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In this work, we propose a novel campaign for constraining relativistically compact massive compact halo object (MACHO) dark matter, such as primordial black holes (PBHs), using the Moon as a detector. PBHs of about 1019 to 1022 g may be sufficiently abundant to have collided with the Moon in the history of the Solar system. We show that the crater profiles of a PBH collision differ from traditional impactors and may be detectable in high-resolution lunar surface scans now available. Any candidates may serve as sites for in situ measurements to identify high-pressure phases of matter which may have formed near the PBH during the encounter. While we primarily consider PBH dark matter, the discussion generalizes to the entire family of MACHO candidates with relativistic compactness. Moreover, we focus on the Moon since it has been studied well, but the same principles can be applied to other rocky bodies in our Solar system without an atmosphere.

The formation of the solar system's terrestrial planets concluded with a period of giant impacts. Previous works examining the volatile loss caused by the impact shock in the moon-forming impact find atmospheric losses of at most 20-30 per cent and essentially no loss of oceans. However, giant impacts also result in thermal heating, which can lead to significant atmospheric escape via a Parker-type wind. Here we show that H2O and other high-mean molecular weight outgassed species can be efficiently lost through this thermal wind if present in a hydrogen-dominated atmosphere, substantially altering the final volatile inventory of terrestrial planets. Specifically, we demonstrate that a giant impact of a Mars-sized embryo with a proto-Earth can remove several Earth oceans' worth of H2O, and other heavier volatile species, together with a primordial hydrogen-dominated atmosphere. These results may offer an explanation for the observed depletion in Earth's light noble gas budget and for its depleted xenon inventory, which suggest that Earth underwent significant atmospheric loss by the end of its accretion. Because planetary embryos are massive enough to accrete primordial hydrogen envelopes and because giant impacts are stochastic and occur concurrently with other early atmospheric evolutionary processes, our results suggest a wide diversity in terrestrial planet volatile budgets., Comment: submitted to MNRAS, 9 pages, 4 figures

Studies of internal structure of gas giant planets suggest that their envelopes are enriched with heavier elements than hydrogen and helium relative to their central stars. Such enrichment likely occurred by solid accretion during late formation stages of gas giant planets in which gas accretion dominates protoplanetary growth. Some previous studies performed orbital integration of planetesimals around a growing protoplanet with the assumption of an uniform circumstellar disc to investigate how efficiently the protoplanet captures planetesimals. However, not only planetesimals but also disc gas are gravitationally perturbed by the protoplanet in its late formation stages, resulting in gap opening in the circumstellar disc. In this study, we investigate the effects of such gap formation on the capture of planetesimals by performing dynamical simulations of planetesimals around a growing proto-gas giant planet. Gap formation reduces the surface density of disc gas, makes a steep pressure gradient and limits the growth rate of the protoplanet. We find that the first effect enhances the capture of planetesimals, while the others reduce it. Consequently the amount of planetesimals captured during the gas accretion is estimated to be at most ~3 Mearth. We conclude that the in-situ capture of planetesimals needs the initial solid surface density more than five times higher than that of the minimum mass solar nebula for explaining the inferred large amount of heavy element in Jupiter. For highly dense warm Jupiters, we would need additional processes enhancing the capture and/or supply of planetesimals., Comment: 15 pages, 13 figures, this is a pre-copyedited, author-produced version of an article accepted for publication in MNRAS following peer review

We explore in situ formation and subsequent evolution of close-in super-Earths and mini-Neptunes. We adopt a steady-state inner protoplanetary gas disc structure that arises from viscous accretion due to the magneto-rotational instability (MRI). We consider the evolution of dust in the inner disc, including growth, radial drift and fragmentation, and find that dust particles that radially drift into the inner disc fragment severely due to the MRI-induced turbulence. This result has two consequences: (1) radial drift of grains within the inner disc is quenched, leading to an enhancement of dust in the inner regions which scales as dust-to-gas-mass-flux-ratio at ~1 AU; (2) however, despite this enhancement, planetesimal formation is impeded by the small grain size. Nevertheless, assuming that planetary cores are present in the inner disc, we then investigate the accretion of atmospheres onto cores and their subsequent photoevaporation. We then compare our results to the observed exoplanet mass-radius relationship. We find that: (1) the low gas surface densities and high temperatures in the inner disc reduce gas accretion onto cores compared to the minimum mass solar nebula, preventing the cores from growing into hot Jupiters, in agreement with the data; (2) however, our predicted envelope masses are still typically larger than observed ones. Finally, we sketch a qualitative picture of how grains may grow and planetesimals form in the inner disc if grain effects on the ionization levels and the MRI and the back-reaction of the dust on the gas (both neglected in our calculations) are accounted for., Comment: Accepted for publication in MNRAS

Comet composition provides critical information on the chemical and physical processes that took place during the formation of the Solar system. We report here on millimetre spectroscopic observations of the long-period bright comet C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1 receiver between 2015 January UT 16.948 to 18.120, when the comet was at heliocentric distance of 1.30 AU and geocentric distance of 0.53 AU. Bright comets allow for sensitive observations of gaseous volatiles that sublimate in their coma. These observations allowed us to detect HCN, CH3OH (multiple transitions), H2CO and CO, and to measure precise molecular production rates. Additionally, sensitive upper limits were derived on the complex molecules acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the strongest lines in the targeted spectral range to improve the signal-to-noise ratio. Gas production rates are derived using a non-LTE molecular excitation calculation involving collisions with H2O and radiative pumping that becomes important in the outer coma due to solar radiation. We find a depletion of CO in C/2014 Q2 (Lovejoy) with a production rate relative to water of 2 per cent, and relatively low abundances of Q(HCN)/Q(H2O), 0.1 per cent, and Q(H2CO)/Q(H2O), 0.2 per cent. In contrast the CH3OH relative abundance Q(CH3OH)/Q(H2O), 2.2 per cent, is close to the mean value observed in other comets. The measured production rates are consistent with values derived for this object from other facilities at similar wavelengths taking into account the difference in the fields of view. Based on the observed mixing ratios of organic molecules in four bright comets including C/2014 Q2, we find some support for atom addition reactions on cold dust being the origin of some of the molecules., 10 pages, 7 figures, to be published in MNRAS

