100 research outputs found
Can dust coagulation trigger streaming instability?
Streaming instability can be a very efficient way of overcoming growth and
drift barriers to planetesimal formation. However, it was shown that strong
clumping, which leads to planetesimal formation, requires a considerable number
of large grains. State-of-the-art streaming instability models do not take into
account realistic size distributions resulting from the collisional evolution
of dust. We investigate whether a sufficient quantity of large aggregates can
be produced by sticking and what the interplay of dust coagulation and
planetesimal formation is. We develop a semi-analytical prescription of
planetesimal formation by streaming instability and implement it in our dust
coagulation code based on the Monte Carlo algorithm with the representative
particles approach. We find that planetesimal formation by streaming
instability may preferentially work outside the snow line, where sticky icy
aggregates are present. The efficiency of the process depends strongly on local
dust abundance and radial pressure gradient, and requires a super-solar
metallicity. If planetesimal formation is possible, the dust coagulation and
settling typically need ~100 orbits to produce sufficiently large and settled
grains and planetesimal formation lasts another ~1000 orbits. We present a
simple analytical model that computes the amount of dust that can be turned
into planetesimals given the parameters of the disk model.Comment: 12 pages, 6 figures, 1 table, accepted for publication in A&A (minor
corrections with respect to v1
Planetesimal formation during protoplanetary disk buildup
Models of dust coagulation and subsequent planetesimal formation are usually
computed on the backdrop of an already fully formed protoplanetary disk model.
At the same time, observational studies suggest that planetesimal formation
should start early, possibly even before the protoplanetary disk is fully
formed. In this paper, we investigate under which conditions planetesimals
already form during the disk buildup stage, in which gas and dust fall onto the
disk from its parent molecular cloud. We couple our earlier planetesimal
formation model at the water snow line to a simple model of disk formation and
evolution. We find that under most conditions planetesimals only form after the
buildup stage when the disk becomes less massive and less hot. However, there
are parameters for which planetesimals already form during the disk buildup.
This occurs when the viscosity driving the disk evolution is intermediate
() while the turbulent mixing of the dust is
reduced compared to that (), and with the assumption
that water vapor is vertically well-mixed with the gas. Such scenario could be expected for layered accretion, where the gas flow
is mostly driven by the active surface layers, while the midplane layers, where
most of the dust resides, are quiescent.Comment: 6 pages, 5 figures, accepted for publication in A&A, minor changes
due to language editio
Forming chondrules in impact splashes. I. Radiative cooling model
The formation of chondrules is one of the oldest unsolved mysteries in
meteoritics and planet formation. Recently an old idea has been revived: the
idea that chondrules form as a result of collisions between planetesimals in
which the ejected molten material forms small droplets which solidify to become
chondrules. Pre-melting of the planetesimals by radioactive decay of 26Al would
help producing sprays of melt even at relatively low impact velocity. In this
paper we study the radiative cooling of a ballistically expanding spherical
cloud of chondrule droplets ejected from the impact site. We present results
from a numerical radiative transfer models as well as analytic approximate
solutions. We find that the temperature after the start of the expansion of the
cloud remains constant for a time t_cool and then drops with time t
approximately as T ~ T_0[(3/5)t/t_cool+ 2/5]^(-5/3) for t>t_cool. The time at
which this temperature drop starts t_cool depends via an analytical formula on
the mass of the cloud, the expansion velocity and the size of the chondrule.
During the early isothermal expansion phase the density is still so high that
we expect the vapor of volatile elements to saturate so that no large volatile
losses are expected
Structure and evolution of protoplanetary disks
We present here a few thoughts on how high-angular resolution observations can give clues to some properties of protoplanetary disks that are fundamental to theories of planet formation. High-angular resolution infrared spectroscopy, either with a large single mirror telescope, or by using infrared interferometry, allows us to probe the abundance of thermally processed dust in the disk as a function of distance to the star. We show that this radial abundance profile can give information about the early evolution of the protoplanetary disk as well as about the nature of the turbulence. Since turbulence is one of the main ingredients in theories of planet formation, this latter result is particularly important. We also show that Nature itself provides an interesting way to perform high-angular resolution observations with intermediate-angular resolution telescopes: if a disk has a (nearly) edge-on orientation and is located in a low-density ambient dusty medium, the disk casts a shadow into this medium, as it blocks the starlight in equatorial direction. We argue how these shadows can be used to characterize the dust in the disk
The effect of Jupiter on the CAI storage problem
By studying the distribution of calcium-aluminium-rich inclusions (CAIs) that
are embedded within meteorites, we can learn about the dynamical history of the
protoplanetary disk from which our Solar System formed. A long-standing problem
concerning CAIs is the CAI storage problem. CAIs are thought to have formed at
high temperatures near the Sun, but they are primarily found in carbonaceous
chondrites, which formed much further out, beyond the orbit of Jupiter.
