414 research outputs found
Fast subsumption checks using anti-links
The concept of "anti-link" is defined, and useful
equivalence-preserving operations based on anti-links are
introduced.These operations eliminate a potentially large number
of subsumed paths in a negation normal form formula.Those
anti-links that directly indicate the presence of subsumed paths
are characterized. The operations have linear time complexity in
the size of that part of the formula containing the anti-link.
The problem of removing all subsumed paths in an NNF formula is
shown to be NP-hard, even though such formulas may be small
relative to the size of their path sets. The general problem of
determining whether there exists a pair of subsumed paths
associated with an arbitrary anti-link is shown to be NP-complete.
Additional techniques based on "strictly pure full blocks" are
introduced and are also shown to eliminate redundant subsumption
checks. The effectiveness of these techniques is examined with
respect to some benchmark examples from the literature
Massive Clumps in the NGC 6334 Star Forming Region
We report observations of dust continuum emission at 1.2 mm toward the star
forming region NGC 6334 made with the SEST SIMBA bolometer array. The
observations cover an area of square degrees with approximately
uniform noise. We detected 181 clumps spanning almost three orders of magnitude
in mass (3\Msun \Msun) and with sizes in the range 0.1--1.0 pc.
We find that the clump mass function is well fit with a power law
of the mass with exponent -0.6 (or equivalently ). The
derived exponent is similar to those obtained from molecular line emission
surveys and is significantly different from that of the stellar initial mass
function. We investigated changes in the mass spectrum by changing the
assumptions on the temperature distribution of the clumps and on the
contribution of free-free emission to the 1.2 mm emission, and found little
changes on the exponent. The Cumulative Mass Distribution Function is also
analyzed giving consistent results in a mass range excluding the high-mass end
where a power-law fit is no longer valid. The masses and sizes of the clumps
observed in NGC 6334 indicate that they are not direct progenitors of stars and
that the process of fragmentation determines the distribution of masses later
on or occurs at smaller spatial scales. The spatial distribution of the clumps
in NGC 6334 reveals clustering which is strikingly similar to that exhibited by
young stars in other star forming regions. A power law fit to the surface
density of companions gives .Comment: 16 pages, 11 figures, 4 tables. To appear in the Astrophysical
Journa
A dynamic logic for every season
This paper introduces a method to build dynamic logics with a graded semantics. The construction is parametrized by a structure to support both the spaces of truth and of the domain of computations. Possible instantiations of the method range from classical assertional) dynamic logic to less common graded logics suitable to deal with programs whose transitional semantics exhibits fuzzy or weighted behaviour.This leads to the systematic derivation of program logics tailored to specific program classes
Mass-losing accretion discs around supermassive black holes
We study the effects of outflow/wind on the gravitational stability of
accretion discs around supermassive black holes using a set of analytical
steady-state solutions. Mass-loss rate by the outflow from the disc is assumed
to be a power-law of the radial distance and the amount of the energy and the
angular momentum which are carried away by the wind are parameterized
phenomenologically. We show that the mass of the first clumps at the
self-gravitating radius linearly decreases with the total mass-loss rate of the
outflow. Except for the case of small viscosity and high accretion rate,
generally, the self-gravitating radius increases as the amount of mass-loss by
the outflow increases. Our solutions show that as more angular momentum is lost
by the outflow, then reduction to the mass of the first clumps is more
significant.Comment: Accepted for publication in Astrophysics & Space Scienc
Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei
Homan & Lister (2006) have recently published circular-polarization (CP)
detections for 34 objects in the MOJAVE sample - a set of bright, compact AGN
being monitored by the Very Long Baseline Array at 15 GHz. We report the
detection of 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114), and
confirm the MOJAVE detection of CP in 1633+382. It is generally believed that
the most likely mechanism for the generation of this CP is Faraday conversion