Made available in DSpace on 2018-12-11T17:34:24Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-12-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) It is widely accepted that knowing the composition and the orbital evolution of asteroids might help us to understand the process of formation of the Solar system. It is also known that asteroids can represent a threat to our planet. Such an important role has made space missions to asteroids a very popular topic in current astrodynamics and astronomy studies. Taking into account the increasing interest in space missions to asteroids, especially to multiple systems, we present a study that aims to characterize the stable and unstable regions around the triple system of asteroids (45) Eugenia. The goal is to characterize the unstable and stable regions of this system and to make a comparison with the system 2001 SN263, which is the target of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) mission. A new concept was used for mapping orbits, by considering the disturbance received by the spacecraft from all perturbing forces individually. This method has also been applied to (45) Eugenia. We present the stable and unstable regions for particles with relative inclination between 0° and 180°. We found that (45) Eugenia presents larger stable regions for both prograde and retrograde cases. This is mainly because the satellites of this system are small when compared to the primary body, and because they are not close to each other. We also present a comparison between these two triple systems, and we discuss how these results can guide us in the planning of future missions. National Institute for Space Research (INPE) Instituto de Ciência e Tecnologia (ICT) Federal University de São Paulo (UNIFESP) Universidade Estadual Paulista (UNESP) Grupo de Dinâmica Orbital e Planetologia Universidade Estadual Paulista (UNESP) Grupo de Dinâmica Orbital e Planetologia CNPq: 150378/2015-7

International audience; We suggest that the large organic molecules found in the dust of comet 67P/CG originated from the interstellar medium (ISM), and that this material is the source of absorption features in stellar spectra known as diffuse interstellar bands (DIBs). These large organic molecules were present in the ISM prior to the emergence of the pre-solar nebula and were conserved during comet formation in the solar nebula due to gentle, hierarchical accretion, a scenario based on many lines of evidences collected with Rosetta rendezvous mission (Davidsson et al. 2016). This is in contrast with the sublimation of H2O ice, according to diverse comet D/H values. While the organic to mineral mass ratios RC for comet Halley and 67P/CG were measured in the range ∼0.32–1.0, we estimate that the DIB carriers alone can provide RISM = 0.32, but that this ratio could be increased by other organic molecules in the ISM that do not show up in absorption. The decrease of DIB during line-of-sight crossings of the dense cores of interstellar clouds and the simultaneous steepening of the far-UV part of the reddening curve suggest that DIB carriers coagulate and are constituent of the very small grains eventually preserved in comets. This conclusion implies that a future sample-return mission of a comet nucleus would not only provide unique information on comets, but also on the exact nature of the interstellar species producing the hundreds of DIBs, an unanswered question since their discovery several decades ago.

Solar system formation models predict that the building blocks of planetesimals were mm- to cm-sized pebbles, aggregates of ices and non-volatile materials, consistent with the compact particles ejected by comet 67P/Churyumov-Gerasimenko (67P hereafter) and detected by GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta spacecraft. Planetesimals were formed by the gentle gravitational accretion of pebbles, so that they have an internal macroporosity of 40 per cent. We measure the average dust bulk density ¿D = 795+840 -65 kg m-3 that, coupled to the nucleus bulk density, provides the average dust-toices mass ratio ¿ = 8.5. We find that the measured densities of the 67P pebbles are consistent with a mixture of (15 ± 6) per cent of ices, (5 ± 2) per cent of Fe-sulphides, (28 ± 5) per cent of silicates, and (52 ± 12) per cent of hydrocarbons, in average volume abundances. This composition matches both the solar and CI-chondritic chemical abundances, thus showing that GIADA has sampled the typical non-volatile composition of the pebbles that formed all planetesimals. The GIADA data do not constrain the abundance of amorphous silicates versus crystalline Mg, Fe-olivines and pyroxenes. We find that the pebbles have a microporosity of (52 ± 8) per cent (internal volume filling factor ¿P = 0.48 ± 0.08), implying an average porosity for the 67P nucleus of (71 ± 8) per cent, lower than previously estimated. © 2016 The Authors., This research was supported by the Italian Space Agency (ASI) within the INAF-ASI agreements I/032/05/0 and I/024/12/0.

Infrared observations provide the dust composition in the protoplanetary discs surface layers, but can not probe the dust chemistry in the midplane, where planet formation occurs. Meteorites show that dynamics was important in determining the dust distribution in the Solar Nebula and needs to be considered if we are to understand the global chemistry in discs. 1D radial condensation sequences can only simulate one disc layer at a time and cannot describe the global chemistry or the complexity of meteorites. To address these limitations, we compute for the first time the two dimensional distribution of condensates in the inner Solar Nebula using a thermodynamic equilibrium model, and derive timescales for vertical settling and radial migration of dust. We find two enstatite-rich zones within 1 AU from the young Sun: a band ~0.1 AU thick in the upper optically-thin layer of the disc interior to 0.8 AU, and in the optically-thick disc midplane out to ~0.4 AU. The two enstatite-rich zones support recent evidence that Mercury and enstatite chondrites shared a bulk material with similar composition. Our results are also consistent with infrared observation of protoplanetary disc which show emission of enstatite-rich dust in the inner surface of discs. The resulting chemistry and dynamics suggests that the formation of the bulk material of enstatite chondrites occurred in the inner surface layer of the disc, within 0.4~AU. We also propose a simple alternative scenario in which gas fractionation and vertical settling of the condensates lead to an enstatite-chondritic bulk material., Astroph version. Accepted in MNRAS on 2015 December 23. Received 2015 December 16; in original form 2015 May 04

We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in turbulent properties extending from the region of thermal ionization to 10 au., Comment: accepted to MNRAS after very minor revisions

One of many challenges in forming giant gas planets via Gravitational disc Instability model (GI) is an inefficient radiative cooling of the pre-collapse fragments. Since fragment contraction times are as long at $10^5 -10^7$ years, the fragments may be tidally destroyed sooner than they contract into gas giant planets. Here we explore the role of "pebble accretion" onto the pre-collapse giant planets and find an unexpected result. Despite larger dust opacity at higher metallicities, addition of metals actually accelerates -- rather than slows down -- collapse of high opacity, relatively low mass giant gas planets ($M_p$ below a few Jupiter masses). A simple analytical theory that explains this result exactly in idealised simplified cases is presented. The theory shows that planets with the central temperature in the range between $\sim$ 1000 to 2000K are especially sensitive to pebble accretion: addition of just $\sim 5$ to 10 % of metals by weight is sufficient to cause their collapse. These results show that dust grain physics and dynamics is essential for an accurate modelling of self-gravitating disc fragments and their near environments in the outer massive and cold protoplanetary discs., published as 2015MNRAS.446..459N

We study torques on migrating low-mass planets in locally isothermal discs. Previous work on low-mass planets generally kept the planet on a fixed orbit, after which the torque on the planet was measured. In addition to these static torques, when the planet is allowed to migrate it experiences dynamical torques, which are proportional to the migration rate and whose sign depends on the background vortensity gradient. We show that in discs a few times more massive than the Minimum Mass Solar Nebula, these dynamical torques can have a profound impact on planet migration. Inward migration can be slowed down significantly, and if static torques lead to outward migration, dynamical torques can take over, taking the planet beyond zero-torque lines set by saturation of the corotation torque in a runaway fashion. This means the region in non-isothermal discs where outward migration is possible can be larger than what would be concluded from static torques alone., 14 pages, 13 figures, accepted for publication in MNRAS

