6,617 research outputs found
A 100 pc Elliptical and Twisted Ring of Cold and Dense Molecular Clouds Revealed by Herschel Around the Galactic Center
Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on board the Herschel satellite, reveal a ~3 Ă 10^7 M_â ring of dense and cold clouds orbiting the Galactic center. Using a simple toy model, an elliptical shape having semi-major axes of 100 and 60 pc is deduced. The major axis of this 100 pc ring is inclined by about 40° with respect to the plane of the sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100 pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr Aâ is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data
Probing halo nucleus structure through intermediate energy elastic scattering
This work addresses the question of precisely what features of few body
models of halo nuclei are probed by elastic scattering on protons at high
centre-of-mass energies. Our treatment is based on a multiple scattering
expansion of the proton-projectile transition amplitude in a form which is well
adapted to the weakly bound cluster picture of halo nuclei. In the specific
case of Li scattering from protons at 800 MeV/u we show that because
core recoil effects are significant, scattering crosssections can not, in
general, be deduced from knowledge of the total matter density alone.
We advocate that the optical potential concept for the scattering of halo
nuclei on protons should be avoided and that the multiple scattering series for
the full transition amplitude should be used instead.Comment: 8 pages REVTeX, 1 eps figure, accepted for publication in Phys. Rev.
Solitary-wave vortices in quadratic nonlinear media
We find families of vortex solitary waves in bulk quadratic nonlinear media under conditions for second-harmonic generation. We show that the vortex solitary waves are azimuthally unstable and that they decay into sets of stable spatial solitons. We calculate the growth rates of the azimuthal perturbations and show how those affect the pattern of output light.
© 1998 Optical Society of AmericaPeer ReviewedPostprint (published version
Multiple scattering effects in quasi free scattering from halo nuclei: a test to Distorted Wave Impulse Approximation
Full Faddeev-type calculations are performed for Be breakup on proton
target at 38.4, 100, and 200 MeV/u incident energies. The convergence of the
multiple scattering expansion is investigated. The results are compared with
those of other frameworks like Distorted Wave Impulse Approximation that are
based on an incomplete and truncated multiple scattering expansion.Comment: 7 pages, 16 figures, to be published in Phys. Rev.
Detection and characterization of a 500 ÎŒm dust emissivity excess in the Galactic plane using Herschel/Hi-GAL observations
Context. Past and recent observations have revealed unexpected variations in the far-infrared â millimeter (FIR-mm) dust emissivity in the interstellar medium. In the Herschel spectral range, those are often referred to as a 500 ÎŒm emission excess. Several dust emission models have been developed to interpret astrophysical data in the FIR-mm domain. However, these are commonly unable to fully reconcile theoretical predictions with observations. In contrast, the recently revised two level system (TLS) model, based on the disordered internal structure of amorphous dust grains, seems to provide a promising way of interpreting existing data.
Aims. The newly available Herschel infrared GALactic (Hi-GAL) data, which covers most of the inner Milky Way, offers a unique opportunity to investigate possible variations in the dust emission properties both with wavelength and environment. The goal of our analysis is to constrain the internal structure of the largest dust grains on Galactic scales, in the framework of the TLS model.
Methods. By combining the IRIS (Improved Reprocessing of the IRAS Survey) 100 ÎŒm with the Hi-GAL 160, 250, 350, and 500 ÎŒm data, we model the dust emission spectra in each pixel of the Hi-GAL maps, using both the TLS model and, for comparison, a single modified black-body fit. The effect of temperature mixing along the line of sight is investigated to test the robustness of our results.
