661 research outputs found
Modeling the Formation of Giant Planet Cores I: Evaluating Key Processes
One of the most challenging problems we face in our understanding of planet
formation is how Jupiter and Saturn could have formed before the the solar
nebula dispersed. The most popular model of giant planet formation is the
so-called 'core accretion' model. In this model a large planetary embryo formed
first, mainly by two-body accretion. This is then followed by a period of
inflow of nebular gas directly onto the growing planet. The core accretion
model has an Achilles heel, namely the very first step. We have undertaken the
most comprehensive study of this process to date. In this study we numerically
integrate the orbits of a number of planetary embryos embedded in a swarm of
planetesimals. In these experiments we have included: 1) aerodynamic gas drag,
2) collisional damping between planetesimals, 3) enhanced embryo cross-sections
due to their atmospheres, 4) planetesimal fragmentation, and 5) planetesimal
driven migration. We find that the gravitational interaction between the
embryos and the planetesimals lead to the wholesale redistribution of material
- regions are cleared of material and gaps open near the embryos. Indeed, in
90% of our simulations without fragmentation, the region near that embryos is
cleared of planetesimals before much growth can occur. The remaining 10%,
however, the embryos undergo a burst of outward migration that significantly
increases growth. On timescales of ~100,000 years, the outer embryo can migrate
~6 AU and grow to roughly 30 Earth-masses. We also find that the inclusion of
planetesimal fragmentation tends to inhibit growth.Comment: Accepted to AJ, 62 pages 11 figure
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
Saturated torque formula for planetary migration in viscous disks with thermal diffusion: recipe for protoplanet population synthesis
We provide torque formulae for low mass planets undergoing type I migration
in gaseous disks. These torque formulae put special emphasis on the horseshoe
drag, which is prone to saturation: the asymptotic value reached by the
horseshoe drag depends on a balance between coorbital dynamics (which tends to
cancel out or saturate the torque) and diffusive processes (which tend to
restore the unperturbed disk profiles, thereby desaturating the torque). We
entertain here the question of this asymptotic value, and we derive torque
formulae which give the total torque as a function of the disk's viscosity and
thermal diffusivity. The horseshoe drag features two components: one which
scales with the vortensity gradient, and one which scales with the entropy
gradient, and which constitutes the most promising candidate for halting inward
type I migration. Our analysis, which is complemented by numerical simulations,
recovers characteristics already noted by numericists, namely that the viscous
timescale across the horseshoe region must be shorter than the libration time
in order to avoid saturation, and that, provided this condition is satisfied,
the entropy related part of the horseshoe drag remains large if the thermal
timescale is shorter than the libration time. Side results include a study of
the Lindblad torque as a function of thermal diffusivity, and a contribution to
the corotation torque arising from vortensity viscously created at the contact
discontinuities that appear at the horseshoe separatrices. For the convenience
of the reader mostly interested in the torque formulae, section 8 is
self-contained.Comment: Affiliation details changed. Fixed equation numbering issue. Biblio
info adde
Oligarchic and giant impact growth of terrestrial planets in the presence of gas giant planet migration
We present the results of N--body simulations which examine the effect that
gas giant planet migration has on the formation of terrestrial planets. The
models incorporate a 0.5 Jupiter mass planet undergoing type II migration
through an inner protoplanet--planetesimal disk, with gas drag included. Each
model is initiated with the inner disk being at successively increased levels
of maturity, so that it is undergoing either oligarchic or giant impact style
growth as the gas giant migrates. In all cases, a large fraction of the disk
mass survives the passage of the giant, either by accreting into massive
terrestrial planets shepherded inward of the giant, or by being scattered into
external orbits. Shepherding is favored in younger disks where there is strong
dynamical friction from planetesimals and gas drag is more influential, whereas
scattering dominates in more mature disks where dissipation is weaker. In each
scenario, sufficient mass is scattered outward to provide for the eventual
accretion of a set of terrestrial planets in external orbits, including within
the system's habitable zone. An interesting result is the generation of
massive, short period, terrestrial planets from compacted material pushed ahead
of the giant. These planets are reminiscent of the short period Neptune mass
planets discovered recently, suggesting that such `hot Neptunes' could form
locally as a by-product of giant planet migration.Comment: 17 pages, 11 figures, to be published in A&A. Higher resolution pdf
available at: http://www.users.globalnet.co.uk/~mfogg/3453fogg.pd
The effect of type I migration on the formation of terrestrial planets in hot-Jupiter systems
Context: Our previous models of a giant planet migrating through an inner
protoplanet/planetesimal disk find that the giant shepherds a portion of the
material it encounters into interior orbits, whilst scattering the rest into
