397 research outputs found
Dust Evolution and the Formation of Planetesimals
The solid content of circumstellar disks is inherited from the interstellar
medium: dust particles of at most a micrometer in size. Protoplanetary disks
are the environment where these dust grains need to grow at least 13 orders of
magnitude in size. Our understanding of this growth process is far from
complete, with different physics seemingly posing obstacles to this growth at
various stages. Yet, the ubiquity of planets in our galaxy suggests that planet
formation is a robust mechanism. This chapter focuses on the earliest stages of
planet formation, the growth of small dust grains towards the gravitationally
bound "planetesimals", the building blocks of planets. We will introduce some
of the key physics involved in the growth processes and discuss how they are
expected to shape the global behavior of the solid content of disks. We will
consider possible pathways towards the formation of larger bodies and conclude
by reviewing some of the recent observational advances in the field.Comment: 43 pages, 6 figures. Chapter in International Space Science Institute
(ISSI) Book on "The Disk in Relation to the Formation of Planets and their
Proto-atmospheres", published in Space Science Reviews by Springe
Lopsided dust rings in transition disks
Context. Particle trapping in local or global pressure maxima in
protoplanetary disks is one of the new paradigms in the theory of the first
stages of planet formation. However, finding observational evidence for this
effect is not easy. Recent work suggests that the large ring-shaped outer disks
observed in transition disk sources may in fact be lopsided and constitute
large banana-shaped vortices.
Aims. We wish to investigate how effective dust can accumulate along the
azimuthal direction. We also want to find out if the size- sorting resulting
from this can produce a detectable signatures at millimeter wavelengths.
Methods. To keep the numerical cost under control we develop a 1+1D method in
which the azimuthal variations are treated sepa- rately from the radial ones.
The azimuthal structure is calculated analytically for a steady-state between
mixing and azimuthal drift. We derive equilibration time scales and compare the
analytical solutions to time-dependent numerical simulations.
Results. We find that weak, but long-lived azimuthal density gradients in the
gas can induce very strong azimuthal accumulations of dust. The strength of the
accumulations depends on the P\'eclet number, which is the relative importance
of advection and diffusion. We apply our model to transition disks and our
simulated observations show that this effect would be easily observable with
ALMA and in principle allows to put constraints on the strength of turbulence
and the local gas density.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter
Gas- and dust evolution in protoplanetary disks
Context. Current models of the size- and radial evolution of dust in
protoplanetary disks generally oversimplify either the radial evolution of the
disk (by focussing at one single radius or by using steady state disk models)
or they assume particle growth to proceed monodispersely or without
fragmentation. Further studies of protoplanetary disks - such as observations,
disk chemistry and structure calculations or planet population synthesis models
- depend on the distribution of dust as a function of grain size and radial
position in the disk.
Aims. We attempt to improve upon current models to be able to investigate how
the initial conditions, the build-up phase, and the evolution of the
protoplanetary disk influence growth and transport of dust.
Methods. We introduce a new version of the model of Brauer et al. (2008) in
which we now include the time-dependent viscous evolution of the gas disk, and
in which more advanced input physics and numerical integration methods are
implemented.
Results. We show that grain properties, the gas pressure gradient, and the
amount of turbulence are much more influencing the evolution of dust than the
initial conditions or the build-up phase of the protoplanetary disk. We
quantify which conditions or environments are favorable for growth beyond the
meter size barrier. High gas surface densities or zonal flows may help to
overcome the problem of radial drift, however already a small amount of
turbulence poses a much stronger obstacle for grain growth.Comment: accepted to A&
The Evolution of Gas and Dust in Protoplanetary Accretion Disks
Dust constitutes only about one percent of the mass of circumstellar disks,
yet it is of crucial importance for the modeling of planet formation, disk
chemistry, radiative transfer and observations. The initial growth of dust from
sub-micron sized grains to planetesimals and also the radial transport of dust
in disks around young stars is the topic of this thesis. Circumstellar dust is
subject to radial drift, vertical settling, turbulent mixing, collisional
growth, fragmentation and erosion.
We approach this subject from three directions: analytical calculations,
numerical simulations, and comparison to observations. We describe the physical
and numerical concepts that go into a model which is able to simulate the
radial and size evolution of dust in a gas disk which is viscously evolving
over several million years. The resulting dust size distributions are compared
to our analytical predictions and a simple recipe for obtaining steady-state
dust size distributions is derived. With the numerical model at hand, we show
that grain fragmentation can explain the fact that circumstellar disks are
observed to be dust-rich for several million years. Finally, we investigate the
challenges that observations present to the theory of grain evolution, namely
that grains of millimeter sizes are observed at large distances from the star.
We have found that under the assumption that radial drift is ineffective, we
can reproduce some of the observed spectral indices and fluxes. Fainter objects
point towards a reduced dust-to-gas ratio or lower dust opacities.Comment: PhD thesis, defended on October 18 201
Can grain growth explain transition disks?
Aims: Grain growth has been suggested as one possible explanation for the
diminished dust optical depths in the inner regions of protoplanetary
"transition" disks. In this work, we directly test this hypothesis in the
context of current models of grain growth and transport.
