2,859 research outputs found
Dynamic System Adaptation by Constraint Orchestration
For Paradigm models, evolution is just-in-time specified coordination
conducted by a special reusable component McPal. Evolution can be treated
consistently and on-the-fly through Paradigm's constraint orchestration, also
for originally unforeseen evolution. UML-like diagrams visually supplement such
migration, as is illustrated for the case of a critical section solution
evolving into a pipeline architecture.Comment: 19 page
Bisimulation of Labelled State-to-Function Transition Systems Coalgebraically
Labeled state-to-function transition systems, FuTS for short, are
characterized by transitions which relate states to functions of states over
general semirings, equipped with a rich set of higher-order operators. As such,
FuTS constitute a convenient modeling instrument to deal with process languages
and their quantitative extensions in particular. In this paper, the notion of
bisimulation induced by a FuTS is addressed from a coalgebraic point of view. A
correspondence result is established stating that FuTS-bisimilarity coincides
with behavioural equivalence of the associated functor. As generic examples,
the equivalences underlying substantial fragments of major examples of
quantitative process algebras are related to the bisimilarity of specific FuTS.
The examples range from a stochastic process language, PEPA, to a language for
Interactive Markov Chains, IML, a (discrete) timed process language, TPC, and a
language for Markov Automata, MAL. The equivalences underlying these languages
are related to the bisimilarity of their specific FuTS. By the correspondence
result coalgebraic justification of the equivalences of these calculi is
obtained. The specific selection of languages, besides covering a large variety
of process interaction models and modelling choices involving quantities,
allows us to show different classes of FuTS, namely so-called simple FuTS,
combined FuTS, nested FuTS, and general FuTS
Combining Insertion and Deletion in RNA-editing Preserves Regularity
Inspired by RNA-editing as occurs in transcriptional processes in the living
cell, we introduce an abstract notion of string adjustment, called guided
rewriting. This formalism allows simultaneously inserting and deleting
elements. We prove that guided rewriting preserves regularity: for every
regular language its closure under guided rewriting is regular too. This
contrasts an earlier abstraction of RNA-editing separating insertion and
deletion for which it was proved that regularity is not preserved. The
particular automaton construction here relies on an auxiliary notion of slice
sequence which enables to sweep from left to right through a completed rewrite
sequence.Comment: In Proceedings MeCBIC 2012, arXiv:1211.347
Narrow absorption features in the co-added XMM-Newton RGS spectra of isolated Neutron Stars
We co-added the available XMM-Newton RGS spectra for each of the isolated
X-ray pulsars RX\,J0720.43125, RX\,J1308.6+2127 (RBS\,1223),
RX\,J1605.3+3249 and RX\,J1856.43754 (four members of the "Magnificent
Seven") and the "Three Musketeers" Geminga, PSR\,B0656+14 and PSR\,B1055-52. We
confirm the detection of a narrow absorption feature at 0.57 keV in the
co-added RGS spectra of RX\,J0720.43125 and RX\,J1605.3+3249 (including most
recent observations). In addition we found similar absorption features in the
spectra of RX\,J1308.6+2127 (at 0.53 keV) and maybe PSR\,B1055-52 (at 0.56
keV). The absorption feature in the spectra of RX\,J1308.6+2127 is broader than
the feature e.g. in RX\,J0720.43125. The narrow absorption features are
detected with 2 to 5.6 significance. Although very bright and
frequently observed, there are no absorption features visible in the spectra of
RX\,J1856.43754 and PSR\,B0656+14, while the co-added XMM-Newton RGS
spectrum of Geminga has not enough counts to detect such a feature. We discuss
a possible origin of these absorption features as lines caused by the presence
of highly ionised oxygen (in particular OVII and/or OVI at 0.57 keV) in the
interstellar medium and absorption in the neutron star atmosphere, namely the
absorption features at 0.57 keV as gravitational redshifted (=1.17)
OVIII.Comment: 14 pages, 10 figures and 10 tables. Accepted for publication by MNRAS
(Sep 12th, 2011
Advances in mass-loss predictions
We present the results of Monte Carlo mass-loss predictions for massive stars
covering a wide range of stellar parameters. We critically test our predictions
against a range of observed mass-loss rates -- in light of the recent
discussions on wind clumping. We also present a model to compute the
clumping-induced polarimetric variability of hot stars and we compare this with
