29 research outputs found
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
Forming Planetesimals in Solar and Extrasolar Nebulae
Planets are built from planetesimals: solids larger than a kilometer which
grow by colliding pairwise. Planetesimals themselves are unlikely to form by
two-body collisions; sub-km objects have gravitational fields individually too
weak, and electrostatic attraction is too feeble for growth beyond a few cm. We
review the possibility that planetesimals form when self-gravity brings
together vast ensembles of small particles. Even when self-gravity is weak,
aerodynamic processes can accumulate solids relative to gas, paving the way for
gravitational collapse. Particles pile up as they drift radially inward. Gas
turbulence stirs particles, but can also seed collapse by clumping them. While
the feedback of solids on gas triggers vertical shear instabilities that
obstruct self-gravity, this same feedback triggers streaming instabilities that
strongly concentrate particles. Numerical simulations find that solids 10-100
cm in size gravitationally collapse in turbulent disks. We outline areas for
progress, including the possibility that still smaller objects self-gravitate.Comment: To appear in Annual Reviews. This review is intended to be both
current and pedagogical. Incorporates suggestions from the community; further
comments welcome. v2: Single-space
Homogeneous Analysis of the Dust Morphology of Transition Disks Observed with ALMA: Investigating Dust Trapping and the Origin of the Cavities
We analyze the dust morphology of 29 transition disks (TDs) observed with
ALMA at (sub-) millimeter-emission. We perform the analysis in the visibility
plane to characterize the total flux, cavity size, and shape of the ring-like
structure. First, we found that the relation is much
flatter for TDs than the observed trends from samples of class II sources in
different star forming regions. This relation demonstrates that cavities open
in high (dust) mass disks, independent of the stellar mass. The flatness of
this relation contradicts the idea that TDs are a more evolved set of disks.
Two potential reasons (not mutually exclusive) may explain this flat relation:
the emission is optically thick or/and millimeter-sized particles are trapped
in a pressure bump. Second, we discuss our results of the cavity size and ring
width in the context of different physical processes for cavity formation.
Photoevaporation is an unlikely leading mechanism for the origin of the cavity
of any of the targets in the sample. Embedded giant planets or dead zones
remain as potential explanations. Although both models predict correlations
between the cavity size and the ring shape for different stellar and disk
properties, we demonstrate that with the current resolution of the
observations, it is difficult to obtain these correlations. Future observations
with higher angular resolution observations of TDs with ALMA will help to
discern between different potential origins of cavities in TDs
Optical Magnetometry
Some of the most sensitive methods of measuring magnetic fields utilize
interactions of resonant light with atomic vapor. Recent developments in this
vibrant field are improving magnetometers in many traditional areas such as
measurement of geomagnetic anomalies and magnetic fields in space, and are
opening the door to new ones, including, dynamical measurements of bio-magnetic
fields, detection of nuclear magnetic resonance (NMR), magnetic-resonance
imaging (MRI), inertial-rotation sensing, magnetic microscopy with cold atoms,
and tests of fundamental symmetries of Nature.Comment: 11 pages; 4 figures; submitted to Nature Physic
Stellar encounters as the origin of distant solar system objects in highly eccentric orbits
The discovery of Sedna places new constraints on the origin and evolution of
our solar system. Here we investigate the possibility that a close encounter
with another star produced the observed edge of the Kuiper belt, at roughly 50
AU, and the highly elliptical orbit of Sedna. We show that a passing star
probably scattered Sedna from the Kuiper Belt into its observed orbit. The
likelihood that a planet at 60-80 AU can be scattered into Sedna's orbit is
roughly 50%; this estimate depends critically on the geometry of the flyby.
Even more interesting, though, is the roughly 10% chance that Sedna was
captured from the outer disk of the passing star. Most captures have very high
inclination orbits; detection of these objects would confirm the presence of
extrasolar planets in our own Solar System.Comment: 9 pages, 3 figure
Planet Populations as a Function of Stellar Properties
Exoplanets around different types of stars provide a window into the diverse
environments in which planets form. This chapter describes the observed
relations between exoplanet populations and stellar properties and how they
connect to planet formation in protoplanetary disks. Giant planets occur more
frequently around more metal-rich and more massive stars. These findings
support the core accretion theory of planet formation, in which the cores of
giant planets form more rapidly in more metal-rich and more massive
protoplanetary disks. Smaller planets, those with sizes roughly between Earth
and Neptune, exhibit different scaling relations with stellar properties. These
planets are found around stars with a wide range of metallicities and occur
more frequently around lower mass stars. This indicates that planet formation
takes place in a wide range of environments, yet it is not clear why planets
form more efficiently around low mass stars. Going forward, exoplanet surveys
targeting M dwarfs will characterize the exoplanet population around the lowest
mass stars. In combination with ongoing stellar characterization, this will
help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet
Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers
We present a review of the interplay between the evolution of circumstellar
disks and the formation of planets, both from the perspective of theoretical
models and dedicated observations. Based on this, we identify and discuss
fundamental questions concerning the formation and evolution of circumstellar
disks and planets which can be addressed in the near future with optical and
infrared long-baseline interferometers. Furthermore, the importance of
complementary observations with long-baseline (sub)millimeter interferometers
and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics
Review"; The final publication is available at http://www.springerlink.co
Connecting Planetary Composition with Formation
The rapid advances in observations of the different populations of
exoplanets, the characterization of their host stars and the links to the
properties of their planetary systems, the detailed studies of protoplanetary
disks, and the experimental study of the interiors and composition of the
massive planets in our solar system provide a firm basis for the next big
question in planet formation theory. How do the elemental and chemical
compositions of planets connect with their formation? The answer to this
requires that the various pieces of planet formation theory be linked together
in an end-to-end picture that is capable of addressing these large data sets.
In this review, we discuss the critical elements of such a picture and how they
affect the chemical and elemental make up of forming planets. Important issues
here include the initial state of forming and evolving disks, chemical and dust
processes within them, the migration of planets and the importance of planet
traps, the nature of angular momentum transport processes involving turbulence
and/or MHD disk winds, planet formation theory, and advanced treatments of disk
astrochemistry. All of these issues affect, and are affected by the chemistry
of disks which is driven by X-ray ionization of the host stars. We discuss how
these processes lead to a coherent end-to-end model and how this may address
the basic question.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018). 46 pages, 10
figure
Planetesimal formation by gravitational instability - The Goldreich-Ward hypothesis revisited
[[abstract]]We consider the formation of planetesimals via gravitational instability. While minimum solar nebula (MSN) models are known to be gravitationally stable, we find that sufficiently metal enriched and/or colder discs can yield planetesimals by the Goldreich-Ward mechanism (GWM). This is because the shear between gas and solids, previously believed to render the GWM ineffective, can only stir a finite amount of solids.[[fileno]]2010118030008[[department]]物理