We present the results of a multi-wavelength study of circumstellar discs around 44 young stellar objects in the 3 Myr old nearby Chamaeleon I star-forming region. In particular, we explore the far-infrared/submm regime using Herschel fluxes. We show that Herschel fluxes at 160-500$\,\mu$m can be used to derive robust estimates of the disc mass. The median disc mass is 0.005$M_{\odot}$ for a sample of 28 Class IIs and 0.006$M_{\odot}$ for 6 transition disks (TDs). The fraction of objects in Chamaeleon-I with at least the `minimum mass solar nebula' is 2-7%. This is consistent with previously published results for Taurus, IC348, $\rho$ Oph. Diagrams of spectral slopes show the effect of specific evolutionary processes in circumstellar discs. Class II objects show a wide scatter that can be explained by dust settling. We identify a continuous trend from Class II to TDs. Including Herschel fluxes in this type of analysis highlights the diversity of TDs. We find that TDs are not significantly different to Class II discs in terms of far-infrared luminosity, disc mass or degree of dust settling. This indicates that inner dust clearing occurs independently from other evolutionary processes in the discs., Comment: 16 pages, 13 figures. Accepted for publication in MNRAS

Short lived radionuclides (SLRs) like $^{26}$Al are synthesized by massive stars and are a byproduct of star formation. The abundances of SLRs in the gas of a star-forming galaxy are inversely proportional to the gas consumption time. The rapid evolution of specific star formation rate (SSFR) of normal galaxies implies they had mean SLR abundances ~3--10 times higher at z = 2. During the epoch of Solar system formation, the background SLR abundances of the Galaxy were up to twice as high as at present, if SLR yields from massive stars do not depend on metallicity. If SLRs are homogenized in the gas of galaxies, the high SSFRs of normal galaxies can partly explain the elevated abundance of SLRs like $^{60}$Fe and $^{26}$Al in the early Solar system. Starburst galaxies have much higher SSFRs still, and have enormous mean abundances of $^{26}$Al ($^{26}$Al/$^{27}$Al ~ $10^{-3}$ for Solar metallicity gas). The main uncertainty is whether the SLRs are mixed with the star-forming molecular gas: they could be trapped in hot gas and decay before entering the colder phases, or be blown out by starburst winds. I consider how variability in star-formation rate affects the SLR abundances, and I discuss how SLR transport may differ in these galaxies. The enhanced $^{26}$Al of starbursts might maintain moderate ionization rates ($10^{-18}$ -- $10^{-17}$ s$^{-1}$), possibly dominating ionization in dense clouds not penetrated by cosmic rays. Similar ionization rates would be maintained in protoplanetary discs of starbursts, if the SLRs are well-mixed, and the radiogenic heating of planetesimals would likewise be much higher. In this way, galaxy evolution can affect the geological history of planetary systems., Published in MNRAS, 12 pages, 3 figures

HD 166191 has been identified by several studies as hosting a rare and extremely bright warm debris disc with an additional outer cool disc component. However, an alternative interpretation is that the star hosts a disc that is currently in transition between a full gas disc and a largely gas-free debris disc. With the help of new optical to mid-IR spectra and Herschel imaging, we argue that the latter interpretation is supported in several ways: i) we show that HD 166191 is co-moving with the ~4 Myr-old Herbig Ae star HD 163296, suggesting that the two have the same age, ii) the disc spectrum of HD 166191 is well matched by a standard radiative transfer model of a gaseous protoplanetary disc with an inner hole, and iii) the HD 166191 mid-IR silicate feature is more consistent with similarly primordial objects. We note some potential issues with the debris disc interpretation that should be considered for such extreme objects, whose lifetime at the current brightness is mush shorter than the stellar age, or in the case of the outer component requires a mass comparable to the solid component of the Solar nebula. These aspects individually and collectively argue that HD 166191 is a 4-5 Myr old star that hosts a gaseous transition disc. Though it does not argue in favour of either scenario, we find strong evidence for 3-5 um disc variability. We place HD 166191 in context with discs at different evolutionary stages, showing that it is a potentially important object for understanding the protoplanetary to debris disc transition., Comment: accepted to MNRAS, fixed typos in abstract and axis label

The Comet Astrobiology Exploration Sample Return (CAESAR) mission is one of two finalists selected by NASA for Phase A study in the New Frontiers program. CAESAR will acquire a minimum of 80 grams of material from the surface of comet 67P/Churuyumov-Gerasimenko and return it to Earth for laboratory analysis. CAESAR preserves much of the science of a cryogenic sample return by retaining volatiles in a dedicated reservoir securely separated from the solid sample. Comet 67P was selected based on its favorable orbital geometry and the risk reduction and scientific context provided by the ESA (European Space Agency)'s Rosetta mission. CAESAR's objectives are to understand the origins of the Solar System starting materials and how these components came together to form planets and give rise to life. We also seek to resolve the conflicting views of comet origins arising from the Stardust and Rosetta missions. While the greater than 1 micron solids returned by Stardust originated in the hot, inner solar nebula, measurements by Rosetta suggest 67P volatiles formed at cryogenic temperatures and remained unchanged for billions of years. This dichotomy provides the rationale for returning both solid and gaseous samples.

Knowledge of water in the solar system is important for understanding of a wide range of evolutionary processes and the thermal history of the solar system. To explore the existence of water in the solar system, it is indispensable to investigate hydrated minerals and/or water ice on asteroids. These water-related materials show absorption features in the 3-$\micron$ band (wavelengths from 2.7 to 3.1 $\micron$). We conducted a spectroscopic survey of asteroids in the 3-$\micron$ band using the Infrared Camera (IRC) on board the Japanese infrared satellite AKARI. In the warm mission period of AKARI, 147 pointed observations were performed for 66 asteroids in the grism mode for wavelengths from 2.5 to 5 $\micron$. According to these observations, most C-complex asteroids have clear absorption features ($> 10\%$ with respect to the continuum) related to hydrated minerals at a peak wavelength of approximately 2.75 $\micron$, while S-complex asteroids have no significant feature in this wavelength range. The present data are released to the public as the Asteroid Catalog using AKARI Spectroscopic Observations (AcuA-spec)., 57 pages, 23 figures, 7 tables, accepted for publication in Publications of the Astronomical Society of Japan

