1,470 research outputs found
Influence of viscosity and the adiabatic index on planetary migration
The strength and direction of migration of low mass embedded planets depends
on the disk's thermodynamic state, where the internal dissipation is balanced
by radiative transport, and the migration can be directed outwards, a process
which extends the lifetime of growing embryos. Very important parameters
determining the structure of disks, and hence the direction of migration, are
the viscosity and the adiabatic index. In this paper we investigate the
influence of different viscosity prescriptions (alpha-type and constant) and
adiabatic indices on disk structures and how this affects the migration rate of
planets embedded in such disks. We perform 3D numerical simulations of
accretion disks with embedded planets. We use the explicit/implicit
hydrodynamical code NIRVANA that includes full tensor viscosity and radiation
transport in the flux-limited diffusion approximation, as well as a proper
equation of state for molecular hydrogen. The migration of embedded 20Earthmass
planets is studied. Low-viscosity disks have cooler temperatures and the
migration rates of embedded planets tend toward the isothermal limit. In these
disks, planets migrate inwards even in the fully radiative case. The effect of
outward migration can only be sustained if the viscosity in the disk is large.
Overall, the differences between the treatments for the equation of state seem
to play a more important role in disks with higher viscosity. A change in the
adiabatic index and in the viscosity changes the zero-torque radius that
separates inward from outward migration. For larger viscosities, temperatures
in the disk become higher and the zero-torque radius moves to larger radii,
allowing outward migration of a 20 Earth-mass planet to persist over an
extended radial range. In combination with large disk masses, this may allow
for an extended period of the outward migration of growing protoplanetary
cores
High-Temperature Processing of Solids Through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles
A fundamental, unsolved problem in Solar System formation is explaining the
melting and crystallization of chondrules found in chondritic meteorites.
Theoretical models of chondrule melting in nebular shocks has been shown to be
consistent with many aspects of thermal histories inferred for chondrules from
laboratory experiments; but, the mechanism driving these shocks is unknown.
Planetesimals and planetary embryos on eccentric orbits can produce bow shocks
as they move supersonically through the disk gas, and are one possible source
of chondrule-melting shocks. We investigate chondrule formation in bow shocks
around planetoids through 3D radiation hydrodynamics simulations. A new
radiation transport algorithm that combines elements of flux-limited diffusion
and Monte Carlo methods is used to capture the complexity of radiative
transport around bow shocks. An equation of state that includes the rotational,
vibrational, and dissociation modes of H is also used. Solids are followed
directly in the simulations and their thermal histories are recorded. Adiabatic
expansion creates rapid cooling of the gas, and tail shocks behind the embryo
can cause secondary heating events. Radiative transport is efficient, and bow
shocks around planetoids can have luminosities few
L. While barred and radial chondrule textures could be produced in
the radiative shocks explored here, porphyritic chondrules may only be possible
in the adiabatic limit. We present a series of predicted cooling curves that
merit investigation in laboratory experiments to determine whether the solids
produced by bow shocks are represented in the meteoritic record by chondrules
or other solids.Comment: Accepted for publication in ApJ. Images have been resized to conform
to arXiv limits, but are all readable upon adjusting the zoom. Changes from
v1: Corrected typos discovered in proofs. Most changes are in the appendi
A COMPARATIVE STUDY OF LARVAL GENE EXPRESSION BETWEEN A PAEDOMORPHIC AND METAMORPHIC SPECIES OF AMBYSTOMATID SALAMANDER
Ambystoma tigrinum undergoes an obligatory metamorphosis while A. mexicanum fails to metamorphose and exhibits paedomorphosis. While it is clear that salamander paedomorphosis is associated with genetic changes that delay developmental timing, it is not clear when and how these changes manifest during development. It is possible that paedomorphic and metamorphic larvae show equivalent patterns of developmental until late in the larval period, when brain regions become competent to stimulate the release of metamorphic hormones. To test this hypothesis, I compared gene expression patterns between the brains of A. mexicanum and A. t. tigrinum larvae. In support of the developmental equivalence hypothesis, 114 differentially expressed genes (DEGs) were identified in common between the species and all but 2 showed the same temporal pattern of expression. However, more DEGs were identified uniquely from each species. In particular, several genes that are associated with the hypothalamus-pituitaryinterrenal axis, which is implicated in metamorphic regulation in amphibians, exhibited significant expression differences between A. mexicanum and A. t. tigrinum larvae. The results show that metamorphic and paedomorphic modes of development are associated with different transcriptional programs in the brain and these programs diverge during early larval development
