56 research outputs found
The opacity of grains in protoplanetary atmospheres
We have computed the size distribution of silicate grains in the outer
radiative region of the envelope of a protoplanet evolving according to the
scenario of Pollack et al. (1996). Our computation includes grain growth due to
Brownian motion and overtake of smaller grains by larger ones. We also include
the input of new grains due to the breakup of planetesimals in the atmosphere.
We follow the procedure of Podolak (2003), but have speeded it up
significantly. This allows us to test the sensitivity of the code to various
parameters. We have also made a more careful estimate of the resulting grain
opacity. We find that the grain opacity is of the order of $10^{-2}\
\mathrm{cm^2 g^{-1}}\sim{1} \mathrm{cm^2 g^{-1}}$. We
discuss the effect of this on the evolution of the models.Comment: 28 pages, 13 Figs., to be published in Icarus (accepted Sep. 2007
Hydrodynamic electron flow in high-mobility wires
Hydrodynamic electron flow is experimentally observed in the differential
resistance of electrostatically defined wires in the two-dimensional electron
gas in (Al,Ga)As heterostructures. In these experiments current heating is used
to induce a controlled increase in the number of electron-electron collisions
in the wire. The interplay between the partly diffusive wire-boundary
scattering and the electron-electron scattering leads first to an increase and
then to a decrease of the resistance of the wire with increasing current. These
effects are the electronic analog of Knudsen and Poiseuille flow in gas
transport, respectively. The electron flow is studied theoretically through a
Boltzmann transport equation, which includes impurity, electron-electron, and
boundary scattering. A solution is obtained for arbitrary scattering
parameters. By calculation of flow profiles inside the wire it is demonstrated
how normal flow evolves into Poiseuille flow. The boundary-scattering
parameters for the gate-defined wires can be deduced from the magnitude of the
Knudsen effect. Good agreement between experiment and theory is obtained.Comment: 25 pages, RevTeX, 9 figure
Saturn's Interior After the Cassini Grand Finale
We present a review of Saturn's interior structure and thermal evolution,
with a particular focus on work in the past 5 years. Data from the Cassini
mission, including a precise determination of the gravity field from the Grand
Finale orbits, and the still ongoing identification of ring wave features in
Saturn's C-ring tied to seismic modes in the planet, have led to dramatic
advances in our understanding of Saturn's structure. Models that match the
gravity field suggest that differential rotation, as seen in the visible
atmosphere, extends down to at least a depth of 10,000 km (1/6 the
planet's radius). At greater depths, a variety of different investigations all
now point to a deep Saturn rotation rate of 10 hours and 33 minutes. There is
very compelling evidence for a central heavy element concentration (``core''),
that in most recent models is 12-20 Earth masses. Ring seismology strongly
suggests that the core is not entirely compact, but is dilute (mixed in with
the overlying H/He), and has a substantial radial extent, perhaps out to around
one-half of the planet's radius. A wide range of thermal evolution scenarios
can match the planet's current luminosity, with progress on better quantifying
the helium rain scenario hampered by Saturn's poorly known atmospheric helium
abundance. We discuss the relevance of magnetic field data on understanding the
planet's current interior structure. We point towards additional future work
that combines seismology and gravity within a framework that includes
differential rotation, and the utility of a Saturn entry probe.Comment: Invited review. Accepted for publication in "Saturn: The Grand
Finale", K. H. Baines et al., eds., Cambridge University Press. All-new
follow-up to previous 2016 (pre-Grand Finale) review chapter here:
arXiv:1609.0632
The collapse of protoplanetary clumps formed through disc instability: 3D simulations of the pre-dissociation phase
We present 3D smoothed particle hydrodynamics simulations of the collapse of
clumps formed through gravitational instability in the outer part of a
protoplanetary disc. The initial conditions are taken directly from a global
disc simulation, and a realistic equation of state is used to follow the clumps
as they contract over several orders of magnitude in density, approaching the
molecular hydrogen dissociation stage. The effects of clump rotation,
asymmetries, and radiative cooling are studied. Rotation provides support
against fast collapse, but non-axisymmetric modes develop and efficiently
transport angular momentum outward, forming a circumplanetary disc. This
transport helps the clump reach the dynamical collapse phase, resulting from
molecular hydrogen dissociation, on a thousand-year timescale, which is smaller
than timescales predicted by some previous spherical 1D collapse models.
