1,334 research outputs found
Noise-Activated Escape from a Sloshing Potential Well
We treat the noise-activated escape from a one-dimensional potential well of
an overdamped particle, to which a periodic force of fixed frequency is
applied. We determine the boundary layer behavior, and the physically relevant
length scales, near the oscillating well top. We show how stochastic behavior
near the well top generalizes the behavior first determined by Kramers, in the
case without forcing. Both the case when the forcing dies away in the weak
noise limit, and the case when it does not, are examined. We also discuss the
relevance of various scaling regimes to recent optical trap experiments.Comment: 9 pages, no figures, REVTeX, expanded versio
Crystal structure of mixed fluorites Ca(1-x)Sr(x)F(2) and Sr(1-x)Ba(x)F(2) and luminescence of Eu(2+) in the crystals
Within the framework of the virtual crystal method implemented in the shell
model and pair potential approximation the crystal structure of mixed fluorites
Ca(1-x)Sr(x)F(2) and Sr(1-x)Ba(x)F(2) has been calculated. The impurity center
Eu(2+) and the distance Eu(2+)-F in this crystals have been also calculated.
The low level position of excited 4f65d configuration of the Eu(2+) ion has
been expressed using phenomenological dependence on distance E(2+)-F. The
dependences of Stokes shift and Huang-Rhys factor on concentration x have been
received for yellow luminescence in Sr(1-x)Ba(x)F(2):Eu(2+). The value x, for
which the eg -level of Eu(2+) ion will be in conduction band in
Sr(1-x)Ba(x)F(2):Eu(2+) has been calculated.Comment: 8 pages, 3 figures. The manuscript is sent to journal 'Physics of the
solid state'. The results will be submitted on inernational conference
SCINTMAT'2002 in oral session (june,20-22,2002,Ekaterinburg,Russia).
Corresponding author e-mail: [email protected]
Galactic Cosmic Rays from Supernova Remnants: II Shock Acceleration of Gas and Dust
This is the second paper (the first was astro-ph/9704267) of a series
analysing the Galactic Cosmic Ray (GCR) composition and origin. In this we
present a quantitative model of GCR origin and acceleration based on the
acceleration of a mixture of interstellar and/or circumstellar gas and dust by
supernova remnant blast waves. We present results from a nonlinear shock model
which includes (i) the direct acceleration of interstellar gas-phase ions, (ii)
a simplified model for the direct acceleration of weakly charged dust grains to
energies of order 100keV/amu simultaneously with the gas ions, (iii) frictional
energy losses of the grains colliding with the gas, (iv) sputtering of ions of
refractory elements from the accelerated grains and (v) the further shock
acceleration of the sputtered ions to cosmic ray energies. The calculated GCR
composition and spectra are in good agreement with observations.Comment: to appear in ApJ, 51 pages, LaTeX with AAS macros, 9 postscript
figures, also available from ftp://wonka.physics.ncsu.edu/pub/elliso
Ten-Micron Observations of Nearby Young Stars
(abridged) We present new 10-micron photometry of 21 nearby young stars
obtained at the Palomar 5-meter and at the Keck I 10-meter telescopes as part
of a program to search for dust in the habitable zone of young stars. Thirteen
of the stars are in the F-K spectral type range ("solar analogs"), 4 have B or
A spectral types, and 4 have spectral type M. We confirm existing IRAS
12-micron and ground-based 10-micron photometry for 10 of the stars, and
present new insight into this spectral regime for the rest. Excess emission at
10 micron is not found in any of the young solar analogs, except for a possible
2.4-sigma detection in the G5V star HD 88638. The G2V star HD 107146, which
does not display a 10-micron excess, is identified as a new Vega-like
candidate, based on our 10-micron photospheric detection, combined with
previously unidentified 60-micron and 100-micron IRAS excesses. Among the
early-type stars, a 10-micron excess is detected only in HD 109573A (HR 4796A),
confirming prior observations; among the M dwarfs, excesses are confirmed in AA
Tau, CD -40 8434, and Hen 3-600A. A previously suggested N band excess in the
M3 dwarf CD -33 7795 is shown to be consistent with photospheric emission.Comment: 40 pages, 4 figures, 5 tables. To appear in the January 1, 2004 issue
of Ap
HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation
We present the results of a comparative study of HI narrow self-absorption
(HINSA), OH, 13CO, and C18O in five dark clouds. The HINSA follows the
distribution of the emission of the carbon monoxide isotopologues, and has a
characteristic size close to that of 13CO. This confirms that the HINSA is
produced by cold HI which is well mixed with molecular gas in well-shielded
regions. The ratio of the atomic hydrogen density to total proton density for
these sources is 5 to 27 x 10^{-4}. Using cloud temperatures and the density of
HI, we set an upper limit to the cosmic ray ionization rate of 10^{-16} s^{-1}.
Comparison of observed and modeled fractional HI abundances indicates ages for
these clouds to be 10^{6.5} to 10^{7} yr. The low values of the HI density we
have determined make it certain that the time scale for evolution from an
atomic to an almost entirely molecular phase, must be a minimum of several
million years. This clearly sets a lower limit to the overall time scale for
star formation and the lifetime of molecular clouds
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