Methods: A set of dust evolution models with different disk shapes, masses,
turbulence parameters, and drift efficiencies is combined with radiative
transfer calculations in order to derive theoretical spectral energy
distributions (SEDs) and images.
Results: We find that grain growth and transport effects can indeed produce
dips in the infrared SED, as typically found in observations of transition
disks. Our models achieve the necessary reduction of mass in small dust by
producing larger grains, yet not large enough to be fragmenting efficiently.
However, this population of large grains is still detectable at millimeter
wavelengths. Even if perfect sticking is assumed and radial drift is neglected,
a large population of dust grains is left behind because the time scales on
which they are swept up by the larger grains are too long. This mechanism thus
fails to reproduce the large emission cavities observed in recent
millimeter-wave interferometric images of accreting transition disks.Comment: 11 pages, 5 figures, accepted to A&
Breaking through: The effects of a velocity distribution on barriers to dust growth
It is unknown how far dust growth can proceed by coagulation. Obstacles to
collisional growth are the fragmentation and bouncing barriers. However, in all
previous simulations of the dust-size evolution in protoplanetary disks, only
the mean collision velocity has been considered, neglecting that a small but
possibly important fraction of the collisions will occur at both much lower and
higher velocities. We study the effect of the probability distribution of
impact velocities on the collisional dust growth barriers. Assuming a
Maxwellian velocity distribution for colliding particles to determine the
fraction of sticking, bouncing, and fragmentation, we implement this in a
dust-size evolution code. We also calculate the probability of growing through
the barriers and the growth timescale in these regimes. We find that the
collisional growth barriers are not as sharp as previously thought. With the
existence of low-velocity collisions, a small fraction of the particles manage
to grow to masses orders of magnitude above the main population. A particle
velocity distribution softens the fragmentation barrier and removes the
bouncing barrier. It broadens the size distribution in a natural way, allowing
the largest particles to become the first seeds that initiate sweep-up growth
towards planetesimal sizes.Comment: 4 pages, 3 figures. Accepted for publication as a Letter in Astronomy
and Astrophysic
High-efficiency receptor-mediated delivery of small and large (48 kilobase gene constructs using the endosome-disruption activity of defective or chemically inactivated adenovirus particles.
One limit to successful receptor-mediated gene delivery is the exit of the endocytosed material from the endosome. We demonstrate here the delivery of marker genes to tissue culture cells using a modification of the receptor-mediated gene delivery technique that exploits the endosomolytic activity of defective adenovirus particles. In particular, greater than 90% of the transfected-cell population is found to express a beta-galactosidase gene, and, most importantly, this high level of expression can be obtained with psoralen-inactivated virus particles. Furthermore, because the delivered gene is not carried within the genome of the adenovirus particle, the size constraints are relieved, and we can, therefore, show the delivery of a 48-kilobase cosmid DNA molecule
Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits
Context. Grains in circumstellar disks are believed to grow by mutual
collisions and subsequent sticking due to surface forces. Results of many
fields of research involving circumstellar disks, such as radiative transfer
calculations, disk chemistry, magneto-hydrodynamic simulations largely depend
on the unknown grain size distribution.
Aims. As detailed calculations of grain growth and fragmentation are both
numerically challenging and computationally expensive, we aim to find simple
recipes and analytical solutions for the grain size distribution in
circumstellar disks for a scenario in which grain growth is limited by
fragmentation and radial drift can be neglected.
Methods. We generalize previous analytical work on self-similar steady-state
grain distributions. Numerical simulations are carried out to identify under
which conditions the grain size distributions can be understood in terms of a
combination of power-law distributions. A physically motivated fitting formula
for grain size distributions is derived using our analytical predictions and
numerical simulations.
Results. We find good agreement between analytical results and numerical
solutions of the Smoluchowski equation for simple shapes of the kernel
function. The results for more complicated and realistic cases can be fitted
with a physically motivated "black box" recipe presented in this paper. Our
results show that the shape of the dust distribution is mostly dominated by the
gas surface density (not the dust-to-gas ratio), the turbulence strength and
the temperature and does not obey an MRN type distribution.Comment: 16 pages, 9 figures, accepted for publication in A&
Trapping dust particles in the outer regions of protoplanetary disks
Aims. We attempt to explain grain growth to mm sized particles and their retention in the outer regions of protoplanetary disks, as observed at sub-mm and mm wavelengths, by investigating whether strong inhomogeneities in the gas density profiles can decelerate excessive radial drift and help the dust particles to grow.
Methods. We use coagulation/fragmentation and disk-structure models, to simulate the evolution of dust in a bumpy surface density profile, which we mimic with a sinusoidal disturbance. For different values of the amplitude and length scale of the bumps, we investigate the ability of this model to produce and retain large particles on million-year timescales. In addition, we compare the pressure inhomogeneities considered in this work with the pressure profiles that come from magnetorotational instability. Using the Common Astronomy Software Applications ALMA simulator, we study whether there are observational signatures of these pressure inhomogeneities that can be seen with ALMA.
Results. We present the conditions required to trap dust particles and the corresponding calculations predicting the spectral slope in the mm-wavelength range, to compare with current observations. Finally, we present simulated images using different antenna configurations of ALMA at different frequencies, to show that the ring structures will be detectable at the distances of either the Taurus Auriga or Ophiucus star-forming regions
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