4,397 research outputs found
Value Iteration for Long-run Average Reward in Markov Decision Processes
Markov decision processes (MDPs) are standard models for probabilistic
systems with non-deterministic behaviours. Long-run average rewards provide a
mathematically elegant formalism for expressing long term performance. Value
iteration (VI) is one of the simplest and most efficient algorithmic approaches
to MDPs with other properties, such as reachability objectives. Unfortunately,
a naive extension of VI does not work for MDPs with long-run average rewards,
as there is no known stopping criterion. In this work our contributions are
threefold. (1) We refute a conjecture related to stopping criteria for MDPs
with long-run average rewards. (2) We present two practical algorithms for MDPs
with long-run average rewards based on VI. First, we show that a combination of
applying VI locally for each maximal end-component (MEC) and VI for
reachability objectives can provide approximation guarantees. Second, extending
the above approach with a simulation-guided on-demand variant of VI, we present
an anytime algorithm that is able to deal with very large models. (3) Finally,
we present experimental results showing that our methods significantly
outperform the standard approaches on several benchmarks
Nuclear break-up of 11Be
The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be
from the GANIL facility on a 48Ti target by measuring correlations between the
10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to
the emission of a neutron at large angle in the laboratory frame is identified
with the towing mode through its characteristic n-fragment correlation. The
experimental spectra are compared with a model where the time dependent
Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A
good agreement is found between experiment and theory for the shapes of neutron
experimental energies and angular distributions. The spectroscopic factor of
the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission
from the 1p and 1d orbitals is also studied
Topology of event distribution as a generalized definition of phase transitions in finite systems
We propose a definition of phase transitions in finite systems based on
topology anomalies of the event distribution in the space of observations. This
generalizes all the definitions based on the curvature anomalies of
thermodynamical potentials and provides a natural definition of order
parameters. The proposed definition is directly operational from the
experimental point of view. It allows to study phase transitions in Gibbs
equilibria as well as in other ensembles such as the Tsallis ensemble.Comment: 4 pages, 3 figure
Test of the Running of in Decays
The decay rate into hadrons of invariant mass smaller than
can be calculated in QCD assuming global
quark--hadron duality. It is shown that this assumption holds for
~GeV. From measurements of the hadronic mass distribution, the
running coupling constant is extracted in the range
0.7~GeV. At , the result is
. The running of is in good
agreement with the QCD prediction.Comment: 9 pages, 3 figures appended; shortened version with new figures, to
appear in Physical Review Letters (April 1996
Electronic state spectroscopy of methyl formate probed by high resolution VUV photoabsorption, He(I) photoelectron spectroscopy and ab initio calculations
The first ab initio calculations of the vertical excitation energies and oscillator strengths are
presented for the neutral electronic transitions of methyl formate, C2H4O2. The highest resolution VUV photoabsorption spectrum of the molecule yet reported is presented over the wavelength
range 115 to 310 nm (10.8 to 4.0 eV) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series, observed in the photoabsorption spectrum, have been assigned in accordance with new theoretical results. The calculations have
been carried out to determine the excitation energies of the lowest energy ionic states of methyl formate and are compared with a newly recorded He(I) photoelectron spectrum (10.4 to 17.0 eV). New vibrational structure is observed in the first photoelectron band. The photoabsorption
cross-sections have been used to calculate the photolysis lifetime of methyl formate in the upper
stratosphere (20-50 km)
The OSACA Database and a Kinematic Analysis of Stars in the Solar Neighborhood
We transformed radial velocities compiled from more than 1400 published
sources, including the Geneva--Copenhagen survey of the solar neighborhood
(CORAVEL-CfA), into a uniform system based on the radial velocities of 854
standard stars in our list. This enabled us to calculate the average weighted
radial velocities for more than 25~000 HIPPARCOS stars located in the local
Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these
radial velocities together with the stars' coordinates, parallaxes, and proper
motions to determine their Galactic coordinates and space velocities. These
quantities, along with other parameters of the stars, are available from the
continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated
database. We perform a kinematic analysis of the stars by applying an
Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single
and multiple main-sequence stars differ substantially. We used distant
(r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular
velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex
deviation,l=13+-2 degrees, and detect a negative K effect. This negative K
effect is most conspicuous in the motion of A0-A5 giants, and is equal to
K=-13.1+-2.0 km/s/kpc.Comment: 16 pages, 8 figure
Spectroscopic and physical parameters of Galactic O-type stars. I. Effects of rotation and spectral resolving power in the spectral classification of dwarfs and giants
The modern-era spectral classification of O-stars relies on either the
Walborn or the Conti-Mathys scheme. Since both of these approaches have been
developed using low-quality photographic data, their application to
high-quality digital data might not be straightforward and be hampered by
problems and complications that have not yet been appreciated. Using
high-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Silla
and following the premises of the Walborn and Conti classification schemes, we
determined the spectral types and luminosity classes of 19 Galactic O-type
stars and compared them to those attributed by Walborn and Mathys based on
low-quality data. Our analysis reveals that the morphological spectral types
assigned using high-resolution data are systematically later (by up to 1.5
subtypes) then those attributed by Walborn. By means of line-profile
simulations, we show that part of this discrepancy is more likely caused by the
combined effect of stellar rotation and high spectral resolution on the depth
of helium lines used as spectral type indicators. In addition, we demonstrate
that at least for narrow-lined stars the "rotational effect" does not disappear
when the high-resolution spectra are degraded to the resolution of the Walborn
standards. We also find evidence of a systematic difference between our
high-resolution quantitative spectral types and those assigned by Mathys.
Rotation and spectral resolution are important third parameters in the spectral
classification of O-type stars. To obtain reliable spectral classes within the
Walborn approach, the unknown and the standard spectra must be compared at the
same resolution and \vsini. Owing to resolution effects, the Conti approach
might also need to be updated.Comment: paper accepted for publication in A&
On the nature of the Be star HR 7409 (7 Vul)
HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt
and is the massive component of a 69-day spectroscopic binary. The binary
parameters and properties of the Be star measured using high-dispersion spectra
obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence
of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young
(<50-80 Myr), then the companion is a K V star, but, following another, older
evolutionary scenario, the companion is a horizontal-branch star or possibly a
white dwarf star. In the latter scenario, a past episode of mass transfer from
an evolved star onto a less massive dwarf star would be responsible for the
peculiar nature of the present-day, fast-rotating Be star.Comment: Accepted for publication in MNRA
Formation scenarios for the young stellar associations between galactic longitudes l = 280-360 deg
We investigate the spatial distribution, the space velocities and age
distribution of the pre-main sequence (PMS) stars belonging to Ophiuchus, Lupus
and Chamaeleon star-forming regions (SFRs), and of the young early-type star
members of the Scorpius-Centaurus OB association. These young stellar
associations extend over the galactic longitude range from 280 deg. to 360
deg., and are at a distance interval of around 100 and 200 pc. This study is
based on a compilation of distances, proper motions and radial velocities from
the literature for the kinematic properties, and of basic stellar data for the
construction of Hertzsprung-Russel diagrams. Although there was no well-known
OB association in Chamaeleon, the distances and the proper motions of a group
of 21 B- and A-type stars, taken from the Hipparcos Catalogue, lead us to
propose that they form a young association. We show that the young early-type
stars of the OB associations and the PMS stars of the SFRs follow a similar
spatial distribution, i.e., there is no separation between the low and the
high-mass young stars. We find no difference in the kinematics nor in the ages
of these two populations studied. We analyze the different scenarios for the
triggering of large-scale star-formation that have been proposed up to now, and
argue that most probably we are observing a spiral arm that passes close to the
Sun. The alignment of young stars and molecular clouds and the average velocity
of the stars in the opposite direction to the Galactic rotation agree with the
expected behavior of star formation in nearby spiral arms.Comment: 14 pages, 14 postscript figures, accepted for publication in A&
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