The Kepler-36 system consists of two planets that are spaced unusually close together, near the 7:6 mean motion resonance. While it is known that mean motion resonances can easily form by convergent migration, Kepler-36 is an extreme case due to the close spacing and the relatively high planet masses of 4 and 8 times that of the Earth. In this paper, we investigate whether such a system can be obtained by interactions with the protoplanetary disc. These discs are thought to be turbulent and exhibit density fluctuations which might originate from the magneto-rotational instability. We adopt a realistic description for stochastic forces due to these density fluctuations and perform both long term hydrodynamical and N-body simulations. Our results show that planets in the Kepler-36 mass range can be naturally assembled into a closely spaced planetary system for a wide range of migration parameters in a turbulent disc similar to the minimum mass solar nebula. The final orbits of our formation scenarios tend to be Lagrange stable, even though large parts of the parameter space are chaotic and unstable., Comment: 13 pages, 8 figures, accepted for publication in MNRAS

The aim of the present paper is to investigate the spatial structure of a protoplanetary disc whose dynamics is governed by magnetorotational turbulence. We perform a series of local 3D chemo-radiative MHD simulations located at different radii of a disc which is twice as massive as the standard minimum mass solar nebula of Hayashi (1981). The ionisation state of the disc is calculated by including collisional ionisation, stellar X-rays, cosmic rays and the decay of radionuclides as ionisation sources, and by solving a simplified chemical network which includes the effect of the absorption of free charges by {\mu}m-sized dust grains. In the region where the ionisation is too low to assure good coupling between matter and magnetic fields, a non-turbulent central "dead zone" forms, which ranges approximately from a distance of 2 AU to 4 AU from the central star. The approach taken in the present work allows for the first time to derive the global spatial structure of a protoplanetary disc from a set of physically realistic numerical simulations.

Trojan asteroids are minor planets that share the orbit of a planet about the Sun and librate around the L4 or L5 Lagrangian points of stability. They are important solar-system fossils because they carry information on early Solar System formation, when collisions between bodies were more frequent. Discovery and study of terrestrial planet Trojans will help constrain models for the distribution of bodies and interactions in the inner Solar System. Since the discovery of the first outer planet Trojan in 1906, several thousand Jupiter Trojans have been found. Of the terrestrial planets there are four known Mars Trojans, and one Earth Trojan has been recently discovered. We present a new model that constrains optimal search areas, and imaging cadences for narrow and wide field survey telescopes including the Gaia satellite for the most efficient use of telescope time to maximize the probability of detecting additional Earth Trojans.

The origins of irregular satellites of the giant planets are an important piece of the giant "puzzle" that is the theory of Solar System formation. It is well established that they are not "in situ" formation objects, around the planet, as are believed to be the regular ones. Then, the most plausible hypothesis to explain their origins is that they formed elsewhere and were captured by the planet. However, captures under restricted three-body problem dynamics have temporary feature, which makes necessary the action of an auxiliary capture mechanism. Nevertheless, there not exist one well established capture mechanism. In this work, we tried to understand which aspects of a binary-asteroid capture mechanism could favor the permanent capture of one member of a binary asteroid. We performed more than eight thousand numerical simulations of capture trajectories considering the four-body dynamical system Sun, Jupiter, Binary-asteroid. We restricted the problem to the circular planar prograde case, and time of integration to 10^4 years. With respect to the binary features, we noted that 1) tighter binaries are much more susceptible to produce permanent captures than the large separation-ones. We also found that 2) the permanent capture probability of the minor member of the binary is much more expressive than the major body permanent capture probability. On the other hand, among the aspects of capture-disruption process, 4) a pseudo eastern-quadrature was noted to be a very likely capture angular configuration at the instant of binary disruptions. In addition, we also found that the 5) capture probability is higher for binary asteroids which disrupt in an inferior-conjunction with Jupiter. These results show that the Sun plays a very important role on the capture dynamic of binary asteroids.

We investigated the probability that an inelastic collision of planetesimals within the Hill sphere of the Jovian planets could explain the presence and orbits of observed irregular satellites. Capture of satellites via this mechanism is highly dependent on not only the mass of the protoplanetary disk, but also the shape of the planetesimal size distribution. We performed 2000 simulations for integrated time intervals $\sim 2$ Myr and found that, given the currently accepted value for the minimum mass solar nebula and planetesimal number density based upon the \citet{Nesvorny2003} and \citet{Charnoz2003} size distribution $dN \sim D^{-3.5} dD$, the collision rates for the different Jovian planets range between $\sim 0.6$ and $\gtrsim 170 \, \Myr^{-1}$ for objects with radii, $1 \, \km \le r \le 10 \, \km$. Additionally, we found that the probability that these collisions remove enough orbital energy to yield a bound orbit was $\lesssim 10^{-5}$ and had very little dependence on the relative size of the planetesimals. Of these collisions, the collision energy between two objects was $\gtrsim 10^3$ times the gravitational binding energy for objects with radii $\sim 100$ km. We find that, capturing irregular satellites via collisions between unbound objects can only account for $\sim 0.1%$ of the observed population, hence can this not be the sole method of producing irregular satellites.

Dawn space mission will provide the first, detailed data of two of the major bodies in the main asteroid belt, Vesta and Ceres. Through its connection with Howardite, Eucrite, Diogenite (HED) meteorites, Vesta is known as one of the first bodies to have accreted and differentiated in the solar nebula, predating the formation of Jupiter and surviving the violent evolution of the early Solar system. The formation time of Ceres instead is unknown, but it should not postdate that of Jupiter by far, since the perturbations of the giant planet stopped planetary accretion in the main asteroid belt. In this work we modelled the collisional histories of Vesta and Ceres at the time of the formation of Jupiter, assumed to be the first giant planet to form. In this first investigation of the evolution of the early Solar system, we did not include the presence of planetary embryos in the disc of planetesimals but concentrated on the role of the forming Jupiter and the effects of its possible inward migration due to disc-planet interactions. Our results clearly indicate that the formation of the giant planet caused an intense early bombardment in the orbital region of the main asteroid belt. We explored the effects of such bombardment on Vesta and Ceres assuming different size distributions of the primordial planetesimals. According to our results, Vesta and Ceres would not have survived the Jovian early bombardment if the disc was populated mainly by large planetesimals like those predicted to form in turbulent circumstellar discs. Discs dominated by small bodies, like those predicted to form in quiescent circumstellar discs, or with a varying fraction of the mass in the form of larger (D ≥ 100 km) planetesimals, represent more favourable environments for the survival of the two asteroids. The abundance of planetesimals, especially the larger ones, proved a critical factor in this regard. The extent of Jupiter's radial migration due to disc-planet interactions proved itself another critical factor. In those scenarios where they survive, both asteroids had their surfaces saturated by craters as big as 150 km and a few as big as 200-300 km. In the case of Vesta, the Jovian early bombardment would have significantly eroded (locally or globally) the crust and possibly caused effusive phenomena similar to the lunar maria, whose crystallization time would then be directly linked to the time of the formation of Jupiter.

