9,879 research outputs found
Directed Explicit Model Checking with HSF-SPIN
We present the explicit state model checker HSF-SPIN which is based on the model checker SPIN and its Promela modeling language. HSF-SPIN incorporates directed search algorithms for checking safety and a large class of LTL-specified liveness properties. We start off from the A* algorithm and define heuristics to accelerate the search into the direction of a specified failure situation. Next we propose an improved nested depth-first search algorithm that exploits the structure of Promela Never-Claims. As a result of both improvements, counterexamples will be shorter and the explored part of the state space will be smaller than with classical approaches, allowing to analyze larger state spaces. We evaluate the impact of the new heuristics and algorithms on a set of protocol models, some of which are real-world industrial protocols
Spontaneous soliton formation and modulational instability in Bose-Einstein condensates
The dynamics of an elongated attractive Bose-Einstein condensate in an
axisymmetric harmonic trap is studied. It is shown that density fringes caused
by self-interference of the condensate order parameter seed modulational
instability. The latter has novel features in contradistinction to the usual
homogeneous case known from nonlinear fiber optics. Several open questions in
the interpretation of the recent creation of the first matter-wave bright
soliton train [Strecker {\it et al.} Nature {\bf 417} 150 (2002)] are
addressed. It is shown that primary transverse collapse, followed by secondary
collapse induced by soliton--soliton interactions, produce bursts of hot atoms
at different time scales.Comment: 4 pages, 3 figures. Phys. Rev. Lett. in pres
Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)
Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can
provide a valuable probe of warm molecular hydrogen within the interstellar
medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer
archive, are strikingly similar to spectral line maps of the H2 pure rotational
transitions that we obtained with the Infrared Spectrograph (IRS) instrument on
Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed
dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of
the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)
and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,
show band ratios that are inconsistent with the presence of gas at a single
temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent
with pure H2 emission from shocked material with a power-law distribution of
gas temperatures. CO vibrational emissions do not contribute significantly to
the observed Band 2 intensity. Assuming that the column density of H2 at
temperatures T to T+dT is proportional to T raised to the power -b for
temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The
power-law index, b, shows variations over the range 3 to 6 within the set of
different sight-lines probed by the maps, with the majority of sight-lines
showing b in the range 4 to 5. The observed power-law index is consistent with
the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap
A pulsed atomic soliton laser
It is shown that simultaneously changing the scattering length of an
elongated, harmonically trapped Bose-Einstein condensate from positive to
negative and inverting the axial portion of the trap, so that it becomes
expulsive, results in a train of self-coherent solitonic pulses. Each pulse is
itself a non-dispersive attractive Bose-Einstein condensate that rapidly
self-cools. The axial trap functions as a waveguide. The solitons can be made
robustly stable with the right choice of trap geometry, number of atoms, and
interaction strength. Theoretical and numerical evidence suggests that such a
pulsed atomic soliton laser can be made in present experiments.Comment: 11 pages, 4 figure
Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters
As a first step for studying star formation in the extreme outer Galaxy
(EOG), we obtained deep near-infrared images of two embedded clusters at the
northern and southern CO peaks of Cloud 2, which is one of the most distant
star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With
high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the
IRCS imager at the Subaru telescope, we detected cluster members with a mass
detection limit of < 0.1 M_{sun}, which is well into the substellar regime.
These high quality data enables a comparison of EOG to those in the solar
neighborhood on the same basis for the first time. Before interpreting the
photometric result, we have first constructed the NIR color-color diagram
(dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the
Mauna Kea Observatory filter system and also for the low metallicity
environment since the metallicity in EOG is much lower than those in the solar
neighborhood. The estimated stellar density suggests that an ``isolated type''
star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation
is ongoing in Cloud 2-S. Despite the difference of the star formation mode,
other characteristics of the two clusters are found to be almost identical: (1)
K-band luminosity function (KLF) of the two clusters are quite similar, as is
the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the
estimated star formation efficiencies (SFEs) for both clusters are typical
compared to those of embedded clusters in the solar neighborhood (~ 10 %). The
similarity of two independent clusters with a large separation (~ 25 pc)
strongly suggest that their star formation activities were triggered by the
same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap
A lower bound on web services composition
A web service is modeled here as a finite state machine. A composition
problem for web services is to decide if a given web service can be constructed
from a given set of web services; where the construction is understood as a
simulation of the specification by a fully asynchronous product of the given
services. We show an EXPTIME-lower bound for this problem, thus matching the
known upper bound. Our result also applies to richer models of web services,
such as the Roman model
Grey solitons in a strongly interacting superfluid Fermi Gas
The Bardeen-Cooper-Schrieffer to Bose-Einstein condensate (BCS to BEC)
crossover problem is solved for stationary grey solitons via the Boguliubov-de
Gennes equations at zero temperature. These \emph{crossover solitons} exhibit a
localized notch in the gap and a characteristic phase difference across the
notch for all interaction strengths, from BEC to BCS regimes. However, they do
not follow the well-known Josephson-like sinusoidal relationship between
velocity and phase difference except in the far BEC limit: at unitary the
velocity has a nearly linear dependence on phase difference over an extended
range. For fixed phase difference the soliton is of nearly constant depth from
the BEC limit to unitarity and then grows progressively shallower into the BCS
limit, and on the BCS side Friedel oscillations are apparent in both gap
amplitude and phase. The crossover soliton appears fundamentally in the gap; we
show, however, that the density closely follows the gap, and the soliton is
therefore observable. We develop an approximate power law relationship to
express this fact: the density of grey crossover solitons varies as the square
of the gap amplitude in the BEC limit and a power of about 1.5 at unitarity.Comment: 10 pages, 6 figures, part of New Journal of Physics focus issue
"Strongly Correlated Quantum Fluids: From Ultracold Quantum Gases to QCD
Plasmas," in pres
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