1,186 research outputs found
Swelling of acetylated wood in organic liquids
To investigate the affinity of acetylated wood for organic liquids, Yezo
spruce wood specimens were acetylated with acetic anhydride, and their swelling
in various liquids were compared to those of untreated specimens. The
acetylated wood was rapidly and remarkably swollen in aprotic organic liquids
such as benzene and toluene in which the untreated wood was swollen only
slightly and/or very slowly. On the other hand, the swelling of wood in water,
ethylene glycol and alcohols remained unchanged or decreased by the
acetylation. Consequently the maximum volume of wood swollen in organic liquids
was always larger than that in water. The effect of acetylation on the maximum
swollen volume of wood was greater in liquids having smaller solubility
parameters. The easier penetration of aprotic organic liquids into the
acetylated wood was considered to be due to the scission of hydrogen bonds
among the amorphous wood constituents by the substitution of hydroxyl groups
with hydrophobic acetyl groups.Comment: to be published in J Wood Science (Japanese wood research society
Effect of superradiance on transport of diffusing photons in cold atomic gases
We show that in atomic gases cooperative effects like superradiance and
subradiance lead to a potential between two atoms that decays like . In
the case of superradiance, this potential is attractive for close enough atoms
and can be interpreted as a coherent mesoscopic effect. The contribution of
superradiant pairs to multiple scattering properties of a dilute gas, such as
photon elastic mean free path and group velocity, is significantly different
from that of independent atoms. We discuss the conditions under which these
effects may be observed and compare our results to recent experiments on photon
transport in cold atomic gases.Comment: 4 pages and 1 figur
Space-frequency correlation of classical waves in disordered media: high-frequency and small scale asymptotics
Two-frequency radiative transfer (2f-RT) theory is developed for geometrical
optics in random media. The space-frequency correlation is described by the
two-frequency Wigner distribution (2f-WD) which satisfies a closed form
equation, the two-frequency Wigner-Moyal equation. In the RT regime it is
proved rigorously that 2f-WD satisfies a Fokker-Planck-like equation with
complex-valued coefficients. By dimensional analysis 2f-RT equation yields the
scaling behavior of three physical parameters: the spatial spread, the
coherence length and the coherence bandwidth. The sub-transport-mean-free-path
behavior is obtained in a closed form by analytically solving a paraxial 2f-RT
equation
Chemical Abundance Gradients in the Star-Forming Ring Galaxies
Ring waves of star formation, propagating outwardly in the galactic disks,
leave chemical abundance gradients in their wakes. We show that the relative
[Fe/O] abundance gradients in ring galaxies can be used as a tool for
determining the role of the SNIa explosions in their chemical enrichment. We
consider two mechanisms which can create outwardly propagating star forming
rings in a purely gaseous disk -- a self-induced wave and a density wave, and
demonstrate that the radial distribution of the relative [Fe/O] abundance
gradients does not depend on the particular mechanism of the wave formation or
on the parameters of the star-forming process. We show that the [Fe/O] profile
is determined by the velocity of the wave, initial mass function, and the
initial chemical composition of the star-forming gas. If the role of SNIa
explosions is negligible in the chemical enrichment, the ratio [Fe/O] remains
constant throughout the galactic disk with a steep gradient at the wave front.
If SNIa stars are important in the production of cosmic iron, the [Fe/O] ratio
has gradient in the wake of the star-forming wave with the value depending on
the frequency of SNIa explosions.Comment: Uses aas2pp4.sty and epsfig.sty, 7 pages including one figure To
appear in Astrophysical Journa
Metallicity Gradients in the Intracluster Gas of Abell 496
Analysis of spatially resolved ASCA spectra of the intracluster gas in Abell
496 confirms there are mild metal abundance enhancements near the center, as
previously found by White et al. (1994) in a joint analysis of Ginga LAC and
Einstein SSS spectra. Simultaneous analysis of spectra from all ASCA
instruments (SIS + GIS) shows that the iron abundance is 0.36 +- 0.03 solar
3-12' from the center of the cluster and rises ~50% to 0.53 +- 0.04 solar
within the central 2'. The F-test shows that this abundance gradient is
significant at the >99.99% level. Nickel and sulfur abundances are also
centrally enhanced. We use a variety of elemental abundance ratios to assess
the relative contribution of SN Ia and SN II to the metal enrichment of the
intracluster gas. We find spatial gradients in several abundance ratios,
indicating that the fraction of iron from SN Ia increases toward the cluster
center, with SN Ia accounting for ~50% of the iron mass 3-12' from the center
and ~70% within 2'. The increased proportion of SN Ia ejecta at the center is
such that the central iron abundance enhancement can be attributed wholly to SN
Ia; we find no significant gradient in SN II ejecta. These spatial gradients in
the proportion of SN Ia/II ejecta imply that the dominant metal enrichment
mechanism near the center is different than in the outer parts of the cluster.