Chemistry in a gravitationally unstable protoplanetary disc
Until now, axisymmetric, alpha-disc models have been adopted for calculations
of the chemical composition of protoplanetary discs. While this approach is
reasonable for many discs, it is not appropriate when self-gravity is
important. In this case, spiral waves and shocks cause temperature and density
variations that affect the chemistry. We have adopted a dynamical model of a
solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc,
similar to those that may be found around Class 0 and early Class I protostars,
in a study of disc chemistry. We find that for each of a number of species,
e.g. H2O, adsorption and desorption dominate the changes in the gas-phase
fractional abundance; because the desorption rates are very sensitive to
temperature, maps of the emissions from such species should reveal the
locations of shocks of varying strengths. The gas-phase fractional abundances
of some other species, e.g. CS, are also affected by gas-phase reactions,
particularly in warm shocked regions. We conclude that the dynamics of massive
discs have a strong impact on how they appear when imaged in the emission lines
of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA
Classification and reduction of pilot error
Human error is a primary or contributing factor in about two-thirds of commercial aviation accidents worldwide. With the ultimate goal of reducing pilot error accidents, this contract effort is aimed at understanding the factors underlying error events and reducing the probability of certain types of errors by modifying underlying factors such as flight deck design and procedures. A review of the literature relevant to error classification was conducted. Classification includes categorizing types of errors, the information processing mechanisms and factors underlying them, and identifying factor-mechanism-error relationships. The classification scheme developed by Jens Rasmussen was adopted because it provided a comprehensive yet basic error classification shell or structure that could easily accommodate addition of details on domain-specific factors. For these purposes, factors specific to the aviation environment were incorporated. Hypotheses concerning the relationship of a small number of underlying factors, information processing mechanisms, and error types types identified in the classification scheme were formulated. ASRS data were reviewed and a simulation experiment was performed to evaluate and quantify the hypotheses
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
Chondrule Formation in Bow Shocks around Eccentric Planetary Embryos
Recent isotopic studies of Martian meteorites by Dauphas & Pourmond (2011)
have established that large (~ 3000 km radius) planetary embryos existed in the
solar nebula at the same time that chondrules - millimeter-sized igneous
inclusions found in meteorites - were forming. We model the formation of
chondrules by passage through bow shocks around such a planetary embryo on an
eccentric orbit. We numerically model the hydrodynamics of the flow, and find
that such large bodies retain an atmosphere, with Kelvin-Helmholtz
instabilities allowing mixing of this atmosphere with the gas and particles
flowing past the embryo. We calculate the trajectories of chondrules flowing
past the body, and find that they are not accreted by the protoplanet, but may
instead flow through volatiles outgassed from the planet's magma ocean. In
contrast, chondrules are accreted onto smaller planetesimals. We calculate the
thermal histories of chondrules passing through the bow shock. We find that
peak temperatures and cooling rates are consistent with the formation of the
dominant, porphyritic texture of most chondrules, assuming a modest enhancement
above the likely solar nebula average value of chondrule densities (by a factor
of 10), attributable to settling of chondrule precursors to the midplane of the
disk or turbulent concentration. We calculate the rate at which a planetary
embryo's eccentricity is damped and conclude that a single planetary embryo
scattered into an eccentric orbit can, over ~ 10e5 years, produce ~ 10e24 g of
chondrules. In principle, a small number (1-10) of eccentric planetary embryos
can melt the observed mass of chondrules in a manner consistent with all known
constraints.Comment: Accepted for publication in The Astrophysical Journa
- …