12,196 research outputs found
1420 MHz Continuum Absorption Towards Extragalactic Sources in the Galactic Plane
We present a 21-cm emission-absorption study towards extragalactic sources in
the Canadian Galactic Plane Survey (CGPS). We have analyzed HI spectra towards
437 sources with S > 150 mJy, giving us a source density of 0.6 sources per
square degree at arcminute resolution. We present the results of a first
analysis of the HI temperatures, densities, and feature statistics. Particular
emphasis is placed on 5 features with observed spin temperatures below 40 K. We
find most spin temperatures in the range from 40 K to 300 K. A simple HI
two-component model constrains the bulk of the cold component to temperatures
(T_c) between 40 K and 100 K. T_c peaks in the Perseus arm region and clearly
drops off with Galactocentric radius, R, beyond that. The HI density follows
this trend, ranging from a local value of 0.4 cm^{-3} to less than 0.1 cm^{-3}
at R = 20 kpc. We find that HI emission alone on average traces about 75% of
the total HI column density, as compared to the total inferred by the emission
and absorption. Comparing the neutral hydrogen absorption to CO emission no
correlation is found in general, but all strong CO emission is accompanied by a
visible HI spectral feature. Finally, the number of spectral HI absorption
features per kpc drop off exponentially with increasing R.Comment: 13 pages, 13 figures, Accepted for March 2004 Ap
Vibrational instability, two-level systems and Boson peak in glasses
We show that the same physical mechanism is fundamental for two seemingly
different phenomena such as the formation of two-level systems in glasses and
the Boson peak in the reduced density of low-frequency vibrational states
g(w)/w^2. This mechanism is the vibrational instability of weakly interacting
harmonic modes. Below some frequency w_c << w_0 (where w_0 is of the order of
Debye frequency) the instability, controlled by the anharmonicity, creates a
new stable universal spectrum of harmonic vibrations with a Boson peak feature
as well as double-well potentials with a wide distribution of barrier heights.
Both are determined by the strength of the interaction I ~ w_c between the
oscillators. Our theory predicts in a natural way a small value for the
important dimensionless parameter C ~ 10^{-4} for two-level systems in glasses.
We show that C ~ I^{-3} and decreases with increasing of the interaction
strength I. We show that the number of active two-level systems is very small,
less than one per ten million of oscillators, in a good agreement with
experiment. Within the unified approach developed in the present paper the
density of the tunneling states and the density of vibrational states at the
Boson peak frequency are interrelated.Comment: 28 pages, 3 figure
Accelerating the convergence of exact diagonalization with the transcorrelated method: Quantum gas in one dimension with contact interactions
Exact diagonalization expansions of Bose or Fermi gases with contact
interactions converge very slowly due to a non-analytic cusp in the wave
function. Here we develop a transcorrelated approach where the cusp is treated
exactly and folded into the many-body Hamiltonian with a similarity
transformation that removes the leading order singularity. The resulting
transcorrelated Hamiltonian is not hermitian but can be treated numerically
with a standard projection approach. The smoothness of the wave function
improves by at least one order and thus the convergence rate for the ground
state energy improves. By numerical investigation of a one-dimensional gas of
spin- fermions we find the error in the transcorrelated energy to
scale as with a single-particle basis of plane waves compared to
for the expansion of the original Hamiltonian and using
conventional lattice renormalization
Renormalization Group Method and Reductive Perturbation Method
It is shown that the renormalization group method does not necessarily
eliminate all secular terms in perturbation series to partial differential
equations and a functional subspace of renormalizable secular solutions
corresponds to a choice of scales of independent variables in the reductive
perturbation method.Comment: 5 pages, late
A study of the neglected Galactic HII region NGC 2579 and its companion ESO 370-9
The Galactic HII region NGC 2579 has stayed undeservedly unexplored due to
identification problems which persisted until recently. Both NGC 2579 and its
companion ESO 370-9 have been misclassified as planetary or reflection nebula,
confused with each other and with other objects. Due to its high surface
brightness, high excitation, angular size of few arcminutes and relatively low
interstellar extinction, NGC 2579 is an ideal object for investigations in the
optical range. Located in the outer Galaxy, NGC 2579 is an excellent object for
studying the Galactic chemical abundance gradients. In this paper we present
the first comprehensive observational study on the nebular and stellar
properties of NGC 2579 and ESO 370-9, including the determination of electron
temperature, density structure, chemical composition, kinematics, distance, and
the identification and spectral classification of the ionizing stars, and
discuss the nature of ESO 370-9. Long slit spectrophotometric data in the
optical range were used to derive the nebular electron temperature, density and
chemical abundances and for the spectral classification of the ionizing star
candidates. Halpha and UBV CCD photometry was carried out to derive stellar
distances from spectroscopic parallax and to measure the ionizing photon flux.Comment: To be published in Astronomy & Astrophysic
The Distances of SNR W41 and overlapping HII regions
New HI images from the VLA Galactic Plane Survey show prominent absorption
features associated with the supernovae remnant G23.3-0.3 (SNR W41). We
highlight the HI absorption spectra and the CO emission spectra of eight
small regions on the face of W41, including four HII regions, three non-thermal
emission regions and one unclassified region. The maximum velocity of
absorption for W41 is 782 km/s and the CO cloud at radial velocity
955 km/s is behind W41. Because an extended TeV source, a diffuse X-ray
enhancement and a large molecular cloud at radial velocity 775 km/s are
also projected at the center of W41, these yield the kinematic distance of 3.9
to 4.5 kpc for W41. For HII regions, our analyses reveal that both G23.42-0.21
and G23.07+0.25 are at the far kinematic distances (9.9 kpc and
10.6 kpc respectively) of their recombination-line velocities (1030.5 km/s
and 89.62.1 km/s respectively), G23.07-0.37 is at the near kinematic
distance (4.40.3 kpc) of its recombination-line velocity (82.72.0
km/s), and G23.27-0.27 is probably at the near kinematic distance (4.10.3
kpc) of its recombination-line velocity (76.10.6 km/s).Comment: 11 pages, 3 figs., 2 tables, accepted by A
The Equilibrium State of Molecular Regions in the Outer Galaxy
A summary of global properties and an evaluation of the equilibrium state of
molecular regions in the outer Galaxy are presented from the decomposition of
the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four
giant molecular cloud complexes. The ensemble of identified objects includes
both small, isolated clouds and clumps within larger cloud complexes. 12CO
velocity dispersions show little variation with cloud sizes for radii less than
10 pc. It is demonstrated that the internal motions of regions with molecular
masses greater than 10**4 msuns are bound by self gravity, yet, the constituent
clumps of cloud complexes and isolated molecular clouds with M < 10**3 msuns
are not in self gravitational equilibrium. The required external pressures to
maintain the equilibrium of this population are (1-2)x10**4 cm-3-K.Comment: To appear in ApJ, 32 pages, 13 figures, 2 table
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