood meal size increases slightly with leech size: 8.4 g for 1-g leeches and 9.7 g for 2-g leeches. However, relative meal size decreases markedly with increasing animal size; from 8.15 times body mass for 1-g to 4.80 times for 2-g leeches. When intact leeches were exposed to micromolar concentrations of serotonin, there was an increase in the rate of pharyngeal peristalsis and the size of the blood meals. Leeches excrete the plasma from their ingested blood meals. Excretion is activated during ingestion, which increases feeding efficiency by increasing the proportion of blood cells in the ingestate. Excretion continues for 4–6 days following ingestion, removing all the remaining plasma from the ingestate. Leech ingestion comprises stereotyped muscular movements, secretion of saliva and excretion of plasma. A strikingly similar feeding physiology is seen in the blood-sucking insect Rhodnius, and we suggest that efficient sanguivory may require the convergent evolution of similar ingestive mechanisms
Vacuum polarization tensor in inhomogeneous magnetic fields
We develop worldline numerical methods, which combine string-inspired with
Monte-Carlo techniques, for the computation of the vacuum polarization tensor
in inhomogeneous background fields for scalar QED. The algorithm satisfies the
Ward identity exactly and operates on the level of renormalized quantities. We
use the algorithm to study for the first time light propagation in a spatially
varying magnetic field. Whereas a local derivative expansion applies to the
limit of small variations compared to the Compton wavelength, the case of a
strongly varying field can be approximated by a derivative expansion for the
averaged field. For rapidly varying fields, the vacuum-magnetic refractive
indices can exhibit a non-monotonic dependence on the local field strength.
This behavior can provide a natural limit on the self-focussing property of the
quantum vacuum.Comment: 12 pages, 9 figure
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
Non-perturbative quenched propagator beyond the infrared approximation
A new approach to the quenched propagator in QED beyond the IR limit is
proposed. The method is based on evolution equations in the proper time.Comment: 13 pages, 1 figure; Misprint on reference correcte
QED in external fields, a functional point of view
A functional partial differential equation is set for the proper graphs
generating functional of QED in external electromagnetic fields. This equation
leads to the evolution of the proper graphs with the external field amplitude
and the external field gauge dependence of the complete fermion propagator and
vertex is derived non-perturbativally.Comment: 8 pages, published versio
Testing Rotational Mixing Predictions with New Boron Abundances in Main Sequence B-type Stars
(Abridged) New boron abundances for seven main-sequence B-type stars are
determined from HST STIS spectroscopy around the BIII 2066A line. Boron
abundances provide a unique and critical test of stellar evolution models that
include rotational mixing since boron is destroyed in the surface layers of
stars through shallow mixing long before other elements are mixed from the
stellar interior through deep mixing. Boron abundances range from 12+log(B/H) =
1.0 to 2.2. The boron abundances are compared to the published values of their
stellar nitrogen abundances (all have 12+log(N/H) < 7.8, i.e., they do not show
significant CNO-mixing) and to their host cluster ages (4 to 16 Myr) to
investigate the predictions from models of massive star evolution with
rotational mixing effects (Heger & Langer 2000). Only three stars (out of 34)
deviate from the model predictions, including HD36591, HD205021, and HD30836.
These three stars suggest that rotational mixing could be more efficient than
currently modelled at the highest rotation rates.Comment: 10 figures, 7 tables; accepted for publication in the Astrophysical
Journa
Counting Complex Disordered States by Efficient Pattern Matching: Chromatic Polynomials and Potts Partition Functions
Counting problems, determining the number of possible states of a large
system under certain constraints, play an important role in many areas of
science. They naturally arise for complex disordered systems in physics and
chemistry, in mathematical graph theory, and in computer science. Counting
problems, however, are among the hardest problems to access computationally.
Here, we suggest a novel method to access a benchmark counting problem, finding
chromatic polynomials of graphs. We develop a vertex-oriented symbolic pattern
matching algorithm that exploits the equivalence between the chromatic
polynomial and the zero-temperature partition function of the Potts
antiferromagnet on the same graph. Implementing this bottom-up algorithm using
appropriate computer algebra, the new method outperforms standard top-down
methods by several orders of magnitude, already for moderately sized graphs. As
a first application, we compute chromatic polynomials of samples of the simple
cubic lattice, for the first time computationally accessing three-dimensional
lattices of physical relevance. The method offers straightforward
generalizations to several other counting problems.Comment: 7 pages, 4 figure
The Two-Loop Euler-Heisenberg Lagrangian in Dimensional Renormalization
We clarify a discrepancy between two previous calculations of the two-loop
QED Euler-Heisenberg Lagrangian, both performed in proper-time regularization,
by calculating this quantity in dimensional regularization.Comment: 12 pages, standard Latex, no figures, uses a4wide.st
Index-free Heat Kernel Coefficients
Using index-free notation, we present the diagonal values of the first five
heat kernel coefficients associated with a general Laplace-type operator on a
compact Riemannian space without boundary. The fifth coefficient appears here
for the first time. For a flat space with a gauge connection, the sixth
coefficient is given too. Also provided are the leading terms for any
coefficient, both in ascending and descending powers of the Yang-Mills and
Riemann curvatures, to the same order as required for the fourth coefficient.
These results are obtained by directly solving the relevant recursion
relations, working in Fock-Schwinger gauge and Riemann normal coordinates. Our
procedure is thus noncovariant, but we show that for any coefficient the
`gauged' respectively `curved' version is found from the corresponding
`non-gauged' respectively `flat' coefficient by making some simple covariant
substitutions. These substitutions being understood, the coefficients retain
their `flat' form and size. In this sense the fifth and sixth coefficient have
only 26 and 75 terms respectively, allowing us to write them down. Using
index-free notation also clarifies the general structure of the heat kernel
coefficients. In particular, in flat space we find that from the fifth
coefficient onward, certain scalars are absent. This may be relevant for the
anomalies of quantum field theories in ten or more dimensions.Comment: 38 pages, LaTe
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