We calculate radial migration rates of protoplanets in laminar minimum mass solar nebula discs using three-dimensional self-gravitating radiation hydrodynamical (RHD) models. The protoplanets are free to migrate, whereupon their migration rates are measured. For low mass protoplanets (10-50 M_\oplus) we find increases in the migration timescales of up to an order of magnitude between locally-isothermal and RHD models. In the high-mass regime the migration rates are changed very little. These results are arrived at by calculating migration rates in locally-isothermal models, before sequentially introducing self-gravity, and radiative transfer, allowing us to isolate the effects of the additional physics. We find that using a locally-isothermal equation of state, without self-gravity, we reproduce the migration rates obtained by previous analytic and numerical models. We explore the impact of different protoplanet models, and changes to their assumed radii, upon migration. The introduction of self-gravity gives a slight reduction of the migration rates, whilst the inertial mass problem, which has been proposed for high mass protoplanets with circumplanetary discs, is reproduced. Upon introducing radiative transfer to models of low mass protoplanets (\approx 10 M_\oplus), modelled as small radius accreting point masses, we find outward migration with a rate of approximately twice the analytic inward rate. However, when modelling such a protoplanet in a more realistic manner, with a surface which enables the formation of a deep envelope, this outward migration is not seen.

The Ortho-to-Para abundance ratio (OPR) of cometary molecules is considered to be one of primordial characters of comeary ices, which has information concerning their formation. In order to determine the OPR of cometary ammonia (NH3), high-dispersion spectra in the optical wavelength region have been the most powerful tool, since NH2 is a photo-dissociation product of NH3 ,a nd NH 2 has a strong electronic transition in the optical region. Here we present an improved procedure to derive the OPR of NH2 (OPR of NH3 is derived from that of NH2) from the high-dispersion spectra of NH2 in comets. We consider (1) the contamination of NH2 lines by the C2 Swan-band lines, (2) the influence by the telluric absorption lines, and (3) multiple measurements of OPR from different vibronic bands of NH2 in the optical region, to achieve a more accurate determination of OPRs than that in previous studies. We applied the new method to the spectrum of comet C/2001 Q4 (NEAT) for which the OPR of NH2 was determined in the previous study. The derived OPR of NH2 is 3.23 ˙ 0.03, and this is more accurate than before. The OPR of NH3 is estimated to be 1.12 ˙ 0.02, which corresponds to a nuclear spin temperature (Tspin )o f 30 +2 � 1 K. The Tspin value of NH3 is basically consistent with the temperature inferred from the mixing ratio of CO with respect to H2O found in the same comet.

Accepted: 2009-10-13, 資料番号: SA1001036000

14 pages, 13 figures.-- Printed version published Oct 2009., ArXiv pre-print available at: http://arxiv.org/abs/0906.3755, The Nice model of Gomes et al. suggests that the migration of the giant planets caused a planetesimal clearing event, which led to the late heavy bombardment (LHB) at 880 Myr. Here, we investigate the infrared emission from the Kuiper belt during the history of the Solar system as described by the Nice model. We describe a method for easily converting the results of N-body planetesimal simulations into observational properties (assuming blackbody grains and a single size distribution) and further modify this method to improve its realism (using realistic grain properties and a three-phase size distribution). We compare our results with observed debris discs and evaluate the plausibility of detecting an LHB-like process in extrasolar systems. Recent surveys have shown that 4 per cent of stars exhibit 24 μm excess and 16 per cent exhibit 70 μm excess. We show that the Solar system would have been amongst the brightest of these systems before the LHB at both 24 and 70 μm. We find a significant increase in 24 μm emission during the LHB, which rapidly drops off and becomes undetectable within 30 Myr, whereas the 70 μm emission remains detectable until 360 Myr after the LHB. Comparison with the statistics of debris disc evolution shows that such depletion events must be rare occurring around less than 12 per cent of Sun-like stars and with this level of incidence we would expect approximately one of the 413 Sun-like field stars so far detected to have a 24 μm excess to be currently going through an LHB. We also find that collisional processes are important in the Solar system before the LHB and that parameters for weak Kuiper belt objects are inconsistent with the Nice model interpretation of the LHB., MB acknowledges the UKPPARC/STFC for a research studentship.

On the basis of their orbital elements, present-day Kuiper belt objects can be grouped into distinct dynamical classes: classical, resonant and scattered ones. Jiang & Yeh have proposed gas-drag-induced resonant capture in a protostellar disc analogous to the primordial solar nebula as a mechanism able to explain the dominant 3:2 resonant population observed in Kuiper belt objects. de la Fuente Marcos & de la Fuente Marcos further investigated the drag-induced mechanism numerically. Our significant contribution is a hydrodynamic theory derivation of results obtained in the Jiang & Yeh and de la Fuente Marcos & de la Fuente Marcos numerical simulations.

The formation of a circumplanetary disc and accretion of angular momentum on to a protoplanetary system are investigated using three-dimensional hydrodynamical simulations. The local region around a protoplanet in a protoplanetary disc is considered with sufficient spatial resolution: the region from outside the Hill sphere to the Jovian radius is covered by the nested-grid method. To investigate the thermal effects of the circumplanetary disc, various equations of state are adopted. Large thermal energy around the protoplanet slightly changes the structure of the circumplanetary disc. Compared with a model adopting an isothermal equation of state, in a model with an adiabatic equation of state, the protoplanet's gas envelope extends farther, and a slightly thick disc appears near the protoplanet. However, different equations of state do not affect the acquisition process of angular momentum for the protoplanetary system. Thus, the specific angular momentum acquired by the system is fitted as a function only of the protoplanet's mass. A large fraction of the total angular momentum contributes to the formation of the circumplanetary disc. The disc forms only in a compact region in very close proximity to the protoplanet. Adapting the results to the Solar system, the proto-Jupiter and Saturn have compact discs in the region of r < 21r Jup (r < 0.028 r H,Jup ) and r < 66r Sat (r < 0.061r H,sat ), respectively, where r Jup (r H,Jup ) and r Sat (r H.Sat ) are the Jovian and Satumian (Hill) radius, respectively. The surface density has a peak in these regions due to the balance between centrifugal force and gravity of the protoplanet. The size of these discs corresponds well to the outermost orbit of regular satellites around Jupiter and Saturn. Regular satellites may form in such compact discs around proto-gas giant planets.

