2,627 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
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
Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves
Many fast supernova remnant shocks show spectra dominated by Balmer lines.
The H profiles have a narrow component explained by direct excitations
and a thermally Doppler broadened component due to atoms that undergo charge
exchange in the post-shock region. However, the standard model does not take
into account the cosmic-ray shock precursor, which compresses and accelerates
plasma ahead of the shock. In strong precursors with sufficiently high
densities, the processes of charge exchange, excitation and ionization will
affect the widths of both narrow and broad line components. Moreover, the
difference in velocity between the neutrals and the precursor plasma gives rise
to frictional heating due to charge exchange and ionization in the precursor.
In extreme cases, all neutrals can be ionized by the precursor.
In this paper we compute the ion and electron heating for a wide range of
shock parameters, along with the velocity distribution of the neutrals that
reach the shock. Our calculations predict very large narrow component widths
for some shocks with efficient acceleration, along with changes in the broad-
to-narrow intensity ratio used as a diagnostic for the electron-ion temperature
ratio. Balmer lines may therefore provide a unique diagnostic of precursor
properties. We show that heating by neutrals in the precursor can account for
the observed H narrow component widths, and that the acceleration
efficiency is modest in most Balmer line shocks observed thus far.Comment: 9 pages, 3 figure
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
Wind modelling of very massive stars up to 300 solar masses
Some studies have claimed a universal stellar upper-mass limit of 150 Msun. A
factor that is often overlooked is that there might be a difference between the
current and initial masses of the most massive stars, as a result of mass loss.
We present Monte Carlo mass-loss predictions for very massive stars in the
range 40-300 Msun, with large luminosities and Eddington factors Gamma. Using
our new dynamical approach, we find an upturn in the mass-loss vs. Gamma
dependence, at the point where the winds become optically thick. This coincides
with the location where wind efficiency numbers surpass the single-scattering
limit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests a
transition from common O-type winds to Wolf-Rayet characteristics at the point
where the winds become optically thick. This transitional behaviour is also
revealed with respect to the wind acceleration parameter beta, which starts at
values below 1 for the optically thin O-stars, and naturally reaches values as
high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding
concerns the transition in spectral morphology of the Of and WN characteristic
He II line at 4686 Angstrom. When we express our mass-loss predictions as a
function of the electron scattering Gamma_e (=L/M) only, we obtain a mass-loss
Gamma dependence that is consistent with a previously reported power-law Mdot
propto Gamma^5 (Vink 2006) that was based on our semi-empirical modelling
approach. When we express Mdot in terms of both Gamma and stellar mass, we find
Mdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm that
the Gamma-effect on the mass-loss predictions is much stronger than that of an
increased helium abundance, calling for a fundamental revision in the way mass
loss is incorporated in evolutionary models of the most massive stars.Comment: minor language changes (Astronomy & Astrophysics in press - 11 pages,
10 figures
The continuum limit of the lattice Gribov problem, and a solution based on Hodge decomposition
We study gauge fixing via the standard local extremization algorithm for
2-dimensional . On a lattice with spherical topology where all
copies are lattice artifacts, we find that the number of these 'Gribov' copies
diverges in the continuum limit. On a torus, we show that lattice artifacts can
lead to the wrong evaluation of the gauge-invariant correlation length, when
measured via a gauge-fixed procedure; this bias does not disappear in the
continuum limit. We then present a new global approach, based on Hodge
decomposition of the gauge field, which produces a unique smooth field in
Landau gauge, and is economically powered by the FFT. We also discuss the use
of this method for examining topological objects, and its extensions to
non-abelian gauge fields.Comment: 6 pages, uuencoded postscript, presented at Lattice 9
Chromomagnetic Instability and Induced Magnetic Field in Neutral Two-Flavor Color Superconductivity
We find that the chromomagnetic instability existing in neutral two- flavor
color superconductivity at moderate densities is removed by the formation of an
inhomogeneous condensate of charged gluons and the corresponding induction of a
magnetic field. It is shown that this inhomogeneous ground state is
energetically favored over a homogeneous one. The spontaneous induction of a
magnetic field in a color superconductor at moderate densities can be of
interest for the astrophysics of compact stellar objects exhibiting strong
magnetic fields as magnetars.Comment: Version to appear in PR
VLTI/AMBER observations of the binary B[e] supergiant HD 327083
