308 research outputs found
Core instability models of giant planet accretion II: forming planetary systems
We develop a simple model for computing planetary formation based on the core
instability model for the gas accretion and the oligarchic growth regime for
the accretion of the solid core. In this model several planets can form
simultaneously in the disc, a fact that has important implications specially
for the changes in the dynamic of the planetesimals and the growth of the cores
since we consider the collision between them as a source of potential growth.
The type I and II migration of the embryos and the migration of the
planetesimals due to the interaction with the disc of gas are also taken into
account. With this model we consider different initial conditions to generate a
variety of planetary systems and analyse them statistically. We explore the
effects of using different type I migration rates on the final number of
planets formed per planetary system such as on the distribution of masses and
semimajor axis of extrasolar planets, where we also analyse the implications of
considering different gas accretion rates. A particularly interesting result is
the generation of a larger population of habitable planets when the gas
accretion rate and type I migration are slower.Comment: 4 figures and 1 table. Accepted for publication in MNRA
Origin of craters on Phoebe: comparison with Cassini's data
Phoebe is one of the irregular satellites of Saturn; the images taken by
Cassini-Huygens spacecraft allowed us to analyze its surface and the craters on
it. We study the craters on Phoebe produced by Centaur objects from the
Scattered Disk (SD) and plutinos escaped from the 3:2 mean motion resonance
with Neptune and compare our results with the observations by Cassini. We use
previous simulations on trans-Neptunian Objects and a method that allows us to
obtain the number of craters and the cratering rate on Phoebe. We obtain the
number of craters and the greatest crater on Phoebe produced by Centaurs in the
present configuration of the Solar System. Moreover, we obtain a present
normalized rate of encounters of Centaurs with Saturn of per year, from which we can infer the current cratering rate on
Phoebe for each crater diameter. Our study and the comparison with the
observations suggest that the main crater features on Phoebe are unlikely to
have been produced in the present configuration of the Solar System and that
they must have been acquired when the SD were depleted in the early Solar
System. If this is what happened and the craters were produced when Phoebe was
a satellite of Saturn, then it had to be captured, very early in the evolution
of the Solar System.Comment: Accepted for publication in Astronomy & Astrophysic
Percolation of diffusionally evolved two-phase systems
Percolation thresholds and critical exponents for universal scaling laws are computed for microstructures that derive from phase-transformation processes in two dimensions. The computed percolation threshold for nucleation and growth processes, p[subscript c] ≈0.6612, is similar to those obtained by random placement of disks and greater than that of spinodal decomposition, p[subscript c] ≈0.4987. Three critical exponents for scaling behavior were computed and do not differ significantly from universal values. The time evolution of a characteristic microstructural length was also computed: For spinodal decomposition, this length grows according to a power law after a short incubation period; for nucleation and growth, there are several transitions in the nature of the growth law. We speculate that the transitions in nucleation and growth derive from competing effects of coalescence at short times and then subsequent coarsening. Short-range order is present, but different, for both classes of microstructural evolution. © 2011 American Physical Society.National Science Foundation (U.S.) (Contract DMR-0855402
Effects of an eccentric inner Jupiter on the dynamical evolution of icy body reservoirs in a planetary scattering scenario
Aims. We analyze the dynamics of small body reservoirs under the effects of an eccentric inner giant planet resulting from a planetary scattering event around a 0.5 M⊙ star. Methods. First, we used a semi-analytical model to define the properties of the protoplanetary disk that lead to the formation of three Jupiter-mass planets. Then, we carried out N-body simulations assuming that the planets are close to their stability limit together with an outer planetesimal disk. In particular, the present work focused on the analysis of N-body simulations in which a single Jupiter-mass planet survives after the dynamical instability event. Results. Our simulations produce outer small body reservoirs with particles on prograde and retrograde orbits, and other ones whose orbital plane flips from prograde to retrograde and back again along their evolution (“Type-F particles”). We find strong correlations between the inclination i and the ascending node longitude Ω of Type-F particles. First, Ω librates around 90° or/and 270°. This property represents a necessary and sufficient condition for the flipping of an orbit. Moreover, the libration periods of i and Ω are equal and they are out to phase by a quarter period. We also remark that the larger the libration amplitude of i, the larger the libration amplitude of Ω. We analyze the orbital parameters of Type-F particles immediately after the instability event (post IE orbital parameters), when a single Jupiter-mass planet survives in the system. Our results suggest that the orbit of a particle can flip for any value of its post IE eccentricity, although we find only two Type-F particles with post IE inclinations i ≲ 17°. Finally, our study indicates that the minimum value of the inclination of the Type-F particles in a given system decreases with an increase in the eccentricity of the giant planet.Fil: Zanardi, Macarena. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: de Elia, Gonzalo Carlos. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Di Sisto, Romina Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Naoz, S.. University of California at Los Angeles; Estados UnidosFil: Li, G.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Guilera, O. M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Brunini, A.. Universidad Nacional de la Patagonia Austral; Argentin
Oligarchic planetesimal accretion and giant planet formation II
The equation of state calculated by Saumon and collaborators has been adopted
in most core-accretion simulations of giant-planet formation performed to date.
Since some minor errors have been found in their original paper, we present
revised simulations of giant-planet formation that considers a corrected
equation of state. We employ the same code as Fortier and collaborators in
repeating our previous simulations of the formation of Jupiter. Although the
general conclusions of Fortier and collaborators remain valid, we obtain
significantly lower core masses and shorter formation times in all cases
considered. The minor errors in the previously published equation of state have
been shown to affect directly the adiabatic gradient and the specific heat,
causing an overestimation of both the core masses and formation times.Comment: 4 pages, 2 figures, Accepted for publication in Astronomy and
Astrophysic
Radiation induced warping of protostellar accretion disks
We examine the consequences of radiatively driven warping of accretion disks
surrounding pre-main-sequence stars. These disks are stable against warping if
the luminosity arises from a steady accretion flow, but are unstable at late
times when the intrinsic luminosity of the star overwhelms that provided by the
disk. Warps can be excited for stars with luminosities of around 10 solar
luminosities or greater, with larger and more severe warps in the more luminous
systems. A twisted inner disk may lead to high extinction towards stars often
viewed through their disks. After the disk at all radii becomes optically thin,
the warp decays gradually on the local viscous timescale, which is likely to be
long. We suggest that radiation induced warping may account for the origin of
the warped dust disk seen in Beta Pictoris, if the star is only around 10-20
Myr old, and could lead to non-coplanar planetary systems around higher mass
stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres
Consequences of the simultaneous formation of giant planets by the core accretion mechanism
The core accretion mechanism is presently the most widely accepted cause of
the formation of giant planets. For simplicity, most models presently assume
that the growth of planetary embryos occurs in isolation. We explore how the
simultaneous growth of two embryos at the present locations of Jupiter and
Saturn affects the outcome of planetary formation. We model planet formation on
the basis of the core accretion scenario and include several key physical
ingredients. We consider a protoplanetary gas disk that exponentially decays
with time. For planetesimals, we allow for a distribution of sizes from 100~m
to 100~km with most of the mass in the smaller objects. We include planetesimal
migration as well as different profiles for the surface density of the
disk. The core growth is computed in the framework of the oligarchic growth
regime and includes the viscous enhancement of the planetesimal capture
cross-section. Planet migration is ignored. By comparing calculations assuming
formation of embryos in isolation to calculations with simultaneous embryo
growth, we find that the growth of one embryo generally significantly affects
the other. This occurs in spite of the feeding zones of each planet never
overlapping. The results may be classified as a function of the gas surface
density profile : if and the protoplanetary
disk is rather massive, Jupiter's formation inhibits the growth of Saturn. If
isolated and simultaneous formation lead to very
similar outcomes; in the the case of Saturn grows
faster and induces a density wave that later acclerates the formation of
Jupiter. Our results indicate that the simultaneous growth of several embryos
impacts the final outcome and should be taken into account by planet formation
models.Comment: Accepted for publication in Astronomy and Astrophysic
Simultaneous formation of Solar System giant planets
In the last few years, the so-called "Nice model" has got a significant
importance in the study of the formation and evolution of the solar system.