Results. We find a slight decrease in the dust temperature with distance from the Galactic center, confirming previous results. We also report the detection of a significant 500 ÎŒm emissivity excess in the peripheral regions of the plane (35° < |l| < 70°) of about 13â15% of the emissivity, which can reach up to 20% in some HII regions. We present the spatial distributions of the best-fit values for the two main parameters of the TLS model, i.e. the charge correlation length, lc, used to characterize the disordered charge distribution (DCD) part of the model, and the amplitude A of the TLS processes with respect to the DCD effect. These distributions illustrate the variations in the dust properties with environment, in particular the plausible existence of an overall gradient with distance to the Galactic center. A comparison with previous findings in the solar neighborhood shows that the local value of the excess is less than expected from the Galactic gradient observed here
Variational approach for walking solitons in birefringent fibres
We use the variational method to obtain approximate analytical expressions
for the stationary pulselike solutions in birefringent fibers when differences
in both phase velocities and group velocities between the two components and
rapidly oscillating terms are taken into account. After checking the validity
of the approximation we study how the soliton pulse shape depends on its
velocity and nonlinear propagation constant. By numerically solving the
propagation equation we have found that most of these stationary solutions are
stable.Comment: LaTeX2e, uses graphicx package, 23 pages with 8 figure
Star Formation and Feedback in Dwarf Galaxies
We examine the star formation history and stellar feedback effects of dwarf
galaxies under the influence of extragalactic ultraviolet radiation. We
consider the dynamical evolution of gas in dwarf galaxies using a
one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute
the effects of radiative transfer and photoionization. We include a
physically-motivated star formation recipe and consider the effects of
feedback. Our results indicate that star formation in the severe environment of
dwarf galaxies is a difficult and inefficient process. For intermediate mass
systems, such as the dSphs around the Galaxy, star formation can proceed with
in early cosmic epochs despite the intense background UV flux. Triggering
processes such as merger events, collisions, and tidal disturbance can lead to
density enhancements, reducing the recombination timescale, allowing gas to
cool and star formation to proceed. However, the star formation and gas
retention efficiency may vary widely in galaxies with similar dark matter
potentials, because they depend on many factors, such as the baryonic fraction,
external perturbation, IMF, and background UV intensity. We suggest that the
presence of very old stars in these dwarf galaxies indicates that their initial
baryonic to dark matter content was comparable to the cosmic value. This
constraint suggests that the initial density fluctuation of baryonic matter may
be correlated with that of the dark matter. For the more massive dwarf
elliptical galaxies, the star formation efficiency and gas retention rate is
much higher. Their mass to light ratio is regulated by star formation feedback,
and is expected to be nearly independent of their absolute luminosity. The
results of our theoretical models reproduce the observed correlation.Comment: 35 pages, 13 figure
Is the Cepheus E Outflow driven by a Class 0 Protostar?
New early release observations of the Cepheus E outflow and its embedded
source, obtained with the Spitzer Space Telescope, are presented. We show the
driving source is detected in all 4 IRAC bands, which suggests that traditional
Class 0 classification, although essentially correct, needs to accommodate the
new high sensitivity infrared arrays and their ability to detected deeply
embedded sources. The IRAC, MIPS 24 and 70 microns new photometric points are
consistent with a spectral energy distribution dominated by a cold, dense
envelope surrounding the protostar. The Cep E outflow, unlike its more famous
cousin the HH 46/47 outflow, displays a very similar morphology in the near and
mid-infrared wavelengths, and is detected at 24 microns. The interface between
the dense molecular gas (where Cep E lies) and less dense interstellar medium,
is well traced by the emission at 8 and 24 microns, and is one of the most
exotic features of the new IRAC and MIPS images. IRS observations of the North
lobe of the flow confirm that most of the emission is due to the excitation of
pure H2 rotational transitions arising from a relatively cold (Tex~700 K) and
dense (N{H}~9.6E20 cm-2 molecular gas.Comment: 14 pages (pre-print format), including 6 figures. Published in ApJ
Special Spitzer Issue (2004
Recommended from our members
Dewey Hodgesâs Research in Structural Dynamics, Aeroelasticity, and Composites: A Personal Perspective
The contributions of Dewey Hodges within the specialized areas of structural dynamics, aeroelasticity, and composites are highlighted in this paper. Dewey Hodges has published 215 journal papers and 170 conference papers covering a wide range of topics in aerospace structures. His research and academic career span nearly five decades and it is difficult, if not impossible, to give a detailed commentary on his colossal achievement in the confines of a single paper. It is widely acknowledged that Dewey Hodgesâs research record is no less than incredible. He has not only given an exceptional account of himself but has also demonstrated his extraordinary versatility in research. The author of this paper has known him both personally and professionally for nearly three decades and, with great humility, he acknowledges the enormous benefit he has received from his association with him. Given the huge volume of work Dewey Hodges has produced and the enormity of the task of commenting on it, the author has understandably been highly selective in choosing which contributions to discuss
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