external orbits. Scattering tends to dominate, leaving behind abundant material
that can accrete into terrestrial planets. Aims: We add to the possible realism
of our model by simulating type I migration forces which cause an inward drift,
and strong eccentricity and inclination damping of protoplanetary bodies. This
extra dissipation might be expected to enhance shepherding at the expense of
scattering, possibly modifying our previous conclusions. Methods: We employ an
N-body code that is linked to a viscous gas disk algorithm capable of
simulating: gas accretion onto the central star; gap formation in the vicinity
of the giant planet; type II migration of the giant planet; type I migration of
protoplanets; and the effect of gas drag on planetesimals. We use the code to
re-run three scenarios from a previous work where type I migration was not
included. Results: The additional dissipation introduced by type I migration
enhances the inward shepherding of material but does not severely reduce
scattering. We find that > 50% of the solids disk material still survives the
migration in scattered exterior orbits: most of it well placed to complete
terrestrial planet formation at < 3 AU. The shepherded portion of the disk
accretes into hot-Earths, which survive in interior orbits for the duration of
our simulations. Conclusions: Water-rich terrestrial planets can form in the
habitable zones of hot-Jupiter systems and hot-Earths and hot-Neptunes may also
be present. These systems should be targets of future planet search missions.Comment: Accepted by A&A. 15 pages, 14 figures. Higher resolution pdf
available at http://www.users.globalnet.co.uk/~mfogg/7950fogg.pd
Formation and Dynamical Evolution of the Neptune Trojans - the Influence of the Initial Solar System Architecture
In this work, we investigate the dynamical stability of pre-formed Neptune
Trojans under the gravitational influence of the four giant planets in compact
planetary architectures, over 10 Myr. In our modelling, the initial orbital
locations of Uranus and Neptune (aN) were varied to produce systems in which
those planets moved on non-resonant orbits, or in which they lay in their
mutual 1:2, 2:3 and 3:4 mean-motion resonances (MMRs). In total, 420
simulations were carried out, examining 42 different architectures, with a
total of 840000 particles across all runs. In the non-resonant cases, the
Trojans suffered only moderate levels of dynamical erosion, with the most
compact systems (those with aN less than or equal 18 AU) losing around 50% of
their Trojans by the end of the integrations. In the 2:3 and 3:4 MMR scenarios,
however, dynamical erosion was much higher with depletion rates typically
greater than 66% and total depletion in the most compact systems. The 1:2
resonant scenarios featured disruption on levels intermediate between the
non-resonant cases and other resonant scenarios, with depletion rates of the
order of tens of percent. Overall, the great majority of plausible
pre-migration planetary architectures resulted in severe levels of depletion of
the Neptunian Trojan clouds. In particular, if Uranus and Neptune formed near
their mutual 2:3 or 3:4 MMR and at heliocentric distances within 18 AU (as
favoured by recent studies), we found that the great majority of pre-formed
Trojans would have been lost prior to Neptune's migration. This strengthens the
case for the great bulk of the current Neptunian Trojan population having been
captured during that migration.Comment: 17 pages, 2 figures, MNRAS (in press). Abstract slightly reduced in
size, but in original form in the PDF fil
Intermediate-mass Black Hole and stellar orbits in the Galactic Center
Many young stars reside within the central half-parsec from SgrA*, the
supermassive black hole in the Galactic Center. The origin of these stars
remains a puzzle. Recently, Hansen and Milosavljevic (2003, HM) have argued
that an Intermediate-Mass Black Hole (IMBH) could have delivered the young
stars to the immediate vicinity of SgrA*. Here we focus on the final stages of
the HM scenario. Namely, we integrate numerically the orbits of stars which are
initially bound to the IMBH, but are stripped from it by the tidal field of
SgrA*. Our numerical algorithm is a symplectic integrator designed specifically
for the problem at hand; however, we have checked our results with SYMBA, a
version of the widely available SWIFT code. We find that the distribution of
the post-inspiral orbital parameters is sensitive to the eccentricity of the
inspiraling IMBH. If the IMBH is on a circular orbit, then the inclinations of
numerically computed orbits relative to the inspiral plane are almost always
smaller than 10 degrees, and therefore (a) the simulations are in good
agreement with the observed motions of stars in a clockwise-moving stellar
disc, (b) the simulations never reproduce the orbits of stars outside this
disc, which include those in the second thick ring of stars and the randomly
oriented unrelaxed orbits of some of the S-stars. If the IMBH's orbital
eccentricity is e=0.6, then approximately half of the stars end up with orbital
inclinations below 10 degrees, and another half have inclinations anywhere
between 0 and 180 degrees; this is somewhat closer to what's observed. We also
show that if IRS13 cluster is bound by an IMBH, as has been argued by Maillard
et. al. 2004, then the same IMBH could not have delivered all of the young
stars to their present location.Comment: 16 pages, 10 figure
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
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