9,059 research outputs found
Size-sorting dust grains in the surface layers of protoplanetary disks
Aims: We wish to investigate what the effect of dust sedimentation is on the
observed 10 mum feature of protoplanetary disks and how this may affect the
interpretation of the observations.
Methods: Using a combination of modeling tools, we simulated the
sedimentation of a dust grain size distribution in an axisymmetric 2-D model of
a turbulent protoplanetary disk, and we used a radiative transfer program to
compute the resulting spectra.
Results: We find that the sedimentation can turn a flat feature into a pointy
one, but only to a limited degree and for a very limited set of particle size
distributions. Only if we have a bimodal size distribution, i.e. a very small
grain population and a bigger grain population, do we find that the
transformation from a flat to a pointy feature upon dust sedimentation is
strong. However, our model shows that, if sedimentation is the sole reason for
the variety of silicate feature strengths observed in protoplanetary disks,
then we would expect to find a correlation such that disks with weak mid- to
far-infrared excess have a stronger 10 mum silicate feature than disks with a
strong mid- to far-infrared excess. If this is contrary to what is observed,
then this would indicate that sedimentation cannot be the main reason for the
variety of 10 mum silicate features observed in protoplanetary disks.Comment: Astronomy and Astrophysics, in pres
libcppa - Designing an Actor Semantic for C++11
Parallel hardware makes concurrency mandatory for efficient program
execution. However, writing concurrent software is both challenging and
error-prone. C++11 provides standard facilities for multiprogramming, such as
atomic operations with acquire/release semantics and RAII mutex locking, but
these primitives remain too low-level. Using them both correctly and
efficiently still requires expert knowledge and hand-crafting. The actor model
replaces implicit communication by sharing with an explicit message passing
mechanism. It applies to concurrency as well as distribution, and a lightweight
actor model implementation that schedules all actors in a properly
pre-dimensioned thread pool can outperform equivalent thread-based
applications. However, the actor model did not enter the domain of native
programming languages yet besides vendor-specific island solutions. With the
open source library libcppa, we want to combine the ability to build reliable
and distributed systems provided by the actor model with the performance and
resource-efficiency of C++11.Comment: 10 page
Astrometric Resolution of Severely Degenerate Binary Microlensing Events
We investigate whether the "close/wide" class of degeneracies in
caustic-crossing binary microlensing events can be broken astrometrically.
Dominik showed that these degeneracies are particularly severe because they
arise from a degeneracy in the lens equation itself rather than a mere
"accidental" mimicking of one light curve by another. A massive observing
campaign of five microlensing collaborations was unable to break this
degeneracy photometrically in the case of the binary lensing event MACHO
98-SMC-1. We show that this degeneracy indeed causes the image centroids of the
wide and close solutions to follow an extremely similar pattern of motion
during the time when the source is in or near the caustic. Nevertheless, the
two image centroids are displaced from one another and this displacement is
detectable by observing the event at late times. Photometric degeneracies
therefore can be resolved astrometrically, even for these most severe cases.Comment: 11 pages, including 4 figures. Submitted to Ap
Effect of Core Cooling on the Radius of Sub-Neptune Planets
Sub-Neptune planets are very common in our galaxy and show a large diversity
in their mass-radius relation. In sub-Neptunes most of the planet mass is in
the rocky part (hereafter core) which is surrounded by a modest hydrogen-helium
envelope. As a result, the total initial heat content of such a planet is
dominated by that of the core. Nonetheless, most studies contend that the core
cooling will only have a minor effect on the radius evolution of the gaseous
envelope, because the core's cooling is in sync with the envelope, i.e., most
of the initial heat is released early on timescales of about 10-100 Myr. In
this Letter we examine the importance of the core cooling rate for the thermal
evolution of the envelope. Thus, we relax the early core cooling assumption and
present a model where the core is characterized by two parameters: the initial
temperature and the cooling time. We find that core cooling can significantly
enhance the radius of the planet when it operates on a timescale similar to the
observed age, i.e. several Gyr. Consequently, the interpretation of
sub-Neptunes' mass-radius observations depends on the assumed core thermal
properties and the uncertainty therein. The degeneracy of composition and core
thermal properties can be reduced by obtaining better estimates of the planet
ages (in addition to their radii and masses) as envisioned by future
observations.Comment: Accepted for publication in A&A Letter