Comet 73P/Schwasmann–Wachmann 3 (hereafter SW3) is known as a split comet, and its fragments are the best targets to investigate the homogeneity of a cometary nucleus. We conducted optical low-dispersion spectroscopic observations of the brightest fragments, B and C, of comet SW3 with the NAYUTA telescope in 2006 early May, in the vicinity of the closest approach to Earth on 2006 May 12, UT. We report on the chemical compositions of these fragments. Both fragments are similar to each other in composition, and are significantly depleted in C2 compared with other comets.

Jiang & Yeh proposed gas-drag-induced resonant capture as a mechanism able to explain the dominant 3:2 resonance observed in the trans-Neptunian belt. Using a model of a disc-star-planet system they concluded that gaseous drag in a protoplanetary disc can trap trans-Neptunian object (TNO) embryos into the 3:2 resonance rather easily although it could not trap objects into the 2:1 resonance. Here we further investigate this scenario using numerical simulations within the context of the planar restricted four-body problem by including both present-day Uranus and Neptune. Our results show that mean motion and corotation resonances are possible and trapping into both the 3:2 and 2:1 resonances as well as other resonances is observed. The associated corotation centres may easily form larger planetesimals from smaller ones. Corotation resonances evolve into pure Lindblad resonances in a time-scale of 0.5 Myr. The non-linear corotation and mean motion resonances produced are very size selective. The 3:2 resonance is dominant for submetric particles but for larger particles the 2:1 resonance is stronger. In summary, our calculations show that confined chaotic motion around the resonances not only increases trapping efficiency but also the orbital eccentricities of the trapped material, modifying the relative abundance of trapped particles in different resonances. If we assume a more compact planetary system, instead of using the present-day values of the orbital elements of Uranus and Neptune, our results remain largely unchanged.

We propose a scenario for the formation of the Main Belt in which asteroids incorporated icy particles formed in the outer Solar Nebula. We calculate the composition of icy planetesimals formed beyond a heliocentric distance of 5 AU in the nebula by assuming that the abundances of all elements, in particular that of oxygen, are solar. As a result, we show that ices formed in the outer Solar Nebula are composed of a mix of clathrate hydrates, hydrates formed above 50 K and pure condensates produced at lower temperatures. We then consider the inward migration of solids initially produced in the outer Solar Nebula and show that a significant fraction may have drifted to the current position of the Main Belt without encountering temperature and pressure conditions high enough to vaporize the ices they contain. We propose that, through the detection and identification of initially buried ices revealed by recent impacts on the surfaces of asteroids, it could be possible to infer the thermodynamic conditions that were present within the Solar Nebula during the accretion of these bodies, and during the inward migration of icy planetesimals. We also investigate the potential influence that the incorporation of ices in asteroids may have on their porosities and densities. In particular, we show how the presence of ices reduces the value of the bulk density of a given body, and consequently modifies its macro-porosity from that which would be expected from a given taxonomic type.

A model of the Oort cloud has been developed by accounting for planetary, stellar and Galactic perturbations using numerical symplectic integrations covering 4.5 Gyr. The model is consistent with the broad dynamical characteristics of the observed cometary populations injected from the Oort cloud into different regions of the Solar system. We show that the majority of observed high-eccentricity trans-Neptunian objects, Centaurs and short-period comets have visited the Oort cloud (a > 1000 au) during their dynamical history. Assuming from observations that the near-parabolic flux from the Oort cloud with absolute magnitudes H 10 < 7, perihelion distances q < 5 au and a > 10 4 au is approximately 1 comet per year, our calculations imply a present Oort cloud population of ∼5 × 10 11 comets with H 10 < 10.9. Roughly half this number have a > 10 4 au. The number of comets reaching the planetary region from the Oort cloud (a > 1000 au) is more than an order of magnitude higher per unit perihelion distance immediately beyond Neptune than in the observable zone q < 5 au. Similarly, the new-comet flux from the Oort cloud per unit perihelion distance is a few tens of times higher in the near-Neptune region than in the observable zone. The present number of high-eccentricity trans-Neptunian objects (q > 30 au and 60 < a < 1000 au) originating from the Oort cloud is in the approximate range 1-3 × 10 10, depending on details of the initial model. A substantial fraction of these have a > 200 au and/or q > 40 au, and they are found mostly to originate from initial orbits with 25 < q < 36 au. Similarly, the number of Centaurs produced from the Oort cloud, where we define Centaurs to have 5 < q < 28 au and a < 1000 au, is smaller by a factor of 20-30. About 90 per cent of these Centaurs have a > 60 au. Objects that have visited the Oort cloud represent a substantial fraction of the Jupiter-family comet population, achieving short-period orbits by a process of gradual dynamical transfer, including a Centaur stage, from the outer Solar system to near-Earth space. A similar mechanism produces Halley-type comets, in addition to the well-known diffusion process operating at small perihelion distances. © 2007 RAS.

Disks in the 6 Myr old cluster eta Chamaeleontis were searched for emission from hot H2. Around the M3 star ECHAJ0843.3-7905 we detect circumstellar gas orbiting at ~2 AU. If the gas is UV-excited, the ro-vibrational line traces a hot gas layer supported by a disk of mass ~0.03Msolar, similar to the minimum mass solar nebula. Such a gas reservoir at 6 Myr would promote the formation and inwards migration of gas giant planets., Accepted for publication in MNRAS. 9 pages

Eris, an object larger than Pluto, is known to reside in the transneptunian region further away than Pluto. One can wonder whether its semimajor orbital axis ts in a generalized Titius-Bode law, in the same way as Pluto does. We performed a new least squares t to a generalized Titius-Bode law including Eris and found that not only does Eris t in the trend, but also that the correlation coecien t improves. In addition, there is a remarkable symmetry of the location of the planetary formation regions with respect to Jupiter when the natural logarithm of the heliocentric distance is used as the metric. The issue of whether the observed patterns have some physical meaning or are due to mere chance is addressed using a Monte Carlo approach identical to that by Lynch (Lynch, P. On the signicance of Titius-Bode law for the distribution of planets. Mon. Not. R. Astron. Soc. 341, 1174-1178. 2003). Although the probability of chance occurrence is highly dependent on the way in which the random congurations of synthetic planetary systems are selected, we nd that in all reasonable scenarios of random planetary systems the probability of chance ocurrence of the observed patterns is small (below 1% in most cases). If the trend were used as a prediction tool, one might expect another planet or dwarf planet or a swarm of bodies with semimajor orbital axis of 120 AU 20 AU. Simple calculations show that the protoplanetary nebula most likely had enough mass to allow the accretion of at least a dwarf planet at that distance. We also found that if the surface density of the nebula decayed with heliocentric distance (r) as a power of -2, the regular spacing in ln(r) in the solar system could be a natural consequence of the existence of a threshold mass for planetary formation.

