540 research outputs found
Atomic motions in the -region of glass-forming polymers: Molecular versus Mode Coupling Theory approach
We present fully atomistic Molecular Dynamics simulation results on a
main-chain polymer, 1,4-Polybutadiene, in the merging region of the -
and -relaxations. A real space analysis reveals the occurrence of
localized motions (``-like'') in addition to the diffusive structural
relaxation. A molecular approach provides a direct connection between the local
conformational changes reflected in the atomic motions and the secondary
relaxations in this polymer. Such local processes occur just in the time window
where the -process of the Mode Coupling Theory is expected. We show that
the application of this theory is still possible, and yields an unusually large
value of the exponent parameter. This result might originate from the
competition between two mechanisms for dynamic arrest: intermolecular packing
and intramolecular barriers for local conformational changes
(``-like'').Comment: 10 pages, 6 figure
Tests of mode coupling theory in a simple model for two-component miscible polymer blends
We present molecular dynamics simulations on the structural relaxation of a
simple bead-spring model for polymer blends. The introduction of a different
monomer size induces a large time scale separation for the dynamics of the two
components. Simulation results for a large set of observables probing density
correlations, Rouse modes, and orientations of bond and chain end-to-end
vectors, are analyzed within the framework of the Mode Coupling Theory (MCT).
An unusually large value of the exponent parameter is obtained. This feature
suggests the possibility of an underlying higher-order MCT scenario for dynamic
arrest.Comment: Revised version. Additional figures and citation
Methyl group dynamics in a confined glass
We present a neutron scattering investigation on methyl group dynamics in
glassy toluene confined in mesoporous silicates of different pore sizes. The
experimental results have been analysed in terms of a barrier distribution
model, such a distribution following from the structural disorder in the glassy
state. Confinement results in a strong decreasing of the average rotational
barrier in comparison to the bulk state. We have roughly separated the
distribution for the confined state in a bulk-like and a surface-like
contribution, corresponding to rotors at a distance from the pore wall
respectively larger and smaller than the spatial range of the interactions
which contribute to the rotational potential for the methyl groups. We have
estimated a distance of 7 Amstrong as a lower limit of the interaction range,
beyond the typical nearest-neighbour distance between centers-of-mass (4.7
Amstrong).Comment: 5 pages, 3 figures. To be published in European Physical Journal E
Direct. Proceedings of the 2nd International Workshop on Dynamics in
Confinemen
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
(abridged) We have investigated 136 Chandra extragalactic sources without
broad optical emission lines, including 93 galaxies with narrow emission lines
(NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray
spectral hardness and optical emission line diagnostics, we have conservatively
classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We
found no statistically significant evolution in Lx/LB, within the limited z
range. We have built log(N)-log(S), after correcting for completeness based on
a series of simulations. The best-fit slope is -1.5 for both S and B energy
bands, which is considerably steeper than that of the AGN-dominated cosmic
background sources, but slightly flatter than the previous estimate, indicating
normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1
cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear
to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting
for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1,
making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG -
X-ray bright, optically normal galaxy candidates) do not appear to be
significantly absorbed, we found two heavily obscured objects, which could be
as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A
galaxy candidates with strong Balmer absorption lines, but no [OII] line. The
X-ray spectra of both galaxies are soft and one of them has a nearby close
companion galaxy, supporting the merger/interaction scenario rather than the
dusty starburst hypothesis.Comment: 31 pages, 9 figures, accepted for publication in ApJ (20 June 2006,
v644), replaced with minor correction
Facies generadoras de carbón en el Westfaliense superior de la Cuenca Carbonífera Central Asturiana
La Cuenca Carbonífera Central Asturiana constituye una cuenca de antepais individualizada en la Zona Cantábrica durante el Carbonifero Superior coincidiendo con la máxima actividad de la orogenia Hercinica. Durante el Westfaliense, se acumularon en ella importantes volúmenes de material clástico procedentes de los relieves en elevación situados al W. Los abanicos aluviales costeros constituyeron unos sistemas deposicionales muy eficientes en la dispersión de estos materiales, generaron extensas cuñas clásticas en relación con las cuales se originaron capas de carbón explotables. En este trabajo se estudian varios litosomas de edad Westfaliense Superior que representan episodios progradantes de estos sistemas sobre ambientes marinos someros v de transición. El análisis e interpretación de sus facies y geometrías permite la reconstrucción de las condiciones ambientales en que se generaron las capas de carbón asociadas a cada una de las unidades estudiadas. Estas se desarrollaron: 1) en zonas marginales a los lóbulos progradantes y zonas interlóbulos y 2) sobre los mismos una vez que éstos fueron abandonados y previamente a su enterramiento
A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties
A complete sample of 60 serendipitous hard X-ray sources with flux in the
range \ecs to \ecs (2 - 10
keV), detected in 87 ASCA GIS2 images, was recently presented in literature.
Using this sample it was possible to extend the description of the 2-10 keV
LogN(>S)-LogS down to a flux limit of \ecs (the
faintest detectable flux), resolving about a quarter of the Cosmic X-ray
Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these
sources to investigate their X-ray spectral properties using the "hardness"
ratios and the "stacked" spectra method. Because of the sample statistical
representativeness, the results presented here, that refer to the faintest hard
X-ray sources that can be studied with the current instrumentation, are
relevant to the understanding of the CXB and of the AGN unification scheme.Comment: 28 pages plus 6 figures, LaTex manuscript, Accepted for publication
in the Astrophysical Journal, Figure 5 can retrieved via anonymous ftp at
ftp://ftp.brera.mi.astro.it/pub/ASCA/paper2/fig5.ps.g
Now you see it, now you don't - the circumstellar disk in the GRO J1008--57 system
Multiwavelength observations are reported here of the Be/X-ray binary pulsar
system GRO J1008-57. Over ten years worth of data are gathered together to show
that the periodic X-ray outbursts are dependant on both the binary motion and
the size of the circumstellar disk. In the first instance an accurate orbital
solution is determined from pulse periods, and in the second case the strength
and shape of the Halpha emission line is shown to be a valuable indicator of
disk size and its behaviour. Furthermore, the shape of the emission line
permits a direct determination of the disk size which is in good agreement with
theoretical estimates. A detailed study of the pulse period variations during
outbursts determined the binary period to be 247.8, in good agreement with the
period determined from the recurrence of the outbursts.Comment: Accepted for publication in MNRA
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