146 research outputs found
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
Planetary migration in evolving planetesimals discs
In the current paper, we further improved the model for the migration of
planets introduced in Del Popolo et al. (2001) and extended to time-dependent
planetesimal accretion disks in Del Popolo and Eksi (2002). In the current
study, the assumption of Del Popolo and Eksi (2002), that the surface density
in planetesimals is proportional to that of gas, is released. In order to
obtain the evolution of planetesimal density, we use a method developed in
Stepinski and Valageas (1997) which is able to simultaneously follow the
evolution of gas and solid particles for up to 10^7 yrs. Then, the disk model
is coupled to migration model introduced in Del Popolo et al. (2001) in order
to obtain the migration rate of the planet in the planetesimal. We find that
the properties of solids known to exist in protoplanetary systems, together
with reasonable density profiles for the disk, lead to a characteristic radius
in the range 0.03-0.2 AU for the final semi-major axis of the giant planet.Comment: IJMP A in prin
Growth and migration of solids in evolving protostellar disks I: Methods and Analytical tests
This series of papers investigates the early stages of planet formation by
modeling the evolution of the gas and solid content of protostellar disks from
the early T Tauri phase until complete dispersal of the gas. In this first
paper, I present a new set of simplified equations modeling the growth and
migration of various species of grains in a gaseous protostellar disk evolving
as a result of the combined effects of viscous accretion and photo-evaporation
from the central star. Using the assumption that the grain size distribution
function always maintains a power-law structure approximating the average
outcome of the exact coagulation/shattering equation, the model focuses on the
calculation of the growth rate of the largest grains only. The coupled
evolution equations for the maximum grain size, the surface density of the gas
and the surface density of solids are then presented and solved
self-consistently using a standard 1+1 dimensional formalism. I show that the
global evolution of solids is controlled by a leaky reservoir of small grains
at large radii, and propose an empirically derived evolution equation for the
total mass of solids, which can be used to estimate the total heavy element
retention efficiency in the planet formation paradigm. Consistency with
observation of the total mass of solids in the Minimum Solar Nebula augmented
with the mass of the Oort cloud sets strong upper limit on the initial grain
size distribution, as well as on the turbulent parameter \alphat. Detailed
comparisons with SED observations are presented in a following paper.Comment: Submitted to ApJ. 23 pages and 13 figure
Oncometabolites:linking altered metabolism with cancer
The discovery of cancer-associated mutations in genes encoding key metabolic enzymes has provided a direct link between altered metabolism and cancer. Advances in mass spectrometry and nuclear magnetic resonance technologies have facilitated high-resolution metabolite profiling of cells and tumors and identified the accumulation of metabolites associated with specific gene defects. Here we review the potential roles of such "oncometabolites" in tumor evolution and as clinical biomarkers for the detection of cancers characterized by metabolic dysregulation
Dusty gas with SPH - II. Implicit timestepping and astrophysical drag regimes
In a companion paper (Laibe & Price 2011b), we have presented an algorithm
for simulating two-fluid gas and dust mixtures in Smoothed Particle
Hydrodynamics (SPH). In this paper, we develop an implicit timestepping method
that preserves the exact conservation of the both linear and angular momentum
in the underlying SPH algorithm, but unlike previous schemes, allows the
iterations to converge to arbitrary accuracy and is suited to the treatment of
non- linear drag regimes. The algorithm presented in Paper I is also extended
to deal with realistic astrophysical drag regimes, including both linear and
non-linear Epstein and Stokes drag. The scheme is benchmarked against the test
suite presented in Paper I, including i) the analytic solutions of the dustybox
problem and ii) solutions of the dustywave, dustyshock, dustysedov and
dustydisc obtained with explicit timestepping. We find that the implicit method
is 1- 10 times faster than the explicit temporal integration when the ratio r
between the the timestep and the drag stopping time is 1 < r < 1000.Comment: Accepted for publication in MNRA
Material enhancement in protoplanetary nebulae by particle drift through evaporation fronts
Solid material in a protoplanetary nebula is subject to vigorous
redistribution processes relative to the nebula gas. Meter-sized particles
drift rapidly inwards near the nebula midplane, and material evaporates when
the particles cross a condensation/evaporation boundary. The material cannot be
removed as fast in its vapor form as it is being supplied in solid form, so its
concentration increases locally by a large factor (more than an order of
magnitude under nominal conditions). As time goes on, the vapor phase
enhancement propagates for long distances inside the evaporation boundary
(potentially all the way in to the star). Meanwhile, material is enhanced in
its solid form over a characteristic lengthscale outside the evaporation
boundary. This effect is applicable to any condensible (water, silicates, {\it
etc.}). Three distinct radial enhancement/depletion regimes can be discerned by
use of a simple model. Meteoritics applications include oxygen fugacity and
isotopic variations, as well as isotopic homogenization in silicates. Planetary
system applications include more robust enhancement of solids in Jupiter's core
formation region than previously suggested. Astrophysical applications include
differential, time-dependent enhancement of vapor phase CO and HO in the
terrestrial planet regions of actively accreting protoplanetary disks.Comment: To appear in Astrophys. J., vol 614, Oct 10 2004 issu
The Eccentricity-Mass Distribution of Exoplanets: Signatures of Different Formation Mechanisms?
