4,709 research outputs found
Saturation properties and incompressibility of nuclear matter: A consistent determination from nuclear masses
Starting with a two-body effective nucleon-nucleon interaction, it is shown
that the infinite nuclear matter model of atomic nuclei is more appropriate
than the conventional Bethe-Weizsacker like mass formulae to extract saturation
properties of nuclear matter from nuclear masses. In particular, the saturation
density thus obtained agrees with that of electron scattering data and the
Hartree-Fock calculations. For the first time using nuclear mass formula, the
radius constant =1.138 fm and binding energy per nucleon = -16.11
MeV, corresponding to the infinite nuclear matter, are consistently obtained
from the same source. An important offshoot of this study is the determination
of nuclear matter incompressibility to be 288 28 MeV using
the same source of nuclear masses as input.Comment: 14 latex pages, five figures available on request ( to appear in Phy.
Rev. C
The Multiphase Intracluster Medium in Galaxy Groups Probed by the Lyman Alpha Forest
The case is made that the intracluster medium (ICM) in spiral-rich galaxy
groups today probably has undergone much slower evolution than that in
elliptical-rich groups and clusters. The environments of proto-clusters and
proto-groups at z > 2 are likely similar to spiral-rich group environments at
lower redshift. Therefore, like the ICM in spiral-rich groups today, the ICM in
proto-groups and proto-clusters at z > 2 is predicted to be significantly
multiphased. The QSO Lyman alpha forest in the vicinity of galaxies is an
effective probe of the ICM at a wide range of redshift. Two recent observations
of Lyman alpha absorption around galaxies by Adelberger et al. and by
Pascarelle et al are reconciled, and it is shown that observations support the
multiphase ICM scenario. Galaxy redshifts must be very accurate for such
studies to succeed. This scenario can also explain the lower metallicity and
lower hot gas fraction in groups.Comment: 4 pages, 1 figure, replaced with the version after proo
2-Nested Simulation is not Finitely Equationally Axiomatizable
2-nested simulation was introduced by Groote and Vaandrager [10] as the coarsest equivalence included in completed trace equivalence for which the tyft/tyxt format is a congruence format. In the lineartime-branching time spectrum of van Glabbeek [8], 2-nested simulationis one of the few equivalences for which no finite equational axiomatization is presented. In this paper we prove that such an axiomatizationdoes not exist for 2-nested simulation.Keywords: Concurrency, process algebra, basic CCS, 2-nested simulation, equational logic, complete axiomatizations
The clock paradox in a static homogeneous gravitational field
The gedanken experiment of the clock paradox is solved exactly using the
general relativistic equations for a static homogeneous gravitational field. We
demonstrate that the general and special relativistic clock paradox solutions
are identical and in particular that they are identical for finite
acceleration. Practical expressions are obtained for proper time and coordinate
time by using the destination distance as the key observable parameter. This
solution provides a formal demonstration of the identity between the special
and general relativistic clock paradox with finite acceleration and where
proper time is assumed to be the same in both formalisms. By solving the
equations of motion for a freely falling clock in a static homogeneous field
elapsed times are calculated for realistic journeys to the stars.Comment: Revision: Posted with the caption included with the figure
Mathisson's helical motions for a spinning particle --- are they unphysical?
It has been asserted in the literature that Mathisson's helical motions are
unphysical, with the argument that their radius can be arbitrarily large. We
revisit Mathisson's helical motions of a free spinning particle, and observe
that such statement is unfounded. Their radius is finite and confined to the
disk of centroids. We argue that the helical motions are perfectly valid and
physically equivalent descriptions of the motion of a spinning body, the
difference between them being the choice of the representative point of the
particle, thus a gauge choice. We discuss the kinematical explanation of these
motions, and we dynamically interpret them through the concept of hidden
momentum. We also show that, contrary to previous claims, the frequency of the
helical motions coincides, even in the relativistic limit, with the
zitterbewegung frequency of the Dirac equation for the electron
The red stellar population in NGC 1569
We present HST NICMOS photometry of the resolved stellar population in NGC
1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars
with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We
describe the data processing which is required to calibrate the instrumental
peculiarities of NICMOS. Two different packages for PSF-fitting photometry are
used to strengthen the photometric results in the crowded stellar field of NGC
1569. The resulting CMD is discussed in terms of the major evolutionary
properties of the resolved stellar populations. For a distance modulus of 26.71
and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo,
corresponding to look-back times > 15 Gyr. This is clear indication of SF
activity spanning an entire Hubble time. The metallicity of the reddest RGB
stars is in better agreement with Z=0.004 as measured in HII regions, than with
Z=0.0004 as expected from the stellar ages. The presence of - yet undetected -
very metal-poor stars embedded in the stellar distribution around J=22.75 and
J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are
preferentially found around the two central super star clusters, whereas the
oldest population has a more uniform spatial distribution. A SFR per unit area
of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the
last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on
the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding
in the NICMOS images of NGC 1569 is a challenge for the photometric analysis.
As a result, optical and NIR images of NGC 1569 sample different populations
and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of
the SF histories derived from the optical and NIR CMDs.Comment: 41 pages including 1 table (Latex) and 14 figures (postscript).
Accepted for publication in the Astronomical Journal, March 2001 issu
VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50
We report the discovery of the gamma-ray burst GRB 000131 and its optical
afterglow. The optical identification was made with the VLT 84 hours after the
burst following a BATSE detection and an Inter Planetary Network localization.
GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal
62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m
follow-up observations. Broad-band and spectroscopic observations of the
spectral energy distribution reveals a sharp break at optical wavelengths which
is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131
at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in
gamma rays alone was approximately 10^54 erg (depending on the assumed
cosmology). The rapid power-law decay of the afterglow (index alpha=2.25,
similar to bursts with a prior break in the lightcurve), however, indicates
collimated outflow, which relaxes the energy requirements by a factor of < 200.
The afterglow of GRB 000131 is the first to be identified with an 8-m class
telescope.Comment: 8 pages, 7 figures, accepted to A&A Letter
Logics for Unranked Trees: An Overview
Labeled unranked trees are used as a model of XML documents, and logical
languages for them have been studied actively over the past several years. Such
logics have different purposes: some are better suited for extracting data,
some for expressing navigational properties, and some make it easy to relate
complex properties of trees to the existence of tree automata for those
properties. Furthermore, logics differ significantly in their model-checking
properties, their automata models, and their behavior on ordered and unordered
trees. In this paper we present a survey of logics for unranked trees
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