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We study the transfer process from the scattered disk (SD) to the high-perihelion scattered disk (HPSD) (defined as the population with perihelion distances q > 40 AU and semimajor axes a > 50 AU) by means of two different models. One model (Model 1) assumes that SD objects (SDOs) were formed closer to the Sun and driven outwards by resonant coupling with the accreting Neptune during the stage of outward migration (Gomes 2003b, Earth, Moon, Planets 92, 29-42.). The other model (Model 2) considers the observed population of SDOs plus clones that try to compensate for observational discovery bias (Ferna´ndez et al. 2004, Icarus, in press). We find that the Kozai mechanism (coupling between the argument of perihelion, eccentricity, and inclination), associated with a mean motion resonance (MMR), is the main responsible for raising both the perihelion distance and the inclination of SDOs. The highest perihelion distance for a body of our samples was found to be q ¼ 69:2 AU. This shows that bodies can be temporarily detached from the planetary region by dynamical interactions with the planets. This phenomenon is temporary since the same coupling of Kozai with a MMR will at some point bring the bodies back to states of lower-q values. However, the dynamical time scale in high-q states may be very long, up to several Gyr. For Model 1, about 10% of the bodies driven away by Neptune get trapped into the HPSD when the resonant coupling Kozai-MMR is disrupted by Neptune's migration. Therefore, Model 1 also supplies a fossil HPSD, whose bodies remain in non-resonant orbits and thus stable for the age of the solar system, in addition to the HPSD formed by temporary captures of SDOs after the giant planets reached their current orbits. We find that about 12-15% of the surviving bodies of our samples are incorporated into the HPSD after about 4-5 Gyr, and that a large fraction of the captures occur for up to the 1:8 MMR (a ' 120 AU), although we record captures up to the 1:24 MMR (a ' 260 AU). Because of the Kozai mechanism, HPSD objects have on average inclinations about 25 -50 , which are higher than those of the classical Edgeworth-Kuiper (EK) belt or the SD. Our results suggest that Sedna belongs to a dynamically distinct population from the HPSD, possibly being a member of the inner core of the Oort cloud. As regards to 2000 CR 105 , it is marginally within the region occupied by HPSD objects in the parametric planes ðq; aÞ and ða; iÞ, so it is not ruled out that it might be a member of the HPSD, though it might as well belong to the inner core.

We study the Jupiter family comet (JFC) population assumed to come from the Scattered Disk and transferred to the Jupiter's zone through gravitational interactions with the Jovian planets. We shall define as JFCs those with orbital periods P < 20 yr and Tisserand parameters in the range 2 < T K 3:1, while those comets coming from the same source, but that do not fulfill the previous criteria (mainly because they have periods P > 20 yr) will be called 'non-JFCs'. We performed a series of numerical simulations of fictitious comets with a purely dynamical model and also with a more complete dynamical-physical model that includes besides nongravitational forces, sublimation and splitting mechanisms. With the dynamical model, we obtain a poor match between the computed distributions of orbital elements and the observed ones. However with the inclusion of physical effects in the complete model we are able to obtain good fits to observations. The best fits are attained with four splitting models with a relative weak dependence on q, and a mass loss in every splitting event that is less when the frequency is high and vice versa. The mean lifetime of JFCs with radii R > 1 km and q < 1:5 AU is found to be of about 150-200 revolutions ($10 3 yrÞ. The total population of JFCs with radii R > 1 km within Jupiter's zone is found to be of 450 AE 50. Yet, the population of non-JFCs with radii R > 1 km in Jupiter-crossing orbits may be $4 times greater, thus leading to a whole population of JFCs + non-JFCs of $2250 AE 250. Most of these comets have perihelia close to Jupiter's orbit. On the other hand, very few non-JFCs reach the Earth's vicinity (perihelion distances q K 2 AU) which gives additional support to the idea that JFCs and Halley-type comets have different dynamical origins. Our model allows us to define the zones of the orbital element space in which we would expect to find a large number of JFCs. This is the first time, to our knowledge, that a physicodynamical model is presented that includes sublimation and different splitting laws. Our work helps to understand the role played by these erosion effects in the distribution of the orbital elements and lifetimes of JFCs.

Context. In the standard scenario of planet formation, terrestrial planets and the cores of the giant planets are formed by accretion of planetesimals. As planetary embryos grow, the planetesimal velocity dispersion increases because of gravitational excitations produced by embryos. The increasing relative velocities of the planetesimal cause them to fragment through mutual collisions. Aims. We study the role of planetesimal fragmentation on giant planet formation. We analyze how planetesimal fragmentation modifies the growth of giant planet cores for a wide range of planetesimal sizes and disk masses. Methods. We incorporated a model of planetesimal fragmentation into our model of in situ giant planet formation. We calculated the evolution of the solid surface density (planetesimals plus fragments) taking into account the accretion by the planet, migration, and fragmentation. Results. Incorporating planetesimal fragmentation significantly modifies the process of planetary formation. If most of the mass loss in planetesimal collisions is distributed in the smaller fragments, planetesimal fragmentation inhibits the growth of the embryo for initial planetesimals of radii smaller than 10 km. Only for initial planetesimals with a radius of 100 km, and disks larger than 0.06 M , embryos achieve masses larger than the mass of Earth. However, even for these large planetesimals and massive disks, planetesimal fragmentation induces the quick formation of massive cores only if most of the mass loss in planetesimal collisions is distributed in the larger fragments. Conclusions. Planetesimal fragmentation seems to play an important role in giant planet formation. The way in which the mass loss in planetesimal collisions is distributed leads to different results, inhibiting or favoring the formation of massive cores.

Context. 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. Aims. We explore how the simultaneous growth of two embryos at the present locations of Jupiter and Saturn affects the outcome of planetary formation. Methods. 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 Σ 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. Results. 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 Σ: if Σ ∝ r −3/2 and the protoplanetary disk is rather massive, Jupiter's formation inhibits the growth of Saturn. If Σ ∝ r −1 isolated and simultaneous formation lead to very similar outcomes; in the the case of Σ ∝ r −1/2 Saturn grows faster and induces a density wave that later accelerates the formation of Jupiter. Conclusions. Our results indicate that the simultaneous growth of several embryos impacts the final outcome and should be taken into account by planet formation models.

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. This growth regime for the core has not been considered before in full evolutionary calculations of this kind. 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 first focus our study on the atmospheric gas drag that the incoming planetesimals suffer. We find that this effect 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. In addition, we also examine the importance of the size of accreted planetesimals on the whole formation process. With regard to this second point, 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.

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