10,332 research outputs found
Toward an initial mass function for giant planets
The distribution of exoplanet masses is not primordial. After the initial
stage of planet formation is complete, the gravitational interactions between
planets can lead to the physical collision of two planets, or the ejection of
one or more planets from the system. When this occurs, the remaining planets
are typically left in more eccentric orbits. Here we use present-day
eccentricities of the observed exoplanet population to reconstruct the initial
mass function of exoplanets before the onset of dynamical instability. We
developed a Bayesian framework that combines data from N-body simulations with
present-day observations to compute a probability distribution for the planets
that were ejected or collided in the past. Integrating across the exoplanet
population, we obtained an estimate of the initial mass function of exoplanets.
We find that the ejected planets are primarily sub-Saturn type planets. While
the present-day distribution appears to be bimodal, with peaks around and , this bimodality does not seem to be
primordial. Instead, planets around appear to be
preferentially removed by dynamical instabilities. Attempts to reproduce
exoplanet populations using population synthesis codes should be mindful of the
fact that the present population has been depleted of intermediate-mass
planets. Future work should explore how the system architecture and
multiplicity might alter our results.Comment: 10 pages, 9 figures; submitted to MNRA
Survival of habitable planets in unstable planetary systems
Many observed giant planets lie on eccentric orbits. Such orbits could be the
result of strong scatterings with other giant planets. The same dynamical
instability that produces these scatterings may also cause habitable planets in
interior orbits to become ejected, destroyed, or be transported out of the
habitable zone. We say that a habitable planet has resilient habitability if it
is able to avoid ejections and collisions and its orbit remains inside the
habitable zone. Here we model the orbital evolution of rocky planets in
planetary systems where giant planets become dynamically unstable. We measure
the resilience of habitable planets as a function of the observed, present-day
masses and orbits of the giant planets. We find that the survival rate of
habitable planets depends strongly on the giant planet architecture. Equal-mass
planetary systems are far more destructive than systems with giant planets of
unequal masses. We also establish a link with observation; we find that giant
planets with present-day eccentricities higher than 0.4 almost never have a
habitable interior planet. For a giant planet with an present-day eccentricity
of 0.2 and semimajor axis of 5 AU orbiting a Sun-like star, 50% of the orbits
in the habitable zone are resilient to the instability. As semimajor axis
increases and eccentricity decreases, a higher fraction of habitable planets
survive and remain habitable. However, if the habitable planet has rocky
siblings, there is a significant risk of rocky planet collisions that would
sterilize the planet.Comment: Accepted to MNRA
How to form planetesimals from mm-sized chondrules and chondrule aggregates
The size distribution of asteroids and Kuiper belt objects in the solar
system is difficult to reconcile with a bottom-up formation scenario due to the
observed scarcity of objects smaller than 100 km in size. Instead,
planetesimals appear to form top-down, with large km bodies forming
from the rapid gravitational collapse of dense clumps of small solid particles.
In this paper we investigate the conditions under which solid particles can
form dense clumps in a protoplanetary disk. We use a hydrodynamic code to model
the interaction between solid particles and the gas inside a shearing box
inside the disk, considering particle sizes from sub-millimeter-sized
chondrules to meter-sized rocks. We find that particles down to millimeter
sizes can form dense particle clouds through the run-away convergence of radial
drift known as the streaming instability. We make a map of the range of
conditions (strength of turbulence, particle mass-loading, disk mass, and
distance to the star) which are prone to producing dense particle clumps.
Finally, we estimate the distribution of collision speeds between mm-sized
particles. We calculate the rate of sticking collisions and obtain a robust
upper limit on the particle growth timescale of years. This means
that mm-sized chondrule aggregates can grow on a timescale much smaller than
the disk accretion timescale ( years). Our results suggest a
pathway from the mm-sized grains found in primitive meteorites to fully formed
asteroids. We speculate that asteroids may form from a positive feedback loop
in which coagualation leads to particle clumping driven by the streaming
instability. This clumping, in turn reduces collision speeds and enhances
coagulation.} Future simulations should model coagulation and the streaming
instability together to explore this feedback loop further.Comment: 20 pages. Accepted for publication in A&
The Global Anomaly Through Level Circling
We discuss a novel manifestation of the global anomaly in an
gauge theory with an odd number of chiral quark doublets and arbitrary Yukawa
couplings. We argue that the massive 4-dim.() Euclidean Dirac operator is
nonhermitean with its spectrum of eigenvalues lying in
pairs in the complex plane. Consequently the existence of an odd number of
normalizable zero modes of the 5-dim.() massive Dirac operator is
equivalent to a fermionic level exchange phenomenon, level ``circling'', under
continuous topologically nontrivial deformations of the external gauge field.
