4,387 research outputs found
Third minima in actinides - do they exist?
We study the existence of third, hyperdeformed minima in a number of
even-even Th, U and Pu nuclei using the Woods-Saxon microscopic-macroscopic
model that very well reproduces first and second minima and fission barriers in
actinides. Deep ( MeV) minima found previously by \'Cwiok et al. are
found spurious after sufficiently general shapes are included. Shallow third
wells may exist in Th, with IIIrd barriers 200 and 330 keV
(respectively). Thus, a problem of qualitative discrepancy between
microscopic-macroscopic and selfconsistent predictions is resolved. Now, an
understanding of experimental results on the apparent third minima in uranium
becomes an issue.Comment: 5 pages, 3 figures, 02. 03. 2012 - submitted to PR
The Serendipitous Discovery of a Group or Cluster of young Galaxies at z=2.40 in Deep Hubble Space Telescope WFPC2 Images
We report the serendipitous discovery of a group or cluster of young galaxies
at 2.40 in a 24-orbit HST/WFPC2 exposure of the field around the weak
radio galaxy 53W002. Potential cluster members were identified on ground-based
narrow-band redshifted Ly images and confirmed via spectroscopy.
In addition to the known weak radio galaxy 53W002 at z=2.390, two other
objects were found to have excess narrow-band Ly emission at
2.40. Both have been spectroscopically confirmed, and one clearly
contains a weak AGN. They are located within one arcminute of 53W002, or
Mpc (=0.5) at 2.40, which is the physical
scale of a group or small cluster of galaxies. Profile fitting of the WFPC2
images shows that the objects are very compact, with scale lengths
0\farcs 1 (kpc), and are rather faint
(luminosities < L*), implying that they may be sub-galactic sized objects. We
discuss these results in the context of galaxy and cluster evolution and the
role that weak AGN may play in the formation of young galaxies.Comment: Accepted for publication in The Astrophysical Journal (Letters). 13
pages of gzip compressed and uuencoded PS. Figures are available at
http://www.phys.unsw.edu.au/~spd/bib.htm
The Properties of Field Elliptical Galaxies at Intermediate Redshift. I: Empirical Scaling Laws
We present measurements of the Fundamental Plane (FP) parameters (the
effective radius, the mean effective surface brightness, and the central
velocity dispersion) of six field elliptical galaxies at intermediate redshift.
The imaging is taken from the Medium Deep Survey of the Hubble Space Telescope,
while the kinematical data are obtained from long-slit spectroscopy using the
3.6-m ESO telescope. The Fundamental Plane appears well defined in the field
even at redshift 0.3. The data show a shift in the FP zero point with
respect to the local relation, possibly indicating modest evolution, consistent
with the result found for intermediate redshift cluster samples. The FP slopes
derived for our field data, plus other cluster ellipticals at intermediate
redshift taken from the literature, differ from the local ones, but are still
consistent with the interpretation of the FP as a result of homology, of the
virial theorem and of the existence of a relation between luminosity and mass,
. We also derive the surface brightness vs. effective
radius relation for nine galaxies with redshift up to , and data
from the literature; the evolution that can be inferred is consistent with what
is found using the FP.Comment: 17 pages, including 9 figures, MNRAS, accepte
Pairing correlations. Part 1: description of odd nuclei in mean-field theories
In order to extract informations on pairing correlations in nuclei from
experimental mass differences, the different contributions to odd-even mass
differences are investigated within the Skyrme HFB method. In this first paper,
the description of odd nuclei within HFB is discussed since it is the key point
for the understanding of the above mentioned contributions. To go from an even
nucleus to an odd one, the advantage of a two steps process is demonstrated and
its physical content is discussed. New results concerning time-reversal
symmetry breaking in odd-nuclei are also reported.
PACS: 21.10Dr; 21.10.Hw; 21.30.-x.
Keywords: Mean-field theories; Pairing correlations; odd nuclei;Comment: 34 pages, 8 figures. Submitted to Phys. Rev.
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
Seeking the Ultraviolet Ionizing Background at z~3 with the Keck Telescope
We describe the initial results of a deep long-slit emission line search for
redshifted (2.7<z<4.1) Lyman-alpha. These observations are used to constrain
the fluorescent Ly-alpha emission from the population of clouds whose
absorption produces the higher-column-density component of the Ly-alpha forest
in quasar spectra. We use the results to set an upper limit on the ultraviolet
ionizing background. Our spectroscopic data obtained with the Keck II telescope
at lambda/(Delta lambda FWHM)~2000 reveals no candidate Ly-alpha emission over
the wavelength range of 4500-6200 Ang along a 3 arcmin slit in a 5400 s
integration. Our 3 sigma upper bound on the mean intensity of the ionizing
background at the Lyman limit is J(nu 0) < 2E-21 erg/s/cm**2/Hz/sr for
2.7<z<3.1 (where we are most sensitive), assuming Lyman limit systems have
typical radii of 70 kpc (q_0=0.5, H_0=50 km/s/Mpc). This constraint is more
than an order of magnitude more stringent than any previously published direct
limit. However, it is still a factor of three above the ultraviolet background
level expected due to the integrated light of known quasars at z~3. This pilot
study confirms the conclusion of Gould \& Weinberg (1996) that integrations of
several hours on a 10-m class telescope should be capable of measuring J(nu 0)
at high redshift.Comment: 22 pages, 2 postscipt figures. Latex requires aaspp4.sty and epsf.sty
(included). Accepted for publication in the Astronomical Journal (Nov 1998
Modelling elliptical galaxies: phase-space constraints on two-component (gamma1,gamma2) models
In the context of the study of the properties of the mutual mass distribution
of the bright and dark matter in elliptical galaxies, present a family of
two-component, spherical, self-consistent galaxy models, where one density
distribution follows a gamma_1 profile, and the other a gamma_2 profile
[(gamma_1,gamma_2) models], with different total masses and ``core'' radii. A
variable amount of Osipkov-Merritt (radial) orbital anisotropy is allowed in
both components. For these models, I derive analytically the necessary and
sufficient conditions that the model parameters must satisfy in order to
correspond to a physical system. Moreover, the possibility of adding a black
hole at the center of radially anisotropic gamma models is discussed,
determining analytically a lower limit of the anisotropy radius as a function
of gamma. The analytical phase-space distribution function for (1,0) models is
presented, together with the solution of the Jeans equations and the quantities
entering the scalar virial theorem. It is proved that a globally isotropic
gamma=1 component is consistent for any mass and core radius of the
superimposed gamma=0 model; on the contrary, only a maximum value of the core
radius is allowed for the gamma=0 model when a gamma=1 density distribution is
added. The combined effects of mass concentration and orbital anisotropy are
investigated, and an interesting behavior of the distribution function of the
anisotropic gamma=0 component is found: there exists a region in the parameter
space where a sufficient amount of anisotropy results in a consistent model,
while the structurally identical but isotropic model would be inconsistent.Comment: 29 pages, LaTex, plus 5 .eps figures and macro aaspp4.sty - accepted
by ApJ, main journa
Towards a Notion of Distributed Time for Petri Nets
We set the ground for research on a timed extension of Petri nets where time parameters are associated with tokens and arcs carry constraints that qualify the age of tokens required for enabling. The novelty is that, rather than a single global clock, we use a set of unrelated clocks --- possibly one per place --- allowing a local timing as well as distributed time synchronisation. We give a formal definition of the model and investigate properties of local versus global timing, including decidability issues and notions of processes of the respective models
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