464 research outputs found
Saving Planetary Systems: Dead Zones & Planetary Migration
The tidal interaction between a disk and a planet leads to the planet's
migration. A long-standing question regarding this mechanism is how to stop the
migration before planets plunge into their central stars. In this paper, we
propose a new, simple mechanism to significantly slow down planet migration,
and test the possibility by using a hybrid numerical integrator to simulate the
disk-planet interaction. The key component of the scenario is the role of low
viscosity regions in protostellar disks known as dead zones, which affect
planetary migration in two ways. First of all, it allows a smaller-mass planet
to open a gap, and hence switch the faster type I migration to the slower type
II migration. Secondly, a low viscosity slows down type II migration itself,
because type II migration is directly proportional to the viscosity. We present
numerical simulations of planetary migration by using a hybrid symplectic
integrator-gas dynamics code. Assuming that the disk viscosity parameter inside
the dead zone is (alpha=1e-4-1e-5), we find that, when a low-mass planet (e.g.
1-10 Earth masses) migrates from outside the dead zone, its migration is
stopped due to the mass accumulation inside the dead zone. When a low-mass
planet migrates from inside the dead zone, it opens a gap and slows down its
migration. A massive planet like Jupiter, on the other hand, opens a gap and
slows down inside the dead zone, independent of its initial orbital radius. The
final orbital radius of a Jupiter mass planet depends on the dead zone's
viscosity. For the range of alpha's noted above, this can vary anywhere from 7
AU, to an orbital radius of 0.1 AU that is characteristic of the hot Jupiters.Comment: 38 pages, 14 figures, some changes in text and figures, accepted for
publication in Ap
Gravitational lens magnification by Abell 1689: Distortion of the background galaxy luminosity function
Gravitational lensing magnifies the luminosity of galaxies behind the lens.
We use this effect to constrain the total mass in the cluster Abell 1689 by
comparing the lensed luminosities of background galaxies with the luminosity
function of an undistorted field. Since galaxies are assumed to be a random
sampling of luminosity space, this method is not limited by clustering noise.
We use photometric redshift information to estimate galaxy distance and
intrinsic luminosity. Knowing the redshift distribution of the background
population allows us to lift the mass/background degeneracy common to lensing
analysis. In this paper we use 9 filters observed over 12 hours with the Calar
Alto 3.5m telescope to determine the redshifts of 1000 galaxies in the field of
Abell 1689. Using a complete sample of 151 background galaxies we measure the
cluster mass profile. We find that the total projected mass interior to
0.25h^(-1)Mpc is (0.48 +/- 0.16) * 10^(15)h^(-1) solar masses, where our error
budget includes uncertainties from the photometric redshift determination, the
uncertainty in the off-set calibration and finite sampling. This result is in
good agreement with that found by number count and shear-based methods and
provides a new and independent method to determine cluster masses.Comment: 13 pages, 10 figures. Submitted to MNRAS (10/99); Replacement with 1
page extra text inc. new section, accepted by MNRA
Dynamical Effects from Asteroid Belts for Planetary Systems
The orbital evolution and stability of planetary systems with interaction
from the belts is studied using the standard phase-plane analysis. In addition
to the fixed point which corresponds to the Keplerian orbit, there are other
fixed points around the inner and outer edges of the belt. Our results show
that for the planets, the probability to move stably around the inner edge is
larger than the one to move around the outer edge. It is also interesting that
there is a limit cycle of semi-attractor for a particular case. Applying our
results to the Solar System, we find that our results could provide a natural
mechanism to do the orbit rearrangement for the larger Kuiper Belt Objects and
thus successfully explain the absence of these objects beyond 50 AU.Comment: accepted by International Journal of Bifurcation and Chaos in Aug.
2003, AAS Latex, 27 pages with 6 color figure
Excitation of Orbital Eccentricities of Extrasolar Planets by Repeated Resonance Crossings
Orbits of known extrasolar planets that are located outside the tidal
circularization regions of their parent stars are often substantially
eccentric. By contrast, planetary orbits in our Solar System are approximately
circular, reflecting planet formation within a nearly axisymmetric, circumsolar
disk. We propose that orbital eccentricities may be generated by divergent
orbital migration of two planets in a viscously accreting circumstellar disk.
The migration is divergent in the sense that the ratio of the orbital period of
the outer planet to that of the inner planet grows. As the period ratio
diverges, the planets traverse, but are not captured into, a series of
mean-motion resonances that amplify their orbital eccentricities in rough
inverse proportion to their masses. Strong viscosity gradients in
protoplanetary disks offer a way to reconcile the circular orbits of Solar
System gas giants with the eccentric orbits of currently known extrasolar
planets.Comment: Final revised version, accepted by ApJ Letters. Includes discussion
from the community at larg
Gas disks to gas giants: Simulating the birth of planetary systems
The ensemble of now more than 250 discovered planetary systems displays a
wide range of masses, orbits and, in multiple systems, dynamical interactions.
