2,257 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
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
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; Países BajosFil: D'argenio, Pedro Ruben. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Baeten, J. C. M.. Technische Universiteit Eindhoven; Países Bajos. Centrum Wiskunde & Informatica; Países BajosFil: De Vink, E. P.. Technische Universiteit Eindhoven; Países Bajos. Centrum Wiskunde & Informatica; Países Bajo
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
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
From Capillary Condensation to Interface Localization Transitions in Colloid Polymer Mixtures Confined in Thin Film Geometry
Monte Carlo simulations of the Asakura-Oosawa (AO) model for colloid-polymer
mixtures confined between two parallel repulsive structureless walls are
presented and analyzed in the light of current theories on capillary
condensation and interface localization transitions. Choosing a polymer to
colloid size ratio of q=0.8 and studying ultrathin films in the range of D=3 to
D=10 colloid diameters thickness, grand canonical Monte Carlo methods are used;
phase transitions are analyzed via finite size scaling, as in previous work on
bulk systems and under confinement between identical types of walls. Unlike the
latter work, inequivalent walls are used here: while the left wall has a
hard-core repulsion for both polymers and colloids, at the right wall an
additional square-well repulsion of variable strength acting only on the
colloids is present. We study how the phase separation into colloid-rich and
colloid-poor phases occurring already in the bulk is modified by such a
confinement. When the asymmetry of the wall-colloid interaction increases, the
character of the transition smoothly changes from capillary condensation-type
to interface localization-type. The critical behavior of these transitions is
discussed, as well as the colloid and polymer density profiles across the film
in the various phases, and the correlation of interfacial fluctuations in the
direction parallel to the confining walls. The experimental observability of
these phenomena also is briefly discussed.Comment: 36 pages, 15 figure
Injective Synchronisation: an extension of the authentication hierarchy
Authentication is one of the foremost goals of many security protocols. It is most often formalised as a form of agreement, which expresses that the communicating partners agree on the values of a number of variables. In this paper we formalise and study an intensional form of authentication which we call synchronisation. Synchronisation expresses that the messages are transmitted exactly as prescribed by the protocol description. Synchronisation is a strictly stronger property than agreement for the standard intruder model, because it can be used to detect preplay attacks. In order to prevent replay attacks on simple protocols, we also define injective synchronisation. Given a synchronising protocol, we show that a sufficient syntactic criterion exists that guarantees that the protocol is injective as well
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
Multiple verification in computational modeling of bone pathologies
We introduce a model checking approach to diagnose the emerging of bone
pathologies. The implementation of a new model of bone remodeling in PRISM has
led to an interesting characterization of osteoporosis as a defective bone
remodeling dynamics with respect to other bone pathologies. Our approach allows
to derive three types of model checking-based diagnostic estimators. The first
diagnostic measure focuses on the level of bone mineral density, which is
currently used in medical practice. In addition, we have introduced a novel
diagnostic estimator which uses the full patient clinical record, here
simulated using the modeling framework. This estimator detects rapid (months)
negative changes in bone mineral density. Independently of the actual bone
mineral density, when the decrease occurs rapidly it is important to alarm the
patient and monitor him/her more closely to detect insurgence of other bone
co-morbidities. A third estimator takes into account the variance of the bone
density, which could address the investigation of metabolic syndromes, diabetes
and cancer. Our implementation could make use of different logical combinations
of these statistical estimators and could incorporate other biomarkers for
other systemic co-morbidities (for example diabetes and thalassemia). We are
delighted to report that the combination of stochastic modeling with formal
methods motivate new diagnostic framework for complex pathologies. In
particular our approach takes into consideration important properties of
biosystems such as multiscale and self-adaptiveness. The multi-diagnosis could
be further expanded, inching towards the complexity of human diseases. Finally,
we briefly introduce self-adaptiveness in formal methods which is a key
property in the regulative mechanisms of biological systems and well known in
other mathematical and engineering areas.Comment: In Proceedings CompMod 2011, arXiv:1109.104
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