We show that the central abundance enhancement is unlikely to be due to ram
pressure stripping of gas from cluster galaxies, or to secularly accumulated
stellar mass loss within the central cD. We suggest that the additional SN Ia
ejecta near the center is the vestige of a secondary SN Ia-driven wind from the
cD (following a more energetic protogalactic SN II-driven wind phase), which
was partially smothered in the cD due to its location at the cluster center.Comment: 25 pages AASTeX; 6 encapsulated PostScript figures; accepted for
publication in ApJ. Replaced with revised versio
Anisotropic multiple scattering in diffuse media
The multiple scattering of scalar waves in diffusive media is investigated by
means of the radiative transfer equation. This approach amounts to a
resummation of the ladder diagrams of the Born series; it does not rely on the
diffusion approximation. Quantitative predictions are obtained, concerning
various observables pertaining to optically thick slabs, such as the mean
angle-resolved reflected and transmitted intensities, and the shape of the
enhanced backscattering cone. Special emphasis is put on the dependence of
these quantities on the anisotropy of the cross-section of the individual
scatterers, and on the internal reflections due to the optical index mismatch
at the boundaries of the sample. The regime of very anisotropic scattering,
where the transport mean free path is much larger than the scattering
mean free path , is studied in full detail. For the first time the
relevant Schwarzschild-Milne equation is solved exactly in the absence of
internal reflections, and asymptotically in the regime of a large index
mismatch. An unexpected outcome concerns the angular width of the enhanced
backscattering cone, which is predicted to scale as
, in contrast with the generally
accepted law, derived within the diffusion approximation.Comment: 53 pages TEX, including 2 tables. The 4 figures are sent at reques
Why Are Rotating Elliptical Galaxies Less Elliptical at X-ray Frequencies?
If mass and angular momentum were conserved in cooling flows associated with
luminous, slowly rotating elliptical galaxies, the inflowing hot gas would spin
up, resulting in disks of cold gas and X-ray images that are highly flattened
along the equatorial plane out to several effective radii. Such X-ray
flattening is not observed at the spatial resolution currently available to
X-ray observations. Evidently mass and angular momentum are not in fact
conserved. If cooling flows are depleted by localized radiative cooling at
numerous sites distributed throughout the flows, then disks of cooled gas do
not form and the X-ray images appear nearly circular. However, the distribution
of young stars formed from the cooled gas is still somewhat flattened relative
to the stellar light. X-ray images of galactic cooling flows can also be
circularized by the turbulent diffusion of angular momentum away from the axis
of rotation, but the effective viscosity of known processes -- stellar mass
loss, supernovae, cooling site evolution, etc. -- is insufficient to
appreciably circularize the X-ray images. Radial gradients in the interstellar
iron abundance are unaffected by the expected level of interstellar turbulence
since these gradients are continuously re-established by Type Ia supernovae.Comment: 17 pages with 6 figures; accepted by Astrophysical Journa
Oxygen Absorption in M87: Evidence for a Warm+Hot ISM
We present a re-analysis of the ROSAT PSPC data within the central 100 kpc of
M87 to search for intrinsic oxygen absorption similar to that recently measured
in several galaxies and groups. Using a spatial-spectral deprojection analysis
we find the strongest evidence to date for intrinsic oxygen absorption in the
hot gas of a galaxy, group, or cluster. Single-phase plasma models modified by
intervening Galactic absorption cannot fit the 0.2-2.2 keV PSPC data as they
under-predict the 0.2-0.4 keV region and over-predict the 0.5-0.8 keV region
where the emission and absorption residuals are obvious upon visual inspection
of the spectral fits. Since the excess emission between 0.2-0.4 keV rules out
intrinsic absorption from cold gas or dust, the most reasonable model for the
excess emission and absorption features is warm, collisionally ionized gas with
a temperature of ~10^6 K. Simple multiphase models (cooling flow, two phases)
modified by both intervening Galactic absorption and by a single oxygen edge
provide good fits and yield temperatures and Fe abundances of the hot gas that
agree with previous determinations by ASCA and SAX. The multiphase models of
M87 inferred from the PSPC can account for the excess EUV emission observed
with EUVE and the excess X-ray absorption inferred from EINSTEIN and ASCA data
above 0.5 keV. Although the total mass of the warm gas implied by the oxygen
absorption is consistent with the matter deposited by a cooling flow, the
suppression of the mass deposition rate and the distortion of the X-ray
isophotes in the region where the radio emission is most pronounced suggest
some feedback effect from the AGN on the cooling gas. (Abridged)Comment: 17 pages (13 figures), Accepted for Publication in The Astrophysical
Journa
Scalar Quantum Field Theory in Disordered Media
A free massive scalar field in inhomogeneous random media is investigated.
The coefficients of the Klein-Gordon equation are taken to be random functions
of the spatial coordinates. The case of an annealed-like disordered medium,
modeled by centered stationary and Gaussian processes, is analyzed. After
performing the averages over the random functions, we obtain the two-point
causal Green's function of the model up to one-loop. The disordered scalar
quantum field theory becomes qualitatively similar to a
self-interacting theory with a frequency-dependent coupling
Intermediate Element Abundances in Galaxy Clusters
We present the average abundances of the intermediate elements obtained by
performing a stacked analysis of all the galaxy clusters in the archive of the
X-ray telescope ASCA. We determine the abundances of Fe, Si, S, and Ni as a
function of cluster temperature (mass) from 1--10 keV, and place strong upper
limits on the abundances of Ca and Ar. In general, Si and Ni are overabundant
with respect to Fe, while Ar and Ca are very underabundant. The discrepancy
between the abundances of Si, S, Ar, and Ca indicate that the alpha-elements do
not behave homogeneously as a single group. We show that the abundances of the
most well-determined elements Fe, Si, and S in conjunction with recent
theoretical supernovae yields do not give a consistent solution for the
fraction of material produced by Type Ia and Type II supernovae at any
temperature or mass. The general trend is for higher temperature clusters to
have more of their metals produced in Type II supernovae than in Type Ias. The
inconsistency of our results with abundances in the Milky Way indicate that
spiral galaxies are not the dominant metal contributors to the intracluster
medium (ICM). The pattern of elemental abundances requires an additional source
of metals beyond standard SNIa and SNII enrichment. The properties of this new
source are well matched to those of Type II supernovae with very massive,
metal-poor progenitor stars. These results are consistent with a significant
fraction of the ICM metals produced by an early generation of population III
stars.Comment: 18 pages, 11 figures, 7 tables. Submitted to Ap
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