4,009 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
Detection of a high frequency break in the X-ray power spectrum of Ark 564
We present a power spectrum analysis of the long ASCA observation of Ark 564
in June/July 2001. The observed power spectrum covers a frequency range of ~
3.5 decades. We detect a high frequency break at ~ 0.002 Hz. The power spectrum
has an rms of ~30% and a slope of ~ -1 and ~ -2 below and above the break
frequency. When combined with the results from a long RXTE observation (Pounds
et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the
low/hard state) are almost identical, showing a similar shape and rms
amplitude. However, the ratio of the high frequency breaks is very small (~
10e{3-4}), implying that these characteristic frequencies are not indicative of
the black hole mass. This result supports the idea of a small black hole
mass/high accretion rate in Ark 564.Comment: 4 pages, 3 figures, A&A (Letters) in pres
S-Duality and Exact Type IIB Superstring Backgrounds
A geometrical approach in the non-symmetric connection framework is employed
to examine the issue of higher order corrections to D=10 type IIB
superstring backgrounds with a covariantly constant null Killing isometry and
non-zero Ramond-Ramond field content. These describe generalized supersymmetric
string waves and were obtained recently by us through the S-duality
transformation of purely NS-NS plane wave backgrounds. We find that the
backgrounds are exact subject to the existence of certain field redefinitions
and provided certain restrictive conditions are satisfied.Comment: 21 Pages, LATEX fil
Four New BL Lac Surveys: Sampling New Populations
The advent of large area deep radio and X-ray surveys is leading to the
creation of many new BL Lac samples. In particular, the ROSAT All-Sky, Green
Bank and FIRST surveys are proving to be rich sources of new BL Lacs. We will
discuss the methods used in four independent BL Lac searches based on these
surveys. Comparison of the broadband spectral energy distributions of these BL
Lacs with those of previously known objects clearly points to the existence of
a large previously unrecognized population of objects with characteristics
intermediate between those exhibited by Low and High energy peaked BL Lacs.Comment: 4 pages, 3 postscript figures, To be published in the Proceedings of
the conference "BL Lac Phenomenon" held in Turku, Finland, June 22-26, 199
Selective Enzymatic Oxidation of Silanes to Silanols
Compared to the biological world's rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wildâtype cytochrome P450 monooxygenase (P450_(BM3) from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to give silanols. Directed evolution was applied to enhance this nonânative activity and create a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as the terminal oxidant. The evolved enzyme leaves CâH bonds present in the silane substrates untouched, and this biotransformation does not lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the native CâH hydroxylation mechanism of the P450 enzyme. This enzymatic silane oxidation extends nature's impressive catalytic repertoire
Vacua of M-theory and string theory
We argue that supersymmetric higher-dimension operators in the effective
actions of M-theory and IIB string theory do not affect the maximally
supersymmetric vacua: and in M-theory and
in IIB string theory. All these vacua are described in
superspace by a fixed point with all components of supertorsion and
supercurvature being supercovariantly constant. This follows from 32 unbroken
supersymmetries and allows us to prove that such vacua are exact.Comment: 16 pages, late
HST Observations of the Serendipitous X-ray Companion to Mrk 273: Cluster at z=0.46?
We have used HST I-band images to identify Mrk 273X, the very unusual
high-redshift X-ray-luminous Seyfert 2 galaxy found by ROSAT in the same
field-of-view as Mrk 273. We have measured the photometric properties of Mrk
273X and have also analyzed the luminosity distribution of the faint galaxy
population seen in the HST image. The luminosity of the galaxy and the
properties of the surrounding environment suggest that Mrk 273X is the
brightest galaxy in a relatively poor cluster at a redshift near 0.46. Its
off-center location in the cluster and the presence of other galaxy groupings
in the HST image may indicate that this is a dynamically young cluster on the
verge of merging with its neighboring clusters. We find that Mrk 273X is a
bright featureless elliptical galaxy with no evidence for a disk. It follows
the de Vaucouleurs (r^{1/4}) surface brightness law very well over a range of 8
magnitudes. Though the surface brightness profile does not appear to be
dominated by the AGN, the galaxy has very blue colors that do appear to be
produced by the AGN. Mrk 273X is most similar to the IC 5063 class of active
galaxies --- a hybrid Sy 2 / powerful radio galaxy.Comment: Accepted for publication in the Astrophysical Journal. 8 pages,
including 4 postscript figures. Uses emulateapj.sty and psfig.sty. Higher
quality version of Figure 1 is available at
http://rings.gsfc.nasa.gov/~borne/fig1-markgals.gi
Supersymmetric String Waves
We present plane-wave-type solutions of the lowest order superstring
effective action which have unbroken space-time supersymmetries. They describe
dilaton, axion and gauge fields in a stringy generalization of the Brinkmann
metric. Some conspiracy between the metric and the axion field is required. We
show that there exists a special class of these solutions, for which
stringy corrections to the effective on-shell action, to the
equations of motion (and therefore to the solutions themselves), and to the
supersymmetry transformations vanish. We call these solutions supersymmetric
string waves (SSW).Comment: 19 pages, LaTeX, SU-ITP-92-30 and UG-10/9
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