3,378 research outputs found
Parameter and Insertion Function Co-synthesis for Opacity Enhancement in Parametric Stochastic Discrete Event Systems
Opacity is a property that characterizes the system's capability to keep its
"secret" from being inferred by an intruder that partially observes the
system's behavior. In this paper, we are concerned with enhancing the opacity
using insertion functions, while at the same time, enforcing the task
specification in a parametric stochastic discrete event system. We first obtain
the parametric Markov decision process that encodes all the possible
insertions. Based on which, we convert this parameter and insertion function
co-synthesis problem into a nonlinear program. We prove that if the output of
this program satisfies all the constraints, it will be a valid solution to our
problem. Therefore, the security and the capability of enforcing the task
specification can be simultaneously guaranteed
Complexity of Detectability, Opacity and A-Diagnosability for Modular Discrete Event Systems
We study the complexity of deciding whether a modular discrete event system
is detectable (resp. opaque, A-diagnosable). Detectability arises in the state
estimation of discrete event systems, opacity is related to the privacy and
security analysis, and A-diagnosability appears in the fault diagnosis of
stochastic discrete event systems. Previously, deciding weak detectability
(opacity, A-diagnosability) for monolithic systems was shown to be
PSPACE-complete. In this paper, we study the complexity of deciding weak
detectability (opacity, A-diagnosability) for modular systems. We show that the
complexities of these problems are significantly worse than in the monolithic
case. Namely, we show that deciding modular weak detectability (opacity,
A-diagnosability) is EXPSPACE-complete. We further discuss a special case where
all unobservable events are private, and show that in this case the problems
are PSPACE-complete. Consequently, if the systems are all fully observable,
then deciding weak detectability (opacity) for modular systems is
PSPACE-complete
Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes
A remarkable discovery of NASA's Kepler mission is the wide diversity in the
average densities of planets of similar mass. After gas disk dissipation, fully
formed planets could interact with nearby planetesimals from a remnant
planetesimal disk. These interactions would often lead to planetesimal
accretion due to the relatively high ratio between the planet size and the hill
radius for typical planets. We present calculations using the open-source
stellar evolution toolkit MESA (Modules for Experiments in Stellar
Astrophysics) modified to include the deposition of planetesimals into the H/He
envelopes of sub-Neptunes (~1-20 MEarth). We show that planetesimal accretion
can alter the mass-radius isochrones for these planets. The same initial planet
as a result of the same total accreted planetesimal mass can have up to ~5%
difference in mean densities several Gyr after the last accretion due to
inherent stochasticity of the accretion process. During the phase of rapid
accretion these differences are more dramatic. The additional energy deposition
from the accreted planetesimals increase the ratio between the planet's radius
to that of the core during rapid accretion, which in turn leads to enhanced
loss of atmospheric mass. As a result, the same initial planet can end up with
very different envelope mass fractions. These differences manifest as
differences in mean densities long after accretion stops. These effects are
particularly important for planets initially less massive than ~10 MEarth and
with envelope mass fraction less than ~10%, thought to be the most common type
of planets discovered by Kepler.Comment: 19 Pages, 10 Figures, 1 Table; Accepted for Publication in the
Astrophysical Journa
The physical properties of extrasolar planets
Tremendous progress in the science of extrasolar planets has been achieved
since the discovery of a Jupiter orbiting the nearby Sun-like star 51 Pegasi in
1995. Theoretical models have now reached enough maturity to predict the
characteristic properties of these new worlds, mass, radius, atmospheric
signatures, and can be confronted with available observations. We review our
current knowledge of the physical properties of exoplanets, internal structure
and composition, atmospheric signatures, including expected biosignatures for
exo-Earth planets, evolution, and the impact of tidal interaction and stellar
irradiation on these properties for the short-period planets. We discuss the
most recent theoretical achievements in the field and the still pending
questions. We critically analyse the different solutions suggested to explain
abnormally large radii of a significant fraction of transiting exoplanets.
Special attention is devoted to the recently discovered transiting objects in
the overlapping mass range between massive planets and low-mass brown dwarfs,
stressing the ambiguous nature of these bodies, and we discuss the possible
observable diagnostics to identify these two distinct populations. We also
review our present understanding of planet formation and critically examine the
different suggested formation mechanisms. We expect the present review to
provide the basic theoretical background to capture the essential of the
physics of exoplanet formation, structure and evolution, and the related
observable signatures.Comment: 62 pages, 15 figures, published in Rep. Prog. Phys, final version
available on http://stacks.iop.org/0034-4885/73/01690
Moving inhomogeneous envelopes of stars
Massive stars are extremely luminous and drive strong winds, blowing a large
part of their matter into the galactic environment before they finally explode
as a supernova. Quantitative knowledge of massive star feedback is required to
understand our Universe as we see it. Traditionally, massive stars have been
studied under the assumption that their winds are homogeneous and stationary,
largely relying on the Sobolev approximation. However, observations with the
newest instruments, together with progress in model calculations, ultimately
dictate a cardinal change of this paradigm: stellar winds are highly
inhomogeneous. Hence, we are now advancing to a new stage in our understanding
of stellar winds. Using the foundations laid by V.V. Sobolev and his school, we
now update and further develop the stellar spectral analysis techniques. New
sophisticated 3-D models of radiation transfer in inhomogeneous expanding media
elucidate the physics of stellar winds and improve classical empiric mass-loss
rate diagnostics. Applications of these new techniques to multiwavelength
observations of massive stars yield consistent and robust stellar wind
parameters.Comment: slightly corrected version of the review for the special issue "V.V.
Sobolev and his Legacy", Journal of Quantitative Spectroscopy and Radiative
Transfe
Optimal Synthesis of Opacity-Enforcing Supervisors for Qualitative and Quantitative Specifications
In this paper, we investigate both qualitative and quantitative synthesis of
optimal privacy-enforcing supervisors for partially-observed discrete-event
systems. We consider a dynamic system whose information-flow is partially
available to an intruder, which is modeled as a passive observer. We assume
that the system has a "secret" that does not want to be revealed to the
intruder. Our goal is to synthesize a supervisor that controls the system in a
least-restrictive manner such that the closed-loop system meets the privacy
requirement. For the qualitative case, we adopt the notion of infinite-step
opacity as the privacy specification by requiring that the intruder can never
determine for sure that the system is/was at a secret state for any specific
instant. If the qualitative synthesis problem is not solvable or the
synthesized solution is too restrictive, then we further investigate the
quantitative synthesis problem so that the secret is revealed (if unavoidable)
as late as possible. Effective algorithms are provided to solve both the
qualitative and quantitative synthesis problems. Specifically, by building
suitable information structures that involve information delays, we show that
the optimal qualitative synthesis problem can be solved as a safety-game. The
optimal quantitative synthesis problem can also be solved as an optimal
total-cost control problem over an augmented information structure. Our work
provides a complete solution to the standard infinite-step opacity control
problem, which has not been solved without assumption on the relationship
between controllable events and observable events. Furthermore, we generalize
the opacity enforcement problem to the numerical setting by introducing the
secret-revelation-time as a new quantitative measure
On Approximate Opacity of Cyber-Physical Systems
Opacity is an important information-flow security property in the analysis of
cyber-physical systems. It captures the plausible deniability of the system's
secret behavior in the presence of an intruder that may access the information
flow. Existing works on opacity only consider non-metric systems by assuming
that the intruder can always distinguish two different outputs precisely. In
this paper, we extend the concept of opacity to systems whose output sets are
equipped with metrics. Such systems are widely used in the modeling of many
real-world systems whose measurements are physical signals. A new concept
called approximate opacity is proposed in order to quantitatively evaluate the
security guarantee level with respect to the measurement precision of the
intruder. Then we propose a new simulation-type relation, called approximate
opacity preserving simulation relation, which characterizes how close two
systems are in terms of the satisfaction of approximate opacity. This allows us
to verify approximate opacity for large-scale, or even infinite systems, using
their abstractions. We also discuss how to construct approximate opacity
preserving symbolic models for a class of discrete-time control systems. Our
results extend the definitions and analysis techniques for opacity from
non-metric systems to metric systems
Triangulating Radiation: Radiative Transfer on Unstructured Grids
We present a new numerical approach that is able to solve the
multi-dimensional radiative transfer equations in all opacity regimes on a
Lagrangian, unstructured network of characteristics based on a stochastic point
process. Our method reverses the limiting procedure used to derive the transfer
equations, by going back to the original Markov process. Thus, we reduce this
highly complex system of coupled differential equations to a simple
one-dimensional random walk on a graph, which is shown to be computationally
very efficient. Specifically, we use a Delaunay graph, which makes it possible
to combine our scheme with a new smoothed particle hydrodynamics (SPH) variant
proposed by Pelupessy et al.(2003). We show that the results of applying a
two-dimensional implementation of our method with various suitable test cases
agree with the analytical results, and we point out the advantages of using our
method with inhomogeneous point distributions, showing examples in the
progress. Hereafter, we present a supplement to our method, which can be useful
in cases where the medium is optically very thin, and we conclude by stating
some anticipated properties of this method in three dimensions, and announce
future extensions.Comment: 19 pages, 19 figures; substantial revision of conten
Giant Impact: An Efficient Mechanism for the Devolatilization of Super-Earths
Mini-Neptunes and volatile-poor super-Earths coexist on adjacent orbits in
proximity to host stars such as Kepler-36 and Kepler-11. Several post-formation
processes have been proposed for explaining the origin of the compositional
diversity: the mass loss via stellar XUV irradiation, degassing of accreted
material, and in-situ accumulation of the disk gas. Close-in planets are also
likely to experience giant impacts during the advanced stage of planet
formation. This study examines the possibility of transforming volatile-rich
super-Earths / mini-Neptunes into volatile-depleted super-Earths through giant
impacts. We present the results of three-dimensional giant impact simulations
in the accretionary and disruptive regimes. Target planets are modeled with a
three-layered structure composed of an iron core, silicate mantle and
hydrogen/helium envelope. In the disruptive case, the giant impact can remove
most of the H/He atmosphere immediately and homogenize the refractory material
in the planetary interior. In the accretionary case, the planet can retain more
than half of the gaseous envelope, while a compositional gradient suppresses
efficient heat transfer as its interior undergoes double-diffusive convection.
After the giant impact, a hot and inflated planet cools and contracts slowly.
The extended atmosphere enhances the mass loss via both a Parker wind induced
by thermal pressure and hydrodynamic escape driven by the stellar XUV
irradiation. As a result, the entire gaseous envelope is expected to be lost
due to the combination of those processes in both cases. We propose that
Kepler-36b may have been significantly devolatilized by giant impacts, while a
substantial fraction of Kepler-36c's atmosphere may remain intact. Furthermore,
the stochastic nature of giant impacts may account for the large dispersion in
the mass--radius relationship of close-in super-Earths and mini-Neptunes.Comment: 8 pages, 8 figures, 1 table, to be published in ApJ, readability
improved according to the proo
On the necessity of composition-dependent low-temperature opacity in metal-poor AGB stars
The vital importance of composition-dependent low-temperature opacity in
low-mass (M < 3Msun) asymptotic giant branch (AGB) stellar models of
metallicity Z > 0.001 has recently been demonstrated (e.g. Marigo 2002; Ventura
& Marigo 2010). Its significance to more metal-poor, intermediate mass (M >
2.5Msun) models has yet to be investigated. We show that its inclusion in
lower-metallicity models ([Fe/H] < -2) is essential, and that there exists no
threshold metallicity below which composition-dependent molecular opacity may
be neglected. We find it to be crucial in all intermediate-mass models
investigated ([Fe/H] < -2 and 2.5 < M/Msun < 5), because of the evolution of
the surface chemistry, including the orders of magnitude increase in the
abundance of molecule-forming species. Its effect on these models mirrors that
previously reported for higher-metallicity models - increase in radius,
decrease in Teff, faster mass loss, shorter thermally pulsing AGB lifetime,
reduced enrichment in third dredge-up products (by a factor of three to ten),
and an increase in the mass limit for hot bottom burning. We show that the
evolution of low-metallicity models with composition-dependent low-temperature
opacity is relatively independent of initial metal abundance because its
contribution to the opacity is far outweighed by changes due to dredge-up. Our
results imply a significant reduction in the expected number of
nitrogen-enhanced metal-poor stars, which may help explain their observed
paucity. We note that these findings are partially a product of the
macrophysics adopted in our models, in particular the Vassiliadis & Wood (1993)
mass loss rate which is strongly dependent on radius.Comment: 13 pages, 13 figures, 2 tables; accepted for publication in The
Astrophysical Journa
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