of linear polarization to CP. A helical jet magnetic-field (B-field) geometry
can facilitate this process - linearly polarized emission from the far side of
the jet is converted to CP as it passes through the magnetised plasma at the
front side of the jet on its way toward the observer. In this case, the sign of
the generated CP is essentially determined by the pitch angle and helicity of
the helical B field. We have determined the pitch-angle regimes and helicities
of the helical jet B fields in 8 AGN for which parsec-scale CP has been
detected, and used them to predict the expected CP signs for these AGN if the
CP is generated via conversion in these helical fields. We have obtained the
intriguing result that our predictions agree with the observed signs in all
eight cases, provided that the longitudinal B-field components in the jets
correspond to South magnetic poles. This clearly non-random pattern
demonstrates that the observed CP in AGN is directly associated with the
presence of helical jet B fields. These results suggest that helical B fields
are ubiquitous in AGN jets.Comment: 24 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
The power output of local obscured and unobscured AGN: crossing the absorption barrier with Swift/BAT and IRAS
The Swift/BAT 9-month catalogue of active galactic nuclei (AGN) provides an
unbiased census of local supermassive black hole accretion, and probes to all
but the highest levels of absorption in AGN. We explore a method for
characterising the bolometric output of both obscured and unobscured AGN by
combining the hard X-ray data from Swift/BAT (14-195keV) with the reprocessed
IR emission as seen with the IRAS all-sky surveys. This approach bypasses the
complex modifications to the SED introduced by absorption in the optical, UV
and 0.1-10 keV regimes and provides a long-term, average picture of the
bolometric output of these sources. We broadly follow the approach of Pozzi et
al. for calculating the bolometric luminosities by adding nuclear IR and hard
X-ray luminosities, and consider different approaches for removing non-nuclear
contamination in the large-aperture IRAS fluxes. Using mass estimates from the
M_BH-L_bulge relation, we present the Eddington ratios \lambda_Edd and 2-10 keV
bolometric corrections for a subsample of 63 AGN (35 obscured and 28
unobscured) from the Swift/BAT catalogue, and confirm previous indications of a
low Eddington ratio distribution for both samples. Importantly, we find a
tendency for low bolometric corrections (typically 10-30) for the obscured AGN
in the sample (with a possible rise from ~15 for \lambda_Edd<0.03 to ~32 above
this), providing a hitherto unseen window onto accretion processes in this
class of AGN. This finding is of key importance in calculating the expected
local black hole mass density from the X-ray background since it is composed of
emission from a significant population of such obscured AGN. Analogous studies
with high resolution IR data and a range of alternative models for the torus
emission will form useful future extensions to this work. (Abridged)Comment: 19 pages, 16 figures, 2 tables. Accepted for publication in MNRA
Isotope shift in the dielectronic recombination of three-electron ^{A}Nd^{57+}
Isotope shifts in dielectronic recombination spectra were studied for Li-like
^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance
positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV
(stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j
transitions were deduced. An evaluation of these values within a full QED
treatment yields a change in the mean-square charge radius of ^{142,150}\delta
= -1.36(1)(3) fm^2. The approach is conceptually new and combines the
advantage of a simple atomic structure with high sensitivity to nuclear size.Comment: 10 pages, 3 figures, accepted for publication in Physical Review
Letter
Faraday Conversion and Rotation In Uniformly Magnetized Relativistic Plasmas
We provide precise fitting formulae for Faraday conversion and rotation
coefficients in uniformly magnetized relativistic plasma. The formulae are
immediately applicable to Rotation Measure and Circular Polarization (CP)
production in jets and hot accretion flows. We show the recipe and results for
arbitrary isotropic particle distributions, in particular thermal and
power-law. The exact Faraday conversion coefficient is found to approach zero
with the increasing particle energy. The non-linear corrections of Faraday
conversion and rotation coefficients are found essential for reliable CP inter-
pretation of Sgr A*.Comment: 21 pages, 10 figues, accepted by MNRA
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