HD 327083 is a luminous B type star which exhibits emission lines and an
infrared excess and is therefore classified as a supergiant B[e] star. In
addition, the star is the primary of a close binary system. It is not clear
whether the B[e] behaviour of HD 327083 is related to its binarity or its
evolutionary state. Here we address this issue by studying its circumstellar
environment with high spatial resolution. To this end, we have observed HD
327083 with the VLTI and AMBER in the medium resolution K-band setting. 13CO
bandhead emission is detected, confirming HD 327083 is a post-main sequence
object. The observations spatially resolve the source of the NIR continuum and
the Br-gamma and CO line emission. In addition, differential phase measurements
allow us to probe the origin of the observed Br-gamma emission with sub-mas
precision. Using geometrical models, we find that the visibilities and closure
phases suggest that the close binary system is surrounded by a circum-binary
disk. We also find that in the case of the binary HD 327083, the relative sizes
of the continuum and Br-gamma emitting regions are different to those of a
single supergiant B[e] star where the standard dual outflow scenario is thought
to apply. These findings are consistent with the hypothesis that the mass loss
of HD 327083 is related to its binary nature.Comment: Accepted in A&
The evolution of rotating very massive stars with LMC composition
We present a dense model grid with tailored input chemical composition
appropriate for the Large Magellanic Cloud. We use a one-dimensional
hydrodynamic stellar evolution code, which accounts for rotation, transport of
angular momentum by magnetic fields, and stellar wind mass loss to compute our
detailed models. We calculate stellar evolution models with initial masses of
70-500 Msun and with initial surface rotational velocities of 0-550 km/s,
covering the core-hydrogen burning phase of evolution. We find our rapid
rotators to be strongly influenced by rotationally induced mixing of helium,
with quasi-chemically homogeneous evolution occurring for the fastest rotating
models. Above 160 Msun, homogeneous evolution is also established through mass
loss, producing pure helium stars at core hydrogen exhaustion independent of
the initial rotation rate. Surface nitrogen enrichment is also found for slower
rotators, even for stars that lose only a small fraction of their initial mass.
For models above 150 MZAMS, and for models in the whole considered mass range
later on, we find a considerable envelope inflation due to the proximity of
these models to their Eddington limit. This leads to a maximum zero-age main
sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of
stars in the mass range 50-100 Msun to the regime of luminous blue variables in
the HR diagram with high internal Eddington factors. Inflation also leads to
decreasing surface temperatures during the chemically homogeneous evolution of
stars above 180 Msun. The cool surface temperatures due to the envelope
inflation in our models lead to an enhanced mass loss, which prevents stars at
LMC metallicity from evolving into pair-instability supernovae. The
corresponding spin-down will also prevent very massive LMC stars to produce
long-duration gamma-ray bursts, which might, however, originate from lower
masses.Comment: 21 pages, 25 figure
Reconciling real and stochastic time: The need for probabilistic refinement
We conservatively extend anACP-style discrete-time process theorywith discrete stochastic delays. The semantics of the timed delays relies on time additivity and time determinism, which are properties that enable us to merge subsequent timed delays and to impose their synchronous expiration. Stochastic delays, however, interact with respect to a so-called race condition that determines the set of delays that expire first, which is guided by an (implicit) probabilistic choice. The race condition precludes the property of time additivity as the merger of stochastic delays alters this probabilistic behavior. To this end, we resolve the race condition using conditionally- distributed unit delays. We give a sound and ground-complete axiomatization of the process theory comprising the standard set of ACP-style operators. In this generalized setting, the alternative composition is no longer associative, so we have to resort to special normal forms that explicitly resolve the underlying race condition. Our treatment succeeds in the initial challenge to conservatively extend standard time with stochastic time. However, the 'dissection' of the stochastic delays to conditionally-distributed unit delays comes at a price, as we can no longer relate the resolved race condition to the original stochastic delays. We seek a solution in the field of probabilistic refinements that enable the interchange of probabilistic and non deterministic choices.Fil: Markovski, J.. Technische Universiteit Eindhoven; PaĂses BajosFil: D'argenio, Pedro Ruben. Universidad Nacional de CĂłrdoba. Facultad de Matemática, AstronomĂa y FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Baeten, J. C. M.. Technische Universiteit Eindhoven; PaĂses Bajos. Centrum Wiskunde & Informatica; PaĂses BajosFil: De Vink, E. P.. Technische Universiteit Eindhoven; PaĂses Bajos. Centrum Wiskunde & Informatica; PaĂses Bajo
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