The results of a numerical study of the growth of solid particles, ranging from 1 micron to 1 millimeter in size, in the vicinity of an azimuthally symmetric density enhancement of a protostellar disk are presented. It is shown that the combined effect of gas drag and pressure gradients, which causes solid objects to rapidly migrate toward the location of the local maximum density, can also enhance the rate of growth of dust particles to larger objects. The results of numerical simulations of such growth processes are presented and the effects of the changes of the physical properties of particles on the rates of their growth are also discussed.

A numerical study of an ensemble of orbits based on observed objects in the near-Neptune high-eccentricity (NNHE) region, with perihelion distances q in the range 28 60 au being roughly an order of magnitude greater than that for a < 60 au, and therefore inconsistent with a source in the NNHE region, which is broadly similar to the so-called ‘Scattered Disc’. The observed distribution of Centaurs with a 60 au is also inconsistent with this source, although it is conceivable that in this region the discrepancies might be explained by factors such as out-gassing, splitting or varying albedo not included in our model. Thus, although Centaurs can be produced from the NNHE region, their numbers and orbital distributions are inconsistent with this region being the dominant source for all Centaurs. We conclude that there must be another source flux, especially for the longer period, more populous group, and suggest that the most likely source for these objects is the Oort cloud. Thus, there are two separate, but overlapping, dynamical classes of Centaurs, one originating from the Oort cloud and the other from the NNHE region. The two source regions produce roughly similar contributions to Centaurs with a 60 au and to the observed Jupiter family of comets.

We revisit the scenario proposed by Duncan and Levison in the late 1990s on the origin of the trans-Neptunian scattered disc. According to this scenario, the current scattered disc population is the remnant of a much more massive population that formed at the beginning of the Solar system, presumably when Neptune grew in mass. In order to compute the expected orbital distribution of the scattered disc bodies in the framework of this model, we have integrated the evolution of several thousands of test particles over the age of the Solar system, and looked at the orbital distribution of those surviving after more than 2 x 10 9 yr from their first scattering event. In order to compare this model distribution with the observed distribution, we have modelled the observational biases by generalizing a method originally introduced recently by Trujillo and Brown. Once the biases are taken into account, the model distribution matches the observed distribution fairly well. The most significant discrepancy is that the observed perihelion distance distribution is somewhat skewed towards larger perihelion distances than our model predicts. This is possibly due to the effects of planet migration (which tends to raise perihelion distances as recently shown by Gomes), which is not taken into account in our simulations.

Most mechanisms proposed for the formation of planets are modified versions of the mechanism proposed for the solar system. Here we argue that, in terms of those planetary systems which have been observed, the case for the solar system being a typical planetary system has yet to be established. We consider the possibility that most observed planetary systems have been formed in some quite different way. If so, it may be that none of the observed planetary systems is likely to harbour an earth-like planet., 6 pages, 3 figures, MNRAS accepted

The dynamical evolution of trans-Neptunian objects (TNOs) to the inner Solar system is investigated. The study is based on the observed sample of high-eccentricity TNOs with perihelia in the near-Neptune region, using a procedure to take account of observational biases. It is shown that observations favour TNOs in high-eccentricity orbits as the main source of Jupiter-family (JF) comets. The relative fraction of objects captured per year from the near-Neptune region to JF comets with perihelion distances q < 1.5 au is estimated as 0.2 x 10 -10 . The maximum lifetime of typical JF comets with q < 1.5 au is approximately 200 revolutions. Based on the observed population of JF comets, there should be ∼10 10 TNOs of cometary size in high-eccentricity orbits with 28 < q < 35.5 au. If this population originated 4.5 Gyr ago, the primordial number must have been at least 20 times as large as the present one.

Panspermia theories require the transport of micro-organisms in a viable form from one astronomical location to another. The evidence of material ejection from planetary surfaces, of dynamical orbit evolution and of potential survival on landing is setting a firm basis for interplanetary panspermia. Pathways for interstellar panspermia are less clear. We compare the direct route, whereby life-bearing planetary ejecta exit the Solar system and risk radiation hazards en route to nearby stellar systems, and an indirect route whereby ejecta hitch a ride within the shielded environment of comets of the Edgeworth-Kuiper Belt that are subsequently expelled from the Solar system. We identify solutions to the delivery problem. Delivery to fully fledged planetary systems of either the direct ejecta or the ejecta borne by comets depends on dynamical capture and is of very low efficiency. However, delivery into a protoplanetary disc of an early solar-type nebula and into pre-stellar molecular clouds is effective, because the solid grains efficiently sputter the incoming material in hypervelocity collisions. The total mass of terrestrial fertile material delivered to nearby pre-stellar systems as the Solar system moves through the Galaxy is from kilograms up to a tonne. Subject to further study of bio-viability under irradiation and fragmenting collisions, a few kg of original grains and sputtered fragments could be sufficient to seed the planetary system with a wide range of Solar system micro-organisms.

Low-dispersion spectroscopic observations of C/2001 A2 (LINEAR) were performed before and after its perihelion passage. C/2001 A2 (LINEAR) showed the first outburst late in 2001 March, and its visual magnitude increased by more than 5mag. In addition to this outburst, three outbursts were recognized around the perihelion passage of the comet. These outbursts are thought to be associated with splitting events of the cometary nucleus. Our first observation was performed on 2001 April 4, during the first outburst. After its perihelion passage, its spectra were obtained on July 2, with its normal activity, and July 12, during an outburst. We determined the gas-to-dust ratios [Q(H2O)/Afρ], as well as gas production rate ratios of CN, C2, and NH2 relative to water [Q(X)/Q(H2O)] for each observation. Obtained gas-to-dust ratios (26.4–26.7) showed that C/LINEAR was a gas-rich comet. Our results showed that there were no significant changes in the gas-to-dust ratio and gas production rate ratios during 3 months around perihelion passage. In comparison with other comets, there were no significant differences in the chemical composition for C/2001 A2 (LINEAR).

The radii of the planetary and satellite orbits are in approximate agreement with geometric progressions. The question of whether the observed patterns have some physical basis or are due to chance may be addressed using a Monte Carlo approach. We find that the estimated probability of chance occurrence depends sensitively on the restrictions imposed on the population of orbits. We argue that it is not possible to conclude unequivocally that laws of Titius‐Bode type are, or are not, significant. Therefore, the possibility of a physical explanation for the observed distributions remains open.

