764 research outputs found
Variable damping and coherence in a high-density magnon gas
We report on the fast relaxation behavior of a high-density magnon gas
created by a parametric amplification process. The magnon gas is probed using
the technique of spin-wave packet recovery by parallel parametric pumping.
Experimental results show a damping behavior which is in disagreement with both
the standard model of exponential decay and with earlier observations of
non-linear damping. In particular, the inherent magnon damping is found to
depend upon the presence of the parametric pumping field. A phenomenological
model which accounts for the dephasing of the earlier injected magnons is in
good agreement with the experimental data
The Gunn-Peterson effect and the Lyman alpha forest
We show that spatial correlations in a stochastic large scale velocity field
in an otherwise smooth intergalactic medium (homogeneous comoving density)
superposed on the general Hubble flow, may cause a `line-like' structure in QSO
spectra similar to the population of unsaturated Ly-alpha forest lines which
usually are attributed to individual clouds with 10^{11} <= N(HI) <= 5*10^{13}
cm^{-2}. Therefore there is no clear observational distinction between a
diffuse intergalactic medium and discrete intergalactic clouds. It follows that
the HI-density in the diffuse intergalactic medium might be substantially
underestimated if it is determined from the observed intensity distribution
near the apparent continuum in high resolution spectra of QSOs. Our tentative
estimate implies a diffuse neutral hydrogen opacity tau_{GP} = 0.3 at z = 3 and
a current baryon density Omega_{IGM} = 0.08$, assuming a Hubble constant H = 70
km s^{-1} Mpc^{-1}.Comment: 9 pages, 3 Postscript figures, a MNRAS Letters submissio
Age as a Variable in an Exercise Program for the Treatment of Simple Urinary Stress Incontinence
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74703/1/j.1552-6909.1987.tb01584.x.pd
Argon and Silicon abundances in the damped Ly-alpha system I Zw 18
We show that the difference between the Ar and Si relative abundance ratio
derived from FUSE absorption spectra and from the HII regions of I Zw 18 is a
consequence of the microturbulent analysis applied to the absorption spectra.
FUSE observations were performed with a large entrance aperture which fully
covered the galaxy. This means that the observed profiles are averaged over the
full body of I Zw 18, implying that large-scale velocity fields influence the
absorption - line profiles. Taking this into account, we show that the
absorption spectra are consistent with the same metal abundances as those
derived from the HII regions. It follows that no significant ionization
correction as suggested by Izotov and collaborators to describe metal contents
in damped Ly-alpha systems (DLA) is required to model abundances in the neutral
gas of I Zw 18 (a local DLA system). Using a mesoturbulent approach and
applying the generalized radiative transfer equation to the ArI1048 and
SiII1020 lines observed by Vidal-Madjar et al., we found that the profiles may
be reproduced with log (Ar/Si) ~= - 0.8 and N(SiII) ~= 4 10^{15} cm^{-2}.Comment: 4 pages, 2 eps figure, submitted to A
A Compendium of Far-Infrared Line and Continuum Emission for 227 Galaxies Observed by the Infrared Space Observatory
Far-infrared line and continuum fluxes are presented for a sample of 227
galaxies observed with the Long Wavelength Spectrometer on the Infrared Space
Observatory. The galaxy sample includes normal star-forming systems,
starbursts, and active galactic nuclei covering a wide range of colors and
morphologies. The dataset spans some 1300 line fluxes, 600 line upper limits,
and 800 continuum fluxes. Several fine structure emission lines are detected
that arise in either photodissociation or HII regions: [OIII]52um, [NIII]57um,
[OI]63um, [OIII]88um, [NII]122um, [OI]145um, and [CII]158um. Molecular lines
such as OH at 53um, 79um, 84um, 119um, and 163um, and H2O at 58um, 66um, 75um,
101um, and 108um are also detected in some galaxies. In addition to those lines
emitted by the target galaxies, serendipitous detections of Milky Way
[CII]158um and an unidentified line near 74um in NGC1068 are also reported.
Finally, continuum fluxes at 52um, 57um, 63um, 88um, 122um, 145um, 158um, and
170um are derived for a subset of galaxies in which the far-infrared emission
is contained within the ~75" ISO LWS beam. The statistics of this large
database of continuum and line fluxes, including trends in line ratios with the
far-infrared color and infrared-to-optical ratio, are explored.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
The D/H ratio at z = 3.57 toward Q 1937-1009
Deuterium abundance re-measurements by Burles and Tytler (1998; hereafter BT)
yielded D/H = (3.3 +/- 0.3) 10^{-5} and the robust upper limit D/H < 3.9
10^{-5} from the z_a = 3.572 system toward Q1937-1009. In this new analysis BT
adopted multicomponent microturbulent models together with the possibility to
vary freely the local continuum level around each HI line to improve the fit.
The procedure failed, however, to fit adequately D Ly-beta without recourse to
an additional H Ly-alpha contamination at the position of D Ly-beta. We show
that this obstacle may be successfully overcome within the framework of the
mesoturbulent model accounting (in contrast to the microturbulent
approximation) for a correlated structure of the large scale velocity field.
Using the same observational data and the original continuum as determined by
Tytler et al. (1996), we obtained good fits. The one-component mesoturbulent
models provide D/H in the range (3.2 - 4.8) 10^{-5} and the total hydrogen
column density N(HI) = (5.6 - 7.0) 10^{17} cm^{-2}. This result is consistent
with that found by us from the z_a = 2.504 and z_a = 0.701 systems toward
Q1009+2956 and Q1718+4807, respectively. The range for D/H common to all three
analyses is D/H = (4.1 - 4.6) 10^{-5}. This value is consistent with standard
big bang nucleosynthesis [SBBN] if the baryon-to-photon ratio, \eta, is in the
range 4.2 10^{-10} <= \eta <= 4.6 10^{-10}, implying 0.0155 <= \Omega_b
h^2_{100} <= 0.0167.Comment: 8 pages, 2 Postscript figures, aaspp4.sty file, submit. ApJ Let
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