Extrapolation to the threshold of the runaway hydrogen dissociation indicates
that the collapse timescales can be shorter than inward migration timescales,
suggesting that clumps could survive tidal disruption and deliver a proto-gas
giant to distances of even a few AU from the central star.Comment: Accepted for publication in MNRA
Gas Giant Formation with Small Cores Triggered by Envelope Pollution by Icy Planetesimals
We have investigated how envelope pollution by icy planetesimals affects the
critical core mass for gas giant formation and the gas accretion time-scales.
In the core-accretion model, runaway gas accretion is triggered after a core
reaches a critical core mass. All the previous studies on the core-accretion
model assumed that the envelope has the solar composition uniformly. In fact,
the envelope is likely polluted by evaporated materials of icy planetesimals
because icy planetesimals going through the envelope experience mass loss via
strong ablation and most of their masses are deposited in the deep envelope. In
this paper, we have demonstrated that envelope pollution in general lowers the
critical core masses and hastens gas accretion on to the protoplanet because of
the increase in the molecular weight and reduction of adiabatic temperature
gradient. Widely- and highly-polluted envelopes allow smaller cores to form
massive envelopes before disc dissipation. Our results suggest that envelope
pollution in the course of planetary accretion has the potential to trigger gas
giant formation with small cores. We propose that it is necessary to take into
account envelope pollution by icy planetesimals when we discuss gas giant
formation based on the core-accretion model.Comment: 12 pages, 9 figures, 4 tables, Accepted for publication in MNRA
Planetary population synthesis
In stellar astrophysics, the technique of population synthesis has been
successfully used for several decades. For planets, it is in contrast still a
young method which only became important in recent years because of the rapid
increase of the number of known extrasolar planets, and the associated growth
of statistical observational constraints. With planetary population synthesis,
the theory of planet formation and evolution can be put to the test against
these constraints. In this review of planetary population synthesis, we first
briefly list key observational constraints. Then, the work flow in the method
and its two main components are presented, namely global end-to-end models that
predict planetary system properties directly from protoplanetary disk
properties and probability distributions for these initial conditions. An
overview of various population synthesis models in the literature is given. The
sub-models for the physical processes considered in global models are
described: the evolution of the protoplanetary disk, the planets' accretion of
solids and gas, orbital migration, and N-body interactions among concurrently
growing protoplanets. Next, typical population synthesis results are
illustrated in the form of new syntheses obtained with the latest generation of
the Bern model. Planetary formation tracks, the distribution of planets in the
mass-distance and radius-distance plane, the planetary mass function, and the
distributions of planetary radii, semimajor axes, and luminosities are shown,
linked to underlying physical processes, and compared with their observational
counterparts. We finish by highlighting the most important predictions made by
population synthesis models and discuss the lessons learned from these
predictions - both those later observationally confirmed and those rejected.Comment: 47 pages, 12 figures. Invited review accepted for publication in the
'Handbook of Exoplanets', planet formation section, section editor: Ralph
Pudritz, Springer reference works, Juan Antonio Belmonte and Hans Deeg, Ed
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Leveraging Code Generation to Improve Code Retrieval and Summarization via Dual Learning
Code summarization generates brief natural language description given a
source code snippet, while code retrieval fetches relevant source code given a
natural language query. Since both tasks aim to model the association between
natural language and programming language, recent studies have combined these
two tasks to improve their performance. However, researchers have yet been able
to effectively leverage the intrinsic connection between the two tasks as they
train these tasks in a separate or pipeline manner, which means their
performance can not be well balanced. In this paper, we propose a novel
end-to-end model for the two tasks by introducing an additional code generation
task. More specifically, we explicitly exploit the probabilistic correlation
between code summarization and code generation with dual learning, and utilize
the two encoders for code summarization and code generation to train the code
retrieval task via multi-task learning. We have carried out extensive
experiments on an existing dataset of SQL and Python, and results show that our
model can significantly improve the results of the code retrieval task over
the-state-of-art models, as well as achieve competitive performance in terms of
BLEU score for the code summarization task.Comment: Published at The Web Conference (WWW) 2020, full pape
Diffusion on percolation clusters with a bias in topological space: non-universal behaviour
Formation of planetary debris discs around white dwarfs – I. Tidal disruption of an extremely eccentric asteroid
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