260 research outputs found
Geodesics around Weyl-Bach's Ring Solution
We explore some of the gravitational features of a uniform ring both in the
Newtonian potential theory and in General Relativity. We use a spacetime
associated to a Weyl static solution of the vacuum Einstein's equations with
ring like singularity. The Newtonian motion for a test particle in the
gravitational field of the ring is studied and compared with the corresponding
geodesic motion in the given spacetime. We have found a relativistic peculiar
attraction: free falling particle geodesics are lead to the inner rim but never
hit the ring.Comment: 8 figures, 14 pages. LaTeX w/ subfigure, graphic
BVRI Surface Photometry of Isolated Spiral Galaxies
A release of multicolor broad band (BVRI) photometry for a subsample of 44
isolated spirals drawn from the Catalogue of Isolated Galaxies (CIG) is
presented. Total magnitudes and colors at various circular apertures, as well
as some global structural/morphological parameters are estimated. Morphology is
reevaluated through optical and sharp/filtered R band images, (B-I) color index
maps, and archive near-IR JHK images from the Two-Micron Survey. The CAS
structural parameters (Concentration, Asymmetry, and Clumpiness) were
calculated from the images in each one of the bands. The fraction of galaxies
with well identified optical/near-IR bars (SB) is 63%, while a 17% more shows
evidence of weak or suspected bars (SAB). The sample average value of the
maximum bar ellipticity is 0.4. Half of the galaxies in the sample shows rings.
We identify two candidates for isolated galaxies with disturbed morphology. The
structural CAS parameters change with the observed band, and the tendencies
they follow with the morphological type and global color are more evident in
the redder bands. In any band, the major difference between our isolated
spirals and a sample of interacting spirals is revealed in the A-S plane. A
deep and uniformly observed sample of isolated galaxies is intended for various
purposes including (i) comparative studies of environmental effects, (ii)
confronting model predictions of galaxy evolution and (iii) evaluating the
change of galaxy properties with redshift.Comment: 44 pages, 9 figures and 7 tables included. To appear in The
Astronomical Journal. For the 43 appendix figures 4.1-4.43 see
http://www.astroscu.unam.mx/~avila/Figs4.1_4.43.tar.gz (7.2 Mb tar.gz file
Unveiling the intruder deformed 0 state in Si
The 0 state in Si has been populated at the {\sc Ganil/Lise3}
facility through the -decay of a newly discovered 1 isomer in
Al of 26(1) ms half-life. The simultaneous detection of pairs
allowed the determination of the excitation energy E(0)=2719(3) keV and
the half-life T=19.4(7) ns, from which an electric monopole strength of
(E0)=13.0(0.9) was deduced. The 2 state is
observed to decay both to the 0 ground state and to the newly observed
0 state (via a 607(2) keV transition) with a ratio
R(2)=1380(717). Gathering all
information, a weak mixing with the 0 and a large deformation parameter
of =0.29(4) are found for the 0 state, in good agreement with
shell model calculations using a new {\sc sdpf-u-mix} interaction allowing
\textit{np-nh} excitations across the N=20 shell gap.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
The Relationship Between Stellar Light Distributions of Galaxies and their Formation Histories
A major problem in extragalactic astronomy is the inability to distinguish in
a robust, physical, and model independent way how galaxy populations are
related to each other and to their formation histories. A similar, but
distinct, and also long standing question is whether the structural appearances
of galaxies, as seen through their stellar light distributions, contain enough
physical information to offer this classification. We argue through the use of
240 images of nearby galaxies that three model independent parameters measured
on a single galaxy image reveal its major ongoing and past formation modes, and
can be used as a robust classification system. These parameters quantitatively
measure: the concentration (C), asymmetry (A) and clumpiness (S) of a galaxy's
stellar light distribution. When combined into a three dimensional `CAS' volume
all major classes of galaxies in various phases of evolution are cleanly
distinguished. We argue that these three parameters correlate with important
modes of galaxy evolution: star formation and major merging activity. This is
argued through the strong correlation of Halpha equivalent width and broad band
colors with the clumpiness parameter, the uniquely large asymmetries of 66
galaxies undergoing mergers, and the correlation of bulge to total light
ratios, and stellar masses, with the concentration index. As an obvious goal is
to use this system at high redshifts to trace evolution, we demonstrate that
these parameters can be measured, within a reasonable and quantifiable
uncertainty, with available data out to z ~ 3 using the Hubble Space Telescope
GOODS ACS and Hubble Deep Field images.Comment: ApJS, in press, 30 pages, Figures 15 and 16 are in color. For a full
resolution version, please go to http://www.astro.caltech.edu/~cc/cas.p
Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383
We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha,
broadband optical and near-infrared) observations of the classic barred spiral
NGC 5383. We compare the observed central gas and dust morphology to the
predictions of recent hydrodynamic simulations. In the nuclear region, our
observations reveal three peaks lying along a S-shaped gas and dust
distribution. In contrast, the model predicts a circumnuclear ring, not the
observed S-shaped distribution; moreover, the predicted surface density
contrast between the central gas accumulation and the bar dust lanes is an
order of magnitude larger than observed. The discrepancies are not due to
unexplored model parameter space or a nuclear bar but are probably due to the
vigorous (7 solar masses per year) star formation activity in the center.