We examine the distributions of eccentricity and host star metallicity of
exoplanets as a function of their mass. Planets with M sin i >~ 4 M_J have an
eccentricity distribution consistent with that of binary stars, while planets
with M sin i <~ 4 M_J are less eccentric than binary stars and more massive
planets. In addition, host star metallicities decrease with planet mass. The
statistical significance of both of these trends is only marginal with the
present sample of exoplanets. To account for these trends, we hypothesize that
there are two populations of gaseous planets: the low-mass population forms by
gas accretion onto a rock-ice core in a circumstellar disk and is more abundant
at high metalliticities, and the high-mass population forms directly by
fragmentation of a pre-stellar cloud. Planets of the first population form in
initially circular orbits and grow their eccentricities later, and may have a
mass upper limit from the total mass of the disk that can be accreted by the
core. The second population may have a mass lower limit resulting from
opacity-limited fragmentation. This would roughly divide the two populations in
mass, although they would likely overlap over some mass range. If most objects
in the second population form before the pre-stellar cloud becomes highly
opaque, they would have to be initially located in orbits larger than ~30 AU,
and would need to migrate to the much smaller orbits in which they are
observed. The higher mean orbital eccentricity of the second population might
be caused by the larger required intervals of radial migration, and the brown
dwarf desert might be due to the inability of high-mass brown dwarfs to migrate
inwards sufficiently in radius.Comment: 7 pages, 4 figures. Version with expanded discussion section.
Accepted for publication in A&
Dust Size Growth and Settling in a Protoplanetary Disk
We have studied dust evolution in a quiescent or turbulent protoplanetary
disk by numerically solving coagulation equation for settling dust particles,
using the minimum mass solar nebular model. As a result, if we assume an
ideally quiescent disk, the dust particles settle toward the disk midplane to
form a gravitationally unstable layer within 2x10^3 - 4x10^4 yr at 1 - 30 AU,
which is in good agreement with an analytic calculation by Nakagawa, Sekiya, &
Hayashi (1986) although they did not take into account the particle size
distribution explicitly. In an opposite extreme case of a globally turbulent
disk, on the other hand, the dust particles fluctuate owing to turbulent motion
of the gas and most particles become large enough to move inward very rapidly
within 70 - 3x10^4 yr at 1 - 30 AU, depending on the strength of turbulence.
Our result suggests that global turbulent motion should cease for the
planetesimal formation in protoplanetary disks.Comment: 27 pages, 8 figures, accepted for publication in the Ap
Dust distribution in protoplanetary disks - Vertical settling and radial migration
We present the results of a three dimensional, locally isothermal,
non-self-gravitating SPH code which models protoplanetary disks with two
fluids: gas and dust. We ran simulations of a 1 Msun star surrounded by a 0.01
Msun disk comprising 99% gas and 1% dust in mass and extending from 0.5 to ~300
AU. The grain size ranges from 0.001 mm to 10 m for the low resolution (~25 000
SPH particles) simulations and from 0.1 mm to 10 cm for the high resolution
(~160 000 SPH particles) simulations. Dust grains are slowed down by the
sub-Keplerian gas and lose angular momentum, forcing them to migrate towards
the central star and settle to the midplane. The gas drag efficiency varies
according to the grain size, with the larger bodies being weakly influenced and
following marginally perturbed Keplerian orbits, while smaller grains are
strongly coupled to the gas. For intermediate sized grains, the drag force
decouples the dust and gas, allowing the dust to preferentially migrate
radially and efficiently settle to the midplane. The resulting dust
distributions for each grain size will indicate, when grain growth is added,
the regions when planets are likely to form.Comment: Accepted for publication in Astronomy & Astrophysics. 11 pages, 6
figure
Derivation of the Mass Distribution of Extrasolar Planets with MAXLIMA - a Maximum Likelihood Algorithm
We construct a maximum-likelihood algorithm - MAXLIMA, to derive the mass
distribution of the extrasolar planets when only the minimum masses are
observed. The algorithm derives the distribution by solving a numerically
stable set of equations, and does not need any iteration or smoothing. Based on
50 minimum masses, MAXLIMA yields a distribution which is approximately flat in
log M, and might rise slightly towards lower masses. The frequency drops off
very sharply when going to masses higher than 10 Jupiter masses, although we
suspect there is still a higher mass tail that extends up to probably 20
Jupiter masses. We estimate that 5% of the G stars in the solar neighborhood
have planets in the range of 1-10 Jupiter masses with periods shorter than 1500
days. For comparison we present the mass distribution of stellar companions in
the range of 100--1000 Jupiter masses, which is also approximately flat in log
M. The two populations are separated by the "brown-dwarf desert", a fact that
strongly supports the idea that these are two distinct populations. Accepting
this definite separation, we point out the conundrum concerning the
similarities between the period, eccentricity and even mass distribution of the
two populations.Comment: 19 pages, 3 figures, submitted to The Astrophysical Journa
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