The migration of planets has usually been considered in relation to circular orbits of protoplanets for which type I and II migration mechanisms are appropriate. There are theories for planetary origin in which protoplanets are produced in extended eccentric orbits requiring a different theoretical treatment. The basic mechanism for the action of a resisting medium involves gravitational interaction with the planet and the simple application of the principle of conservation of momentum. A gaseous resisting medium will be in the form of a flared disc with a three-dimensional structure. Such a medium is simulated by a distribution of non-interacting particles, initially in a stable orbit around the central star. Their gravitational interactions with the protoplanet are softened, as for particles in smoothed-particle hydrodynamics, to avoid gravitational singularities. Computations, in which parameters are varied in a systematic manner, give a range of final orbits similar to those observed and inferred for exoplanets. When the motion of the medium is affected by high luminosity and/or a strong stellar wind from an active young star, orbits of high eccentricity may be an outcome, as is observed for some exoplanets.

A symplectic integrator is used to study the evolution of high-eccentricity trans-Neptunian objects (TNOs) over the age of the Solar system. For 26 objects, a few cloned orbits were integrated. We demonstrate the existence of several known bodies that are in relatively stable orbits located far from Neptune for the age of the Solar system. Thus, we provide an indication of the structure of the protoplanetary disc immediately after the period of planet formation. The orbits of these bodies cannot be explained by a model in which a near-Neptune disc of planetesimals is gravitationally scattered by Neptune. In this paper, therefore, we demonstrate the existence of a new, and populous, class of ‘outer’ TNOs which have substantially different dynamical characteristics from those of scattered disc objects.

Assuming the Jovian planets experienced smooth orbital migrations, Malhotra successfully explained some important features of the Kuiper Belt by the mechanism of resonance capture. However, the migration should really be stochastic. In this paper we numerically simulate numerous orbital evolutions of test particles under such stochastic planet migrations. The main results are as follows. Given the proper parameters, the concentration of objects in the 3 : 2 resonance is distinct, while very few objects enter the 2 : 1 resonance, and consequently, the orbital distribution of Kuiper Belt objects matches the observational data very well. The stochastic effects excite the orbital eccentricities and inclinations of objects in the non-resonant regions. The introduction of stochastic effects also gives possible explanations of the mass depletion, and of the sources of the classical and scattered Kuiper Belt objects.

The 2 : 1 mean motion resonance with Jupiter in the main asteroid belt is associated to one of the largest Kirkwood gaps: the so-called Hecuba gap. Centred at about 3.3 au, the Hecuba gap is characterized by a very small number of asteroids when compared to its neighbourhoods. Long-term instabilities caused by resonant planetary perturbations are thought to be responsible for the almost lack of bodies in the gap. However, current observations suggest a significant population of asteroids in the 2 : 1 resonance. The origin of these bodies is puzzling. Do we observe the few lucky survivors of a much larger population formed in the resonance in primordial times? Do the resonant orbits of the observed asteroids have a more recent origin? To understand these issues, we performed numerical simulations of the orbital evolution of both real and fictitious asteroids in the 2 : 1 resonance. Our models include gravitational perturbations by the major planets. Based on the dynamical lifetimes, we classify the observed resonant asteroids into three groups: (i) the Zhongguos, which seems to be stable over the age of the Solar system; (ii) the Griquas, with typical lifetimes in the resonance of the order of some 100 Myr; and (iii) the strongly unstable asteroids, which escape from the resonance in a few 10 Myr or less. Our simulations confirm that the Zhongguos may be primordial asteroids, located in the 2 : 1 resonance since the formation of the Solar system. The dynamics of the Zhongguos constitute a typical example of slow chaotic evolution confined to a small region of the resonance. On the other hand, an analysis of the size distribution of the Zhongguos reveals a rather steep distribution. Such a distribution would not be compatible with a long collisional history, rather suggesting that the Zhongguos are likely to be the outcomes of a recent breakup event. Thus, while dynamics points toward a primordial resonant origin, the size distribution rather points to a recent origin. A possible explanation is that the Zhongguos formed by the cratering/ fragmentation of a large resonant or near-resonant asteroid.

This paper presents a model for the outcome of collisions between planetesimals in a debris disk and assesses the impact of collisional processes on the structure and size distribution of the disk. The model is presented by its application to Fomalhaut's collisionally replenished dust disk; a recent 450 micron image of this disk shows a clump embedded within it with a flux ~5 per cent of the total. The following conclusions are drawn: (i) SED modelling is consistent with Fomalhaut's disk having a collisional cascade size distribution extending from bodies 0.2 m in diameter down to 7 micron-sized dust. (ii) Collisional lifetime arguments imply that the cascade starts with planetesimals 1.5-4 km in diameter. Any larger bodies must be predominantly primordial. (iii) Constraints on the timescale for the ignition of the cascade are consistent with these primordial planetesimals having a distribution that extends up to 1000km, resulting in a disk mass of 5-10 times the minimum mass solar nebula. (iv) The debris disk is expected to be intrinsically clumpy, since planetesimal collisions result in dust clumps. The intrinsic clumpiness of Fomalhaut's disk is below current detection limits, but could be detectable by future observatories such as the ALMA, and could provide the only way of determining the primordial planetesimal population. (v) The observed clump could have originated in a collision between two runaway planetesimals, both larger than 1400 km diameter. It is unlikely that we should witness such an event unless both the formation of these runaways and the ignition of the collisional cascade occurred within the last ~10 Myr. (vi) Another explanation for Fomalhaut's clump is that ~5 per cent of the planetesimals in the ring are trapped in 1:2 resonance with a planet orbiting at 80 AU., Comment: 21 pages, 13 figures, accepted by MNRAS

We have performed N-body numerical simulations of the exchange of angular momentum between a massive planet and a 3D Keplerian disc of planetesimals. Our interest is directed at the study of the classical analytical expressions of the lineal theory of density waves, as representative of the dynamical friction in discs 'dominated by the planet' and the orbital migration of the planets with regard to this effect. By means of a numerical integration of the equations of motion, we have carried out a set of numerical experiments with a large number of particles (N ≥ 10 000), and planets with the mass of Jupiter, Saturn and one core mass of the giant planets in the Solar system (Mc = 10M⊕). The torque, measured in a phase in which a 'steady forcing' is clearly measurable, yields inward migration in a minimum-mass solar disc (∑ ∼ 10 g cm-2 ), with a characteristic drift time of ∼ a few 106 yr. The planets predate the disc, but the orbital decay rate is not sufficient to allow accretion in a time-scale relevant to the formation of giant planets. We found reductions of the measured torque on the planet, with respect to the linear theory, by a factor of 0.38 for Mc, 0.04 for Saturn and 0.01 for Jupiter, due to the increase in the perturbation on the disc. The behaviour of planets whose mass is larger than Mc is similar to the one of type II migrators in gaseous discs. Our results suggest that, in a minimum mass, solar planetesimals disc, type I migrations occur for masses smaller than Mc, whereas for this mass value it could be a transition zone between the two types of migration., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata