6,146 research outputs found
Observation of coherent electroproduction on deuterons at large momentum transfer
The first experimental results for coherent -electroproduction on the
deuteron, , at large momentum transfer, are reported. The
experiment was performed at Jefferson Laboratory at an incident electron energy
of 4.05 GeV. A large pion production yield has been observed in a kinematical
region for 1.11.8 GeV, from threshold to 200 MeV excitation energy
in the system. The -dependence is compared with theoretical
predictions.Comment: 26 page
Oscillations and instabilities of fast and differentially rotating relativistic stars
We study non-axisymmetric oscillations of rapidly and differentially rotating
relativistic stars in the Cowling approximation. Our equilibrium models are
sequences of relativistic polytropes, where the differential rotation is
described by the relativistic -constant law. We show that a small degree of
differential rotation raises the critical rotation value for which the
quadrupolar f-mode becomes prone to the CFS instability, while the critical
value of at the mass-shedding limit is raised even more. For softer
equations of state these effects are even more pronounced. When increasing
differential rotation further to a high degree, the neutral point of the CFS
instability first reaches a local maximum and is lowered afterwards. For stars
with a rather high compactness we find that for a high degree of differential
rotation the absolute value of the critical is below the corresponding
value for rigid rotation. We conclude that the parameter space where the CFS
instability is able to drive the neutron star unstable is increased for a small
degree of differential rotation and for a large degree at least in stars with a
higher compactness.Comment: 16 pages, 11 figures; paper accepted for publication in Phys. Rev. D
(81.084019
15-Ketodihydro-PGF2α, Progesterone and Cortisol Profiles in Heifers after Induction of Parturition by Injection of Dexamethasone
In order to study rapid changes in 15-ketodihydro-PGF2α, cortisol and progesterone in the period preceding parturition in cattle, pre-term parturition was induced in 4 late pregnant heifers. Parturitions were induced by 2 intramuscular injections of 20 mg dexamethasone with a 24-h interval. The first injection was made on days 254, 258, 264 and 265 in gestation, respectively. Twenty-four h before the first injection an intravenous polyurethane cannula was inserted. Blood samples were collected at least every hour until 12 h after parturition and during the second stage of labour at least 6 times per hour. Plasma was analysed for 15-ketodihydro-PGF2α and progesterone by radioimmunoassays, and for cortisol by an ELISA. The average time from injection to parturition was 7.7 (6.6–8.9) days (mean (range)). Two of the heifers had retained foetal membranes (RFM). At the start of the experiment the levels of PGF2α metabolite were low (< 300 pmol/L) and increased slowly to levels between 1000 and 2000 pmol/L at one day before parturition. During the last day, however, the levels increased rapidly and the highest levels (>10000 pmol/L) were reached at the time of delivery. No pulsatile release was seen. Immediately after foetal expulsion the PG-metabolite levels decreased rapidly in all animals. In the 2 animals with RFM, however, this decline ceased within a few h. The PG-metabolite levels in these animals then started to increase and reached levels as high as during parturition. Luteolysis occurred between 1.6 and 0.4 days before parturition in all animals. The cortisol profile showed a distinct peak at the time of parturition in the RFM heifers. This peak was absent in the non-RFM heifers. This study shows that the PGF2α release at prepartal luteolysis and parturition is not pulsatile in cattle and that cortisol profiles in heifers with retained foetal membranes might differ from the profiles in non-RFM heifers at the time of parturition
Mira's wind explored in scattering infrared CO lines
We have observed the intermediate regions of the circumstellar envelope of
Mira (o Ceti) in photospheric light scattered by three vibration-rotation
transitions of the fundamental band of CO, from low-excited rotational levels
of the ground vibrational state, at an angular distance of beta = 2"-7" away
from the star. The data were obtained with the Phoenix spectrometer mounted on
the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately
0.5" and seeing limited. Our observations provide absolute fluxes, leading to
an independent new estimate of the mass-loss rate of approximately 3e-7
Msun/yr, as derived from a simple analytic wind model. We find that the
scattered intensity from the wind of Mira for 2" < beta < 7" decreases as
beta^-3, which suggests a time constant mass-loss rate, when averaged over 100
years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa
Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
Chemically peculiar stars define a class of stars that show unusual elemental
abundances due to stellar photospheric effects and not due to natal variations.
In this paper, we compare the elemental abundance patterns of the ultra
metal-poor stars with metallicities [Fe/H] to those of a subclass of
chemically peculiar stars. These include post-AGB stars, RV Tauri variable
stars, and the Lambda Bootis stars, which range in mass, age, binarity, and
evolutionary status, yet can have iron abundance determinations as low as
[Fe/H] . These chemical peculiarities are interpreted as due to the
separation of gas and dust beyond the stellar surface, followed by the
accretion of dust depleted-gas. Contrary to this, the elemental abundances in
the ultra metal-poor stars are thought to represent yields of the most
metal-poor supernova and, therefore, observationally constrain the earliest
stages of chemical evolution in the Universe. The abundance of the elements in
the photospheres of the ultra metal-poor stars appear to be related to the
condensation temperature of that element; if so, then their CNO abundances
suggest true metallicities of [X/H]~ -2 to -4, rather than their present
metallicities of [Fe/H] < -5.Comment: Accepted for ApJ. 17 pages, 10 figure
Oscillations of rapidly rotating relativistic stars
Non-axisymmetric oscillations of rapidly rotating relativistic stars are
studied using the Cowling approximation. The oscillation spectra have been
estimated by Fourier transforming the evolution equations describing the
perturbations. This is the first study of its kind and provides information on
the effect of fast rotation on the oscillation spectra while it offers the
possibility in studying the complete problem by including spacetime
perturbations. Our study includes both axisymmetric and non-axisymmetric
perturbations and provides limits for the onset of the secular bar mode
rotational instability. We also present approximate formulae for the dependence
of the oscillation spectrum from rotation. The results suggest that it is
possible to extract the relativistic star's parameters from the observed
gravitational wave spectrum.Comment: this article will be published in Physical Review
Scaling Limits for Internal Aggregation Models with Multiple Sources
We study the scaling limits of three different aggregation models on Z^d:
internal DLA, in which particles perform random walks until reaching an
unoccupied site; the rotor-router model, in which particles perform
deterministic analogues of random walks; and the divisible sandpile, in which
each site distributes its excess mass equally among its neighbors. As the
lattice spacing tends to zero, all three models are found to have the same
scaling limit, which we describe as the solution to a certain PDE free boundary
problem in R^d. In particular, internal DLA has a deterministic scaling limit.
We find that the scaling limits are quadrature domains, which have arisen
independently in many fields such as potential theory and fluid dynamics. Our
results apply both to the case of multiple point sources and to the
Diaconis-Fulton smash sum of domains.Comment: 74 pages, 4 figures, to appear in J. d'Analyse Math. Main changes in
v2: added "least action principle" (Lemma 3.2); small corrections in section
4, and corrected the proof of Lemma 5.3 (Lemma 5.4 in the new version);
expanded section 6.
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