82 research outputs found
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
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
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
A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap
We combined hydrodynamical simulations of planet-disk interactions with dust
evolution models that include coagulation and fragmentation of dust grains over
a large range of radii and derived observational properties using radiative
transfer calculations. We studied the role of the snow line in the survival of
the inner disk of transition disks. Inside the snow line, the lack of ice
mantles in dust particles decreases the sticking efficiency between grains. As
a consequence, particles fragment at lower collision velocities than in regions
beyond the snow line. This effect allows small particles to be maintained for
up to a few Myrs within the first astronomical unit. These particles are
closely coupled to the gas and do not drift significantly with respect to the
gas. For lower mass planets (1), the pre-transition appearance
can be maintained even longer because dust still trickles through the gap
created by the planet, moves invisibly and quickly in the form of relatively
large grains through the gap, and becomes visible again as it fragments and
gets slowed down inside of the snow line. The global study of dust evolution of
a disk with an embedded planet, including the changes of the dust aerodynamics
near the snow line, can explain the concentration of millimetre-sized particles
in the outer disk and the survival of the dust in the inner disk if a large
dust trap is present in the outer disk. This behaviour solves the conundrum of
the combination of both near-infrared excess and ring-like millimetre emission
observed in several transition disks.Comment: Accepted for publication in A&A (including acknowledgments
Recommended from our members
Evolutionary Signatures In The Formation Of Low-Mass Protostars. II. Toward Reconciling Models And Observations
A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L(bol), T(bol), L(bol)/L(smm)) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T(bol) and L(bol)/L(smm)) can be considered reliable indicators of physical stage.NASA 1224608, 1288664, 1288658, RSA 1377304, NNX 07-AJ72GNSF AST0607793UT Austin University Continuing FellowshipAstronom
Long-term infrared variability of the UX Ori-type star SV Cep
We investigate the long-term optical-infrared variability of SV Cep, and
explain it in the context of an existing UX Ori (UXOR) model. A 25-month
monitoring programme was completed with the Infrared Space Observatory in the
3.3-100 um wavelength range. Following a careful data reduction, the infrared
light curves were correlated with the variations of SV Cep in the V-band. A
remarkable correlation was found between the optical and the far-infrared light
curves. In the mid-infrared regime the amplitude of variations is lower, with a
hint for a weak anti-correlation with the optical changes. In order to
interpret the observations, we modelled the spectral energy distribution of SV
Cep assuming a self-shadowed disc with a puffed-up inner rim, using a
2-dimensional radiative transfer code. We found that modifying the height of
the inner rim, the wavelength-dependence of the long-term optical-infrared
variations is well reproduced, except the mid-infrared domain. The origin of
variation of the rim height might be fluctuation in the accretion rate in the
outer disc. In order to model the mid-infrared behaviour we tested to add an
optically thin envelope to the system, but this model failed to explain the
far-infrared variability. Infrared variability is a powerful tool to
discriminate between models of the circumstellar environment. The proposed
mechanism of variable rim height may not be restricted to UXOR stars, but might
be a general characteristic of intermediate-mass young stars.Comment: 11 pages, 9 figures, accepted for publiction in Monthly Notices of
the Royal Astronomical Societ
High-resolution spectroscopic view of planet formation sites
Theories of planet formation predict the birth of giant planets in the inner,
dense, and gas-rich regions of the circumstellar disks around young stars.
These are the regions from which strong CO emission is expected. Observations
have so far been unable to confirm the presence of planets caught in formation.
We have developed a novel method to detect a giant planet still embedded in a
circumstellar disk by the distortions of the CO molecular line profiles
emerging from the protoplanetary disk's surface. The method is based on the
fact that a giant planet significantly perturbs the gas velocity flow in
addition to distorting the disk surface density. We have calculated the
emerging molecular line profiles by combining hydrodynamical models with
semianalytic radiative transfer calculations. Our results have shown that a
giant Jupiter-like planet can be detected using contemporary or future
high-resolution near-IR spectrographs such as VLT/CRIRES or ELT/METIS. We have
also studied the effects of binarity on disk perturbations. The most
interesting results have been found for eccentric circumprimary disks in
mid-separation binaries, for which the disk eccentricity - detectable from the
asymmetric line profiles - arises from the gravitational effects of the
companion star. Our detailed simulations shed new light on how to constrain the
disk kinematical state as well as its eccentricity profile. Recent findings by
independent groups have shown that core-accretion is severely affected by disk
eccentricity, hence detection of an eccentric protoplanetary disk in a young
binary system would further constrain planet formation theories.Comment: IAU Symposium 276 (contributed talk
Impact splash chondrule formation during planetesimal recycling
Chondrules are the dominant bulk silicate constituent of chondritic
meteorites and originate from highly energetic, local processes during the
first million years after the birth of the Sun. So far, an astrophysically
consistent chondrule formation scenario, explaining major chemical, isotopic
and textural features, remains elusive. Here, we examine the prospect of
forming chondrules from planetesimal collisions. We show that intensely melted
bodies with interior magma oceans became rapidly chemically equilibrated and
physically differentiated. Therefore, collisional interactions among such
bodies would have resulted in chondrule-like but basaltic spherules, which are
not observed in the meteoritic record. This inconsistency with the expected
dynamical interactions hints at an incomplete understanding of the planetary
growth regime during the protoplanetary disk phase. To resolve this conundrum,
we examine how the observed chemical and isotopic features of chondrules
constrain the dynamical environment of accreting chondrite parent bodies by
interpreting the meteoritic record as an impact-generated proxy of
planetesimals that underwent repeated collision and reaccretion cycles. Using a
coupled evolution-collision model we demonstrate that the vast majority of
collisional debris feeding the asteroid main belt must be derived from
planetesimals which were partially molten at maximum. Therefore, the precursors
of chondrite parent bodies either formed primarily small, from sub-canonical
aluminum-26 reservoirs, or collisional destruction mechanisms were efficient
enough to shatter planetesimals before they reached the magma ocean phase.
Finally, we outline the window in parameter space for which chondrule formation
from planetesimal collisions can be reconciled with the meteoritic record and
how our results can be used to further constrain early solar system dynamics.Comment: 20 pages, 11 figures, 2 tables; accepted for publication in Icarus;
associated blog article at goo.gl/5bDqG
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