According to this model, the initial orbital configuration of the giant planets
was much more compact than the one we observe today. We study the formation of
the giant planets in connection with some parameters that describe the
protoplanetary disk. The aim of this study is to establish the conditions that
favor their simultaneous formation in line with the initial configuration
proposed by the Nice model. We focus in the conditions that lead to the
simultaneous formation of two massive cores, corresponding to Jupiter and
Saturn, able to achieve the cross-over mass (where the mass of the envelope of
the giant planet equals the mass of the core, and gaseous runway starts) while
Uranus and Neptune have to be able to grow to their current masses. We compute
the in situ planetary formation, employing the numerical code introduced in our
previous work, for different density profiles of the protoplanetary disk.
Planetesimal migration is taken into account and planetesimals are considered
to follow a size distribution between (free parameter) and
km. The core's growth is computed according to the oligarchic
growth regime. The simultaneous formation of the giant planets was successfully
completed for several initial conditions of the disk. We find that for
protoplanetary disks characterized by a power law (),
smooth surface density profiles () favor the simultaneous
formation. However, for steep slopes (, as previously proposed by
other authors) the simultaneous formation of the solar system giant planets is
unlikely ...Comment: Accepted for publication in Astronomy and Astrophysic
Dynamical evolution of escaped plutinos, another source of Centaurs
It was shown in previous works the existence of weakly chaotic orbits in the
plutino population that diffuse very slowly. These orbits correspond to
long-term plutino escapers and then represent the plutinos that are escaping
from the resonance at present. In this paper we perform numerical simulations
in order to explore the dynamical evolution of plutinos recently escaped from
the resonance. The numerical simulations were divided in two parts. In the
first one we evolved 20,000 test particles in the resonance in order to detect
and select the long-term escapers. In the second one, we numerically integrate
the selected escaped plutinos in order to study their dynamical post escaped
behavior. Our main results include the characterization of the routes of escape
of plutinos and their evolution in the Centaur zone. We obtained a present rate
of escape of plutinos between 1 and 10 every 10 years. The escaped plutinos
have a mean lifetime in the Centaur zone of 108 Myr and their contribution to
the Centaur population would be a fraction of less than 6 % of the total
Centaur population. In this way, escaped plutinos would be a secondary source
of Centaurs.Comment: Accepted for publication in A&
Oligarchic planetesimal accretion and giant planet formation
Aims. In the context of the core instability model, we present calculations
of in situ giant planet formation. The oligarchic growth regime of solid
protoplanets is the model adopted for the growth of the core. Methods. The full
differential equations of giant planet formation were numerically solved with
an adaptation of a Henyey-type code. The planetesimals accretion rate was
coupled in a self-consistent way to the envelope's evolution. Results. We
performed several simulations for the formation of a Jupiter-like object by
assuming various surface densities for the protoplanetary disc and two
different sizes for the accreted planetesimals. We find that the atmospheric
gas drag gives rise to a major enhancement on the effective capture radius of
the protoplanet, thus leading to an average timescale reduction of 30% -- 55%
and ultimately to an increase by a factor of 2 of the final mass of solids
accreted as compared to the situation in which drag effects are neglected. With
regard to the size of accreted planetesimals, we find that for a swarm of
planetesimals having a radius of 10 km, the formation time is a factor 2 to 3
shorter than that of planetesimals of 100 km, the factor depending on the
surface density of the nebula. Moreover, planetesimal size does not seem to
have a significant impact on the final mass of the core.Comment: 12 pages, 10 figures, accepted for publication in A&
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