These represent the end point of a complex sequence of events, wherein an
entire protostellar disk converts itself into a small number of planetary
bodies. Here, we present self-consistent numerical simulations of this process,
which produce results in agreement with some of the key trends observed in the
properties of the exoplanets. Analogs to our own solar system do not appear to
be common, originating from disks near the boundary between barren and (giant)
planet-forming.Comment: Science, August 8 issue. Published version and Supporting Online
material incl. movies are at
http://www.sciencemag.org/cgi/content/abstract/321/5890/81
Theory Challenges of the Accelerating Universe
The accelerating expansion of the universe presents an exciting, fundamental
challenge to the standard models of particle physics and cosmology. I highlight
some of the outstanding challenges in both developing theoretical models and
interpreting without bias the observational results from precision cosmology
experiments in the next decade that will return data to help reveal the nature
of the new physics. Examples given focus on distinguishing a new component of
energy from a new law of gravity, and the effect of early dark energy on baryon
acoustic oscillations.Comment: 10 pages, 4 figures; minor changes to match J. Phys. A versio
An optical study of the GRB 970111 field beginning 19 hours after the Gamma-Ray Burst
We present the results of the monitoring of the GRB 970111 field that started
19 hours after the event. This observation represents the fastest ground-based
follow-up performed for GRB 970111 in all wavelengths. As soon as the detection
of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998)
we reanalyzed the optical data collected for the GRB 970111 field. Although we
detect small magnitude variability in some objects, no convincing optical
counterpart is found inside the WFC error box. Any change in brightness 19
hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8.
The bluest object found in the field is coincident with 1SAXJ1528.8+1937.
Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with
redshift z=0.657, which we propose as the optical counterpart of the X-ray
source.
Further observations allowed to perform multicolour photometry for objects in
the GRB 970111 error box. The colour-colour diagrams do not show any object
with unusual colours. We applied a photometric classification method to the
objects inside the GRB error box, that can distinguish stars from galaxies and
estimate redshifts. We were able to estimate photometric redshifts in the range
0.2 < z < 1.4 for several galaxies in this field and we did not find any
conspicuous unusual object.
We note that GRB 970111 and GRB 980329 could belong to the same class of
GRBs, which may be related to nearby sources (z ~1) in which high intrinsic
absorption leads to faint optical afterglows.Comment: 10 pages with 11 encapsulated PostScript figures included. Uses
Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy
& Astrophysic
The Formation of Uranus and Neptune in Solid-Rich Feeding Zones: Connecting Chemistry and Dynamics
The core accretion theory of planet formation has at least two fundamental
problems explaining the origins of Uranus and Neptune: (1) dynamical times in
the trans-Saturnian solar nebula are so long that core growth can take > 15
Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10
Earth masses necessitates a sudden gas accretion cutoff just as the ice giant
cores reach critical mass. Both problems may be resolved by allowing the ice
giants to migrate outward after their formation in solid-rich feeding zones
with planetesimal surface densities well above the minimum-mass solar nebula.
We present new simulations of the formation of Uranus and Neptune in the
solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor
axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005;
Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15
AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the
outermost after 5.3-6 Myr, a considerable time decrease from previous
one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay
subcritical, eliminating the need for a sudden gas accretion cutoff. Our
calculated carbon mass fractions of 22% are in excellent agreement with the ice
giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based
on the requirement that the ice giant-forming planetesimals contain >10% mass
fractions of methane ice, we can reject any solar system formation model that
initially places Uranus and Neptune inside the orbit of Saturn. We also
demonstrate that a large population of planetesimals must be present in both
ice giant feeding zones throughout the lifetime of the gaseous nebula.Comment: Accepted for publication in Icarus. 9 pages, including 3 figure
The Growth & Migration of Jovian Planets in Evolving Protostellar Disks with Dead Zones
The growth of Jovian mass planets during migration in their protoplanetary
disks is one of the most important problems that needs to be solved in light of
observations of the exosolar planets. Studies of the migration of planets in
standard gas disk models routinely show that migration is too fast to form
Jovian planets, and that such migrating planetary cores generally plunge into
the central stars in less than a Myr. In previous work, we have shown that a
poorly ionized, less viscous region in a protoplanetary disk called a dead zone
slows down the migration of fixed-mass planets. In this paper, we extend our
numerical calculations to include dead zone evolution along with the disk, as
well as planet formation via accretion of rocky and gaseous materials. Using
our symplectic-integrator-gas dynamics code, we find that dead zones, even in
evolving disks wherein migrating planets grow by accretion, still play a
fundamental role in saving planetary systems. We demonstrate that Jovian
planets form within 2.5 Myr for disks that are ten times more massive than a
minimum mass solar nebula (MMSN) with an opacity reduction and without slowing
down migration artificially. Our simulations indicate that protoplanetary disks
with an initial mass comparable to the MMSN only produce Neptunian mass
planets. We also find that planet migration does not help core accretion as
much in the oligarchic planetesimal accretion scenario as it was expected in
the runaway accretion scenario. Therefore we expect that an opacity reduction
(or some other mechanisms) is needed to solve the formation timescale problem
even for migrating protoplanets, as long as we consider the oligarchic growth.
We also point out a possible role of a dead zone in explaining long-lived,
strongly accreting gas disks.Comment: 16 pages, 15 figures, accepted for publication in Ap
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