12,753 research outputs found
Fragmentation, infall, and outflow around the showcase massive protostar NGC7538 IRS1 at 500 AU resolution
Aims: Revealing the fragmentation, infall, and outflow processes in the
immediate environment around massive young stellar objects is crucial for
understanding the formation of the most massive stars. Methods: With this goal
in mind we present the so far highest spatial-resolution thermal submm line and
continuum observations toward the young high-mass protostar NGC7538 IRS1. Using
the Plateau de Bure Interferometer in its most extended configuration at 843mum
wavelength, we achieved a spatial resolution of 0.2"x0.17", corresponding to
~500AU at a distance of 2.7\,kpc. Results: For the first time, we have observed
the fragmentation of the dense inner core of this region with at least three
subsources within the inner 3000 AU. The outflow exhibits blue- and red-shifted
emission on both sides of the central source indicating that the current
orientation has to be close to the line-of-sight, which differs from other
recent models. We observe rotational signatures in northeast-southwest
direction; however, even on scales of 500 AU, we do not identify any Keplerian
rotation signatures. This implies that during the early evolutionary stages any
stable Keplerian inner disk has to be very small (<=500 AU). The high-energy
line HCN(4-3)v2=1 (E_u/k=1050K) is detected over an extent of approximately
3000 AU. In addition to this, the detection of red-shifted absorption from this
line toward the central dust continuum peak position allows us to estimate
infall rates of ~1.8x10^(-3)Msun/yr on the smallest spatial scales. Although
all that gas will not necessarily be accreted onto the central protostar,
nevertheless, such inner core infall rates are among the best proxies of the
actual accretion rates one can derive during the early embedded star formation
phase. These data are consistent with collapse simulations and the observed
high multiplicity of massive stars.Comment: Accepted for Astronomy & Astrophysics, 8 pages, also available at
http://www.mpia.de/homes/beuther/papers.htm
The Delta-Hole model at Finite Temperature
The spectral function of pions interacting with a gas of nucleons and
Delta-33-resonances is investigated using the formalism of Thermo Field
Dynamics. After a discussion of the zero Delta-width approximation at finite
temperature, we take into account a constant width of the resonance. Apart from
a full numerical calculation, we give analytical approximations to the pionic
spectral function including such a width. They are found to be different from
previous approximations, and require an increase of the effective Delta-width
in hot compressed nuclear matter. The results are summarized in an effective
dispersion relation for interacting pions.Comment: 34 pages in standard LaTeX GSI-preprint No. GSI-93-2
Diagonalization of full finite temperature Green's function by quasi-particles
For thermal systems, standard perturbation theory breaks down because of the
absence of stable, observable asymptotic states. We show, how the introduction
of {\it statistical} quasi-particles (stable, but not observable) gives rise to
a consistent description. Statistical and spectral information can be cleanly
separated also for interacting systems.Comment: 9 pages in standard LaTe
Carbonates in space - The challenge of low temperature data
Carbonates have repeatedly been discussed as possible carriers of stardust
emission bands. However, the band assignments proposed so far were mainly based
on room temperature powder transmission spectra of the respective minerals.
Since very cold calcite grains have been claimed to be present in protostars
and in Planetary Nebulae such as NGC 6302, the changes of their dielectric
functions at low temperatures are relevant from an astronomical point of view.
We have derived the IR optical constants of calcite and dolomite from
reflectance spectra - measured at 300, 200, 100 and 10K - and calculated small
particle spectra for different grain shapes, with the following results: i) The
absorption efficiency factors both of calcite and dolomite are extremely
dependent on the particle shapes. This is due to the high peak values of the
optical constants of CaCO3 and CaMg[CO3]2. ii) The far infrared properties of
calcite and dolomite depend also very significantly on the temperature. Below
200K, a pronounced sharpening and increase in the band strengths of the FIR
resonances occurs. iii) In view of the intrinsic strength and sharpening of the
44 mum band of calcite at 200-100K, the absence of this band -- inferred from
Infrared Space Observatory data -- in PNe requires dust temperatures below 45K.
iv) Calcite grains at such low temperatures can account for the '92' mum band,
while our data rule out dolomite as the carrier of the 60-65 mum band. The
optical constants here presented are publicly available in the electronic
database http://www.astro.uni-jena.de/Laboratory/OCDBComment: 20 pages, 10 figures, accepted by ApJ, corrected typo
Quasiclassical theory for the superconducting proximity effect in Dirac materials
We derive the quasiclassical non-equilibrium Eilenberger and Usadel equations
to first order in quantities small compared to the Fermi energy, valid for
Dirac edge and surface electrons with spin-momentum locking, as relevant for
topological insulators. We discuss in detail several of the key technical
points and assumptions of the derivation, and provide a Riccati-parametrization
of the equations. Solving first the equilibrium equations for S/N and S/F
bilayers and Josephson junctions, we study the superconducting proximity effect
in Dirac materials. Similarly to related works, we find that the effect of an
exchange field depends strongly on the direction of the field. Only components
normal to the transport direction lead to attenuation of the Cooper pair
wavefunction inside the F. Fields parallel to the transport direction lead to
phase-shifts in the dependence on the superconducting phase difference for both
the charge current and density of states in an S/F/S-junction. Moreover, we
compute the differential conductance in S/N and S/F bilayers with an applied
voltage bias, and determine the dependence on the length of the N and F regions
and the exchange field.Comment: 13 pages, 5 figures. Accepted for publication in Phys. Rev.
Meson and Quark Degrees of Freedom and the Radius of the Deuteron
The existing experimental data for the deuteron charge radius are discussed.
The data of elastic electron scattering are inconsistent with the value
obtained in a recent atomic physics experiment. Theoretical predictions based
on a nonrelativistic description of the deuteron with realistic nucleon-nucleon
potentials and with a rather complete set of meson-exchange contributions to
the charge operator are presented. Corrections arising from the quark-gluon
substructure of the nucleon are explored in a nonrelativistic quark model; the
quark-gluon corrections, not accounted for by meson exchange, are small. Our
prediction for the deuteron charge radius favors the value of a recent atomic
physics experiment.Comment: 20 pages, LaTeX, 4 Postscript figures, to appear in Few-Body-System
TRADE GAINS AND WELFARE COSTS OF INCOME STABILIZATION PROGRAMS FOR HOG PRODUCERS IN QUEBEC
The welfare costs of deficiency payments for an exported commodity may, under certain conditions, outweigh the gains from trade. The potential welfare impacts of stabilization programs in the hog sector in Quebec are estimated, based on a partial equilibrium framework, and elasticity estimates drawn from other sources. The results indicate that the loss in surplus in Quebec as a result of deficiency payments is very modest at approximately 14 million.Livestock Production/Industries,
Shear banding in nematogenic fluids with oscillating orientational dynamics
We investigate the occurrence of shear banding in nematogenic fluids under
planar Couette flow, based on mesoscopic dynamical equations for the
orientational order parameter and the shear stress. We focus on parameter
values where the sheared homogeneous system exhibits regular oscillatory
orientational dynamics, whereas the equilibrium system is either isotropic
(albeit close to the isotropic--nematic transition) or deep in its nematic
phase. The numerical calculations are restricted to spatial variations in shear
gradient direction. We find several new types of shear banded states
characterized by regions with regular oscillatory orientational dynamics. In
all cases shear banding is accompanied by a non--monotonicity of the flow curve
of the homogeneous system; however, only in the case of the initially isotropic
system this curve has the typical --like shape. We also analyze the
influence of different orientational boundary conditions and of the spatial
correlation length.Comment: 12 pages, 10 figure
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