Additionally, radial drift of CAI particles should have removed them from the
solar protoplanetary disk several million years before the parent bodies of
meteorites in which they are encountered would have accreted. We revisit a
previously suggested solution to the CAI storage problem by Desch, Kalyaan, and
Alexander which proposed that CAIs were mixed radially outward through the disk
and subsequently got trapped in a pressure maximum created by Jupiter's growing
core opening a planet gap. Our aim is to investigate whether their solution
still works when we take into account the infall phase during which the disk
builds up from the collapse of a molecular cloud core. We build a 1D numerical
code in Python using the DISKLAB package to simulate the evolution of the solar
protoplanetary disk, starting with a collapsing molecular cloud. We find that
outward transport of CAIs during the infall phase is very efficient, possibly
mixing them all the way into the far outer disk. Subsequent inward radial drift
collects CAIs in the pressure maximum beyond Jupiter's orbit while draining the
inner disk, roughly reproducing parts of the result by Desch et al. By
introducing CAI formation so early, abundances out to 100 AU remain
significant, possibly not consistent with some meteoritic data. It is possible
to create a disk that does not expand as far out and also does not push CAIs as
far out by using a very slowly rotating cloud
Model atmospheres of irradiated exoplanets: The influence of stellar parameters, metallicity, and the C/O ratio
Many parameters constraining the spectral appearance of exoplanets are still
poorly understood. We therefore study the properties of irradiated exoplanet
atmospheres over a wide parameter range including metallicity, C/O ratio and
host spectral type. We calculate a grid of 1-d radiative-convective atmospheres
and emission spectra. We perform the calculations with our new
Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary
Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric
structures and spectra are made available online. We find that atmospheres of
planets with C/O ratios 1 and 1500 K can exhibit
inversions due to heating by the alkalis because the main coolants CH,
HO and HCN are depleted. Therefore, temperature inversions possibly occur
without the presence of additional absorbers like TiO and VO. At low
temperatures we find that the pressure level of the photosphere strongly
influences whether the atmospheric opacity is dominated by either water (for
low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot,
carbon-rich objects this pressure level governs whether the atmosphere is
dominated by methane or HCN. Further we find that host stars of late spectral
type lead to planetary atmospheres which have shallower, more isothermal
temperature profiles. In agreement with prior work we find that for planets
with 1750 K the transition between water or methane dominated
spectra occurs at C/O 0.7, instead of 1, because condensation
preferentially removes oxygen.Comment: 30 pages, 20 figures. Accepted for publication in Ap
Self-Sustaining Vortices in Protoplanetary Disks: Setting the Stage for Planetary System Formation
The core accretion scenario of planet formation assumes that planetesimals
and planetary embryos are formed during the primordial, gaseous phases of the
protoplanetary disk. However, how the dust particles overcome the traditional
growth barriers is not well understood. The recently proposed viscous
ring-instability may explain the concentric rings observed in protoplanetary
disks by assuming that the dust grains can reduce the gas conductivity, which
can weaken the magneto-rotational instability. We present an analysis of this
model with the help of GPU-based numerical hydrodynamic simulations of coupled
gas and dust in the thin-disk limit. During the evolution of the disk the dusty
rings become Rossby unstable and break up into a cascade of small-scale
vortices. The vortices form secularly stable dusty structures, which could be
sites of planetesimal formation by the streaming instability as well as direct
gravitational collapse. The phenomenon of self-sustaining vortices is
consistent with observational constraints of exoplanets and sets a favorable
environment for planetary system formation.Comment: 10 pages, accepted for publication in MNRA
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