observations of Luminous Blue Variables, for which polarimetric variability is
larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an
ideal testbed for studies of wind clumping and wind geometry, as well as for
wind strength calculations, and we propose they may be direct supernova
progenitors.Comment: 3 pages, 3 figures, to appear in the proceedings of workshop
'Clumping in Hot Star Winds', eds. W.-R. Hamann, A. Feldmeier, & L. Oskinov
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
Logical Characterization of Bisimulation for Transition Relations over Probability Distributions with Internal Actions
In recent years the study of probabilistic transition systems has shifted to transition relations over distributions to allow for a smooth adaptation of the standard non-probabilistic apparatus. In this paper we study transition relations over probability distributions in a setting with internal actions. We provide new logics that characterize probabilistic strong, weak and branching bisimulation. Because these semantics may be considered too strong in the probabilistic context, Eisentraut et al. recently proposed weak distribution bisimulation. To show the flexibility of our approach based on the framework of van Glabbeek for the non-deterministic setting, we provide a novel logical characterization for the latter probabilistic equivalence as well
Rooted branching bisimulation as a congruence for probabilistic transition systems
Ponencia presentada en el 13 International Workshop on Quantitative Aspects of Programming Languages and Systems. London, United Kingdom, April 11-12, 2015.We propose a probabilistic transition system specification format, referred to as probabilistic RBB safe, for which rooted branching bisimulation is a congruence. The congruence theorem is based on the approach of Fokkink for the qualitative case. For this to work, the theory of transition system specifications in the setting of labeled transition systems needs to be extended to deal with probability distributions, both syntactically and semantically. We provide a scheduler-free characterization of probabilistic branching bisimulation as adapted from work of Andova et al. for the alternating model. Counter examples are given to justify the various conditions required by the format.Fil: Lee, Matías David. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: De Vink, Erik P. Eindhoven University of Technology; The Netherlands.Fil: De Vink, Erik P. Centrum Wiskunde & Informatica; The Netherlands.Ciencias de la Computació
Evidence for precession of the isolated neutron star RX J0720.4-3125
The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 obtained
over 4.5 years can be described by sinusoidal variations in the inferred
blackbody temperature, the size of the emitting area and the depth of the
absorption line with a period of 7.1 +/- 0.5 years, which we suggest to be the
precession period of the neutron star. Precession of a neutron star with two
hot spots of different temperature and size, probably not located exactly in
antipodal positions, may account for the variations in the X-ray spectra,
changes in the pulsed fraction, shape of the light curve and the phase-lag
between soft and hard energy bands observed from RX J0720.4-3125. An
independent sinusoidal fit to published and new pulse timing residuals from a
coherent analysis covering ~12 years yields a consistent period of 7.7 +/- 0.6
years supporting the precession model.Comment: Accepted for publication in A&A Letters, 5 pages, 5 figure
Optical and X-ray observations of candidate isolated neutron stars in the G315.4-2.3 SNR
G315.4-2.3 is a young Galactic supernova remnant (SNR), whose identification
as the remains of a Type-II supernova (SN) explosion has been debated for a
long time. In particular, recent multi-wavelength observations suggest that it
is the result of a Type Ia SN, based on spectroscopy of the SNR shell and the
lack of a compact stellar remnant.However, two X-ray sources, one detected by
Einstein and ROSAT (Source V) and the other by Chandra (Source N) have been
proposed as possible isolated neutron star candidates. In both cases, no clear
optical identification was available and, therefore, we performed an optical
and X-ray study to determine the nature of these two sources. Based on Chandra
astrometry, Source V is associated with a bright V~14 star, which had been
suggested based on the less accurate ROSAT position. Similarly, from VLT
archival observations, we found that Source N is associated with a relatively
bright star (). These likely identifications suggest that both X-ray
sources cannot be isolated neutron stars.Comment: 4 pages, 4 figures, accepted for publication in MNRA
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