As is common in similar bars, the star formation rate in the bar between the
bar ends and the central region is low (~0.5 solar masses per yr), despite the
high gas column density in the bar dust lanes; this is generally attributed to
shear and shocks. We note a tendency for the HII regions to be associated with
the spurs feeding the main bar dust lanes, but these are located on the leading
side of the bar. We propose that stars form in the spurs, which provide a high
column density but low shear environment. HII regions can therefore be found
even on the leading side of the bar because the ionizing stars pass
ballistically through the dust laneComment: Accepted for publication in The Astrophysical Journal, 33 pages
(includes 10 figures
Polygonal Structures in the Gaseous Disk: Numerical Simulations
The results of numerical simulations of a gaseous disk in the potential of a
stellar spiral density wave are presented. The conditions under which
straightened spiral arm segments (rows) form in the gas component are studied.
These features of the spiral structure were identified in a series of works by
A.D. Chernin with coauthors. Gas-dynamic simulations have been performed for a
wide range of model parameters: the pitch angle of the spiral pattern, the
amplitude of the stellar spiral density wave, the disk rotation speed, and the
temperature of the gas component. The results of 2D- and 3D-disk simulations
are compared. The rows in the numerical simulations are shown to be an
essentially nonstationary phenomenon. A statistical analysis of the
distribution of geometric parameters for spiral patterns with rows in the
observed galaxies and the constructed hydrodynamic models shows good agreement.
In particular, the numerical simulations and observations of galaxies give
for the average angles between straight segments.Comment: 22 pages, 10 figure
Lenticular galaxies with UV-rings
By using the public UV imaging data obtained by the GALEX (Galaxy Ultraviolet
Explorer) for nearby galaxies, we have compiled a list of lenticular galaxies
possessing ultraviolet rings - starforming regions tightly confined to
particular radial distances from galactic centers. We have studied large-scale
structure of these galaxies in the optical bands by using the data of the SDSS
(Sloan Digital Sky Survey): we have decomposed the galactic images into
large-scale disks and bulges, have measured the ring optical colours from the
residual images after subtracting model disks and bulges, and have compared the
sizes of the rings in the optical light and in the UV-band. The probable origin
of the outer starforming ring appearances in unbarred galaxies demonstrating
otherwise the regular structure and homogeneously old stellar population beyond
the rings is discussed.Comment: 9 pages plus one big colour figure in the Appendix; the slightly
expanded version of the paper accepted to Astronomy Letter
New pathway to bypass the 15O waiting point
We propose the sequential reaction process
O(,)O as a new pathway to bypass of the
O waiting point. This exotic reaction is found to have a surprisingly
high cross section, approximately 10 times higher than the
O(,)O. These cross sections were calculated after
precise measurements of energies and widths of the proton-unbound F low
lying states, obtained using the H(O,p)O reaction. The large
cross section can be understood to arise from the more
efficient feeding of the low energy wing of the ground state resonance by the
gamma decay. The implications of the new reaction in novae explosions and X-ray
bursts are discussed.Comment: submitte
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