7,364 research outputs found
The Generation of Fullerenes
We describe an efficient new algorithm for the generation of fullerenes. Our
implementation of this algorithm is more than 3.5 times faster than the
previously fastest generator for fullerenes -- fullgen -- and the first program
since fullgen to be useful for more than 100 vertices. We also note a
programming error in fullgen that caused problems for 136 or more vertices. We
tabulate the numbers of fullerenes and IPR fullerenes up to 400 vertices. We
also check up to 316 vertices a conjecture of Barnette that cubic planar graphs
with maximum face size 6 are hamiltonian and verify that the smallest
counterexample to the spiral conjecture has 380 vertices.Comment: 21 pages; added a not
Arrest stress of uniformly sheared wet granular matter
We conduct extensive independent numerical experiments considering
frictionless disks without internal degrees of freedom (rotation etc.) in two
dimensions. We report here that for a large range of the packing fractions
below random-close packing, all components of the stress tensor of wet granular
materials remain finite in the limit of zero shear rate. This is direct
evidence for a fluid-to-solid arrest transition. The offset value of the shear
stress characterizes plastic deformation of the arrested state {which
corresponds to {\em dynamic yield stress} of the system}. {Based on an
analytical line of argument, we propose that the mean number of capillary
bridges per particle, , follows a non-trivial dependence on the packing
fraction, , and the capillary energy, \vareps. Most noticeably, we show
that is a generic and universal quantity which does not depend on the
driving protocol.} Using this universal quantity, we calculate the arrest
stress, , analytically based on a balance of the energy injection
rate due to the external force driving the flow and the dissipation rate
accounting for the rupture of capillary bridges. The resulting prediction of
is a non-linear function of the packing fraction , and the
capillary energy \vareps. This formula provides an excellent, parameter-free
prediction of the numerical data. Corrections to the theory for small and large
packing fractions are connected to the emergence of shear bands and of
contributions to the stress from repulsive particle interactions, respectively.Comment: 7 pages, g figure
How Saturated are Absorption Lines in the Broad Absorption Line Quasar PG 1411+442 ?
Recently, convincing evidence was found for extremely large X-ray absorption
by column densities in broad absorption line quasars. One
consequence of this is that any soft X-ray emission from these QSOs would be
the scattered light or leaked light from partially covering absorbing material.
A detection of the unabsorbed soft X-ray and absorbed hard X-ray compo nent
will allow to determine the total column density as well as the effective
covering factor of the absorbing material, which can be hardly obtained from
the UV absorption lines. Brinkmann et al. (1999) showed that both the
unabsorbed and absorbed components are detected in the nearby very bright broad
absorption line quasar PG 1411+442. In this letter, we make a further analysis
of the broad band X-ray spectrum and the UV spectrum from HST, and demonstrate
that broad absorption lines are completely saturated at the bottom of
absorption troughs.Comment: 6 pages, 3 postscript figures. to appear in Astrophy. J. Letter
Recent X-ray observations of intermediate BL Lac objects
We present recent ROSAT, ASCA and SAX observations of intermediate BL Lac
objects (IBLs), i.e. BL Lacs which are located between high-energy and
low-energy peaked BL Lac objects with respect to alpha_rx. Both the statistical
properties of IBLs from the RGB sample and a detailed broad band X-ray spectral
analysis of two objects (1424+2401, 1055+5644) point towards a continuous
distribution of synchrotron emission peak frequencies among BL Lac objects.Comment: 4 pages, 4 figures; to appear in the proceedings of the conference
"BL Lac Phenomenon" held in Turku, Finland, June 22-26, 199
SDSS AGNs with X-ray Emission from ROSAT PSPC Pointed Observations
We present a sample of 1744 type 1 active galactic nuclei (AGNs) from the
Sloan Digital Sky Survey (SDSS DR4) spectroscopic catalog with X-ray
counterparts in the White-Giommi-Angelini Catalog (WGACAT) of ROSAT PSPC
pointed observations. Of 1744 X-ray sources, 1410 (80.9%) are new AGN
identifications. Of 4574 SDSS DR4 AGNs for which we found radio matches in the
catalog of radio sources from the FIRST catalog, 224 turned up in our sample of
SDSS X-ray AGNs. The sample objects are given in a catalog that contains
optical and X-ray parameters along with radio emission parameters where
available. We illustrate the content of our catalog and its potential for AGN
science by providing statistical relationships for the catalog data. The
potential of the morphological information is emphasized by confronting the
statistics of optically resolved and unresolved AGNs. The immediate properties
of the catalog objects include significant correlation of X-ray and optical
fluxes, which is consistent with expectations. Also expected is the decrease of
X-ray flux toward higher redshifts. The X-ray to optical flux ratio for the
unresolved AGNs exhibits a decline toward higher redshifts, in agreement with
previous results. The resolved AGNs, however, display the opposite trend. At a
given optical brightness, X-ray fluxes of radio-quiet AGNs by a factor of 2. We
caution, however, that because of the variety of selection effects present in
both the WGACAT and the SDSS, the interpretation of any relationships based on
our sample of X-ray AGNs requires a careful analysis of these effects.Comment: 34 pages, 18 figure
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