HoCHC: A Refutationally Complete and Semantically Invariant System of Higher-order Logic Modulo Theories
We present a simple resolution proof system for higher-order constrained Horn
clauses (HoCHC) - a system of higher-order logic modulo theories - and prove
its soundness and refutational completeness w.r.t. the standard semantics. As
corollaries, we obtain the compactness theorem and semi-decidability of HoCHC
for semi-decidable background theories, and we prove that HoCHC satisfies a
canonical model property. Moreover a variant of the well-known translation from
higher-order to 1st-order logic is shown to be sound and complete for HoCHC in
standard semantics. We illustrate how to transfer decidability results for
(fragments of) 1st-order logic modulo theories to our higher-order setting,
using as example the Bernays-Schonfinkel-Ramsey fragment of HoCHC modulo a
restricted form of Linear Integer Arithmetic
Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 micron interferometers
We present simulations of the interferometric visibilities of Herbig Ae star
disks. We investigate whether interferometric measurements in the 10 micrometer
atmospheric window are sensitive to the presence of an increased scale height
at the inner disk edge, predicted by recent models. Furthermore, we investigate
whether such measurements can discriminate between disks with a ``flaring''
geometry and disks with a ``flat'' geometry. We show that both these questions
can be addressed, using measurements at a small number of appropriately chosen
baselines. The classification of Herbig Ae stars in two groups, based on the
appearance of the spectral energy distribution (SED), has been attributed to a
difference in disk geometry. Sources with a group I SED would have a flaring
outer disk geometry, whereas the disk of group II sources is proposed to be
flat (or ``self-shadowed''). We show that this hypothesis can be tested using
long-baseline interferometric measurements in the micrometer atmospheric
window.Comment: 10 pages, 7 figures, accepted for publiction in Astronomy and
Astrophysic
On the Nature and Location of the Microlenses
This paper uses the caustic crossing events in the microlens data sets to
explore the nature and location of the lenses. We conclude that the large
majority of lenses, whether they are luminous or dark, are likely to be
binaries. Further, we demonstrate that blending is an important feature of all
the data sets. An additional interpretation suggested by the data, that the
caustic crossing events along the directions to the Magellanic Clouds are due
to lenses located in the Clouds, implies that most of the LMC/SMC events to
date are due to lenses in the Magellanic Clouds. All of these conclusions can
be tested. If they are correct, a large fraction of lenses along the direction
to the LMC may be ordinary stellar binary systems, just as are the majority of
the lenses along the direction to the Bulge. Thus, a better understanding of
the larger-than-anticipated value derived for the Bulge optical depth may allow
us to better interpret the large value derived for the optical depth to the
LMC. Indeed, binarity and blending in the data sets may illuminate connections
among several other puzzles: the dearth of binary-source light curves, the
dearth of non-caustic-crossing perturbed binary-lens events, and the dearth of
obviously blended point-lens events.Comment: 15 pages, 2 figures. Submitted to the Astrophysical Journal Letters,
4 January 199
Revisiting Actor Programming in C++
The actor model of computation has gained significant popularity over the
last decade. Its high level of abstraction makes it appealing for concurrent
applications in parallel and distributed systems. However, designing a
real-world actor framework that subsumes full scalability, strong reliability,
and high resource efficiency requires many conceptual and algorithmic additives
to the original model.
In this paper, we report on designing and building CAF, the "C++ Actor
Framework". CAF targets at providing a concurrent and distributed native
environment for scaling up to very large, high-performance applications, and
equally well down to small constrained systems. We present the key
specifications and design concepts---in particular a message-transparent
architecture, type-safe message interfaces, and pattern matching
facilities---that make native actors a viable approach for many robust,
elastic, and highly distributed developments. We demonstrate the feasibility of
CAF in three scenarios: first for elastic, upscaling environments, second for
including heterogeneous hardware like GPGPUs, and third for distributed runtime
systems. Extensive performance evaluations indicate ideal runtime behaviour for
up to 64 cores at very low memory footprint, or in the presence of GPUs. In
these tests, CAF continuously outperforms the competing actor environments
Erlang, Charm++, SalsaLite, Scala, ActorFoundry, and even the OpenMPI.Comment: 33 page
Growth of Dust as the Initial Step Toward Planet Formation
We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.Comment: Protostars and Planets V (PPV) review. 18 pages, 5 figure
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