More generally global anomalies are a manifestation of fermionic level
``circling'' in any gauge theory with an odd number of massive
fermions in the spinor representation and arbitrary Yukawa couplings.Comment: 14 pages, NBI-HE-93-5
On sphaleron deformations induced by Yukawa interactions
Due to the presence of the chiral anomaly sphalerons with Chern-Simons number
a half (CS=1/2) are the only static configurations that allow for a fermion
level crossing in the two-dimensional Abelian-Higgs model with massless
fermions, i.e. in the absence of Yukawa interactions. In the presence of
fermion-Higgs interactions we demonstrate the existence of zero energy
solutions to the one-dimensional Dirac equation at deformed sphalerons with
CS Induced level crossing due to Yukawa interactions illustrates a
non-trivial generalization of the Atiyah-Patodi-Singer index theorem and of the
equivalence between parity anomaly in odd and the chiral anomaly in even
dimensions. We discuss a subtle manifestation of this effect in the standard
electroweak theory at finite temperatures.Comment: 14 pages, Latex, NBI-HE-93-7
The destruction of inner planetary systems during high-eccentricity migration of gas giants
Hot Jupiters are giant planets on orbits a few hundredths of an AU. They do
not share their system with low-mass close-in planets, despite these latter
being exceedingly common. Two migration channels for hot Jupiters have been
proposed: through a protoplanetary gas disc or by tidal circularisation of
highly-eccentric planets. We show that highly-eccentric giant planets that will
become hot Jupiters clear out any low-mass inner planets in the system,
explaining the observed lack of such companions to hot Jupiters. A less common
outcome of the interaction is that the giant planet is ejected by the inner
planets. Furthermore, the interaction can implant giant planets on
moderately-high eccentricities at semimajor axes AU, a region otherwise
hard to populate. Our work supports the hypothesis that most hot Jupiters
reached their current orbits following a phase of high eccentricity, possibly
excited by other planetary or stellar companions.Comment: Replaced with accepted versio
The effects of external planets on inner systems: multiplicities, inclinations, and pathways to eccentric warm Jupiters
We study how close-in systems such as those detected by Kepler are affected
by the dynamics of bodies in the outer system. We consider two scenarios: outer
systems of giant planets potentially unstable to planet--planet scattering, and
wide binaries that may be capable of driving Kozai or other secular variations
of outer planets' eccentricities. Dynamical excitation of planets in the outer
system reduces the multiplicity of Kepler-detectable planets in the inner
system in of our systems. Accounting for the occurrence rates of
wide-orbit planets and binary stars, of close-in systems could be
destabilised by their outer companions in this way. This provides some
contribution to the apparent excess of systems with a single transiting planet
compared to multiple, however, it only contributes at most of the
excess. The effects of the outer dynamics can generate systems similar to
Kepler-56 (two coplanar planets significantly misaligned with the host star)
and Kepler-108 (two significantly non-coplanar planets in a binary). We also
identify three pathways to the formation of eccentric warm Jupiters resulting
from the interaction between outer and inner systems: direct inelastic
collision between an eccentric outer and an inner planet, secular eccentricity
oscillations that may "freeze out" when scattering resolves in the outer
system; and scattering in the inner system followed by "uplift", where inner
planets are removed by interaction with the outer planets. In these scenarios,
the formation of eccentric warm Jupiters is a signature of a past history of
violent dynamics among massive planets beyond au.Comment: 24 pages, 19 figures. Accepted to MNRA
Planetesimal formation by the streaming instability in a photoevaporating disk
Recent years have seen growing interest in the streaming instability as a
candidate mechanism to produce planetesimals. However, these investigations
have been limited to small-scale simulations. We now present the results of a
global protoplanetary disk evolution model that incorporates planetesimal
formation by the streaming instability, along with viscous accretion,
photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice
line, and stratified turbulence. Our simulations produce massive (60-130
) planetesimal belts beyond 100 au and up to of
planetesimals in the middle regions (3-100 au). Our most comprehensive model
forms 8 of planetesimals inside 3 au, where they can give rise to
terrestrial planets. The planetesimal mass formed in the inner disk depends
critically on the timing of the formation of an inner cavity in the disk by
high-energy photons. Our results show that the combination of photoevaporation
and the streaming instability are efficient at converting the solid component
of protoplanetary disks into planetesimals. Our model, however, does not form
enough early planetesimals in the inner and middle regions of the disk to give
rise to giant planets and super-Earths with gaseous envelopes. Additional
processes such as particle pileups and mass loss driven by MHD winds may be
needed to drive the formation of early planetesimal generations in the planet
forming regions of protoplanetary disks.Comment: 20 pages, 12 figures; accepted to Ap
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