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Endogenous or exogenous, dry or wet, various scenarios have been so far depicted for the origin of water on our Solar Systems rocky bodies. Hydrated silicates found in meteorites and in interplanetary dust particles together with observations of abundant water reservoirs in the habitable zone of protoplanetary disks are evidences that support aqueous alteration of silicate dust grains by water vapor condensation in a nebular setting. We investigate the thermodynamics (temperature and pressure dependencies) and kinetics (adsorption rates and energies, surface diffusion and cluster formation) of water adsorption on surfaces of forsterite grains. (abbreviated) The protoplanetary disk model (ProDiMo) code is tuned to simulate the thermochemical disk structure of the early solar nebula at three evolutionary stages. Pressure, temperature and water vapor abundance within 1 au from the protosun were extracted and used as input for a Monte Carlo code to model water associative adsorption using adsorption energies that resemble the forsterite [100] crystal lattice. Hydration of forsterite surfaces by water vapor adsorption could have occurred within the nebula lifetime already at a density of 1e8 cm-3, with increasing surface coverage for higher water vapor densities. Full surface coverage is attained for temperatures lower than 500 K, while for hotter grain surfaces water cluster formation plays a crucial role. (abbreviated) This work shows that water cluster formation enhances the water surface coverage and enables a stable water layer to form at high temperature and low water vapor density conditions. Finally, surface diffusion of physisorbed water molecules shortens the timescale for reaching steady state, enabling phyllosilicate formation within the solar nebula timescale., Comment: A&A accepted

Context.The discovery of distant trans-Neptunian objects has led to heated discussions about the structure of the outer Solar System.Aims.We study the dynamical evolution of small bodies from the Hill regions of migrating giant gaseous clumps that form in the outer solar nebula via gravitational fragmentation. We attempt to determine whether the observed features of the orbital distribution of distant trans-Neptunian objects could be caused by this process.Methods.We consider a simple model that includes the Sun, two point-like giant clumps with masses of ∼10 Jupiter masses, and a set of massless objects initially located in the Hill regions of these clumps. We carry out numerical simulations of the motions of small bodies under gravitational perturbations from two giant clumps that move in elliptical orbits and approach each other. The orbital distribution of these small bodies is compared with the observed distribution of distant trans-Neptunian objects.Results.In addition to the known grouping in longitudes of perihelion, we note new features for observed distant trans-Neptunian objects. The observed orbital distribution points to the existence of two groups of distant trans-Neptunian objects with different dynamical characteristics. We show that the main features of the orbital distribution of distant trans-Neptunian objects can be explained by their origin in the Hill regions of migrating giant gaseous clumps. Small bodies are ejected from the Hill regions when the giant clumps move in high-eccentricity orbits and have a close encounter with each other.Conclusions.The resulting orbital distribution of small bodies in our model and the observed distribution of distant trans-Neptunian objects have similar features.

Context. When we wish to understand planetesimal formation, the only data set we have is our own Solar System. The Solar System is particularly interesting because so far, it is the only planetary system we know of that developed life. Understanding the conditions under which the solar nebula evolved is crucial in order to understand the different processes in the disk and the subsequent dynamical interaction between (proto-)planets after the gas disk has dissolved. Aims. Protoplanetary disks provide a plethora of different parameters to explore. The question is whether this parameter space can be constrained, allowing simulations to reproduce the Solar System. Methods. Models and observations of planet formation provide constraints on the initial planetesimal mass in certain regions of the solar nebula. By making use of pebble flux-regulated planetesimal formation, we performed a parameter study with nine different disk parameters such as the initial disk mass, the initial disk size, the initial dust-to-gas ratio, the turbulence level, and others. Results. We find that the distribution of mass in planetesimals in the disk depends on the timescales of planetesimal formation and pebble drift. Multiple disk parameters can affect the pebble properties and thus planetesimal formation. However, it is still possible to draw some conclusions on potential parameter ranges. Conclusions. Pebble flux-regulated planetesimal formation appears to be very robust, allowing simulations with a wide range of parameters to meet the initial planetesimal constraints for the solar nebula. This means that it does not require much fine-tuning.

We study the dynamics and growth of dust particles in circumstellar disks of different masses that are prone to gravitational instability during the critical first Myr of their evolution. The dust component is made up of two different components: micron-sized dust and grown dust of evolving size. For the dust component, we considered the dust coagulation, fragmentation, momentum exchange with the gas, and dust self-gravity. We found that the micron-sized dust particles grow rapidly in the circumstellar disk, reaching a few cm in size in the inner 100 au of the disk during less than 100 kyr after the disk formation, provided that fragmentation velocity is $30\rm~ms^{-1}$. Due to the accretion of micron dust particles from the surrounding envelope, which serves as a micron dust reservoir, the approximately cm-sized dust particles continue to be present in the disk for more than 900 kyr after the disk formation and maintain a dust-to-gas ratio close to 0.01. We show that a strong correlation exists between the gas and pebble fluxes in the disk. We find that radial surface density distribution of pebbles in the disk shows power-law distribution with an index similar to that of the Minimum-mass solar nebula (MMSN) regardless the disk mass. We also show that the gas surface density in our models agrees well with measurements of dust in protoplanetary disks of AS 209, HD 163296, and DoAr 25 systems. Pebbles are formed during the very early stages of protoplanetary disk evolution. They play a crucial role in the planet formation process. Our disc simulations reveal the early onset ($, 13 pages, 10 figures

Context. The landing of the Philae probe as part of the ESA Rosetta mission made it possible to study the magnetization of comet 67P/Churyumov-Gerasimenko (67P) by combining observations from the lander and orbiter. In this work, we revisit the magnetic properties with information gained during the progression of the mission for a comprehensive understanding of the circumstances of Philae’s descent and landing. Aims. The aim is to derive a limit for any possible magnetization of the cometary material on the surface of 67P. To achieve this, the surface contacts of Philae were analyzed. Combined with a more detailed understanding of the background magnetic field, this allows us to interpret the underlying magnetic measurements in detail. Methods. We combined magnetic field observations from the ROMAP magnetometer on board Philae with observations from the RPC-MAG instrument on board the Rosetta orbiter. To facilitate this, a correlation analysis was used to correct phase shifts between the observed signals. Additionally, in-flight calibration of the ROMAP offsets was performed using information about the dynamics of Philae during flight. These corrections made it possible to use the orbiter measurements as reference for the comet-based Philae observations. We assumed a simple dipole model and used the magnetic field observations to derive an upper limit for the magnetization of the cometary material. Results. An upper limit of 0.9 nT for the observed magnetic field on the surface of 67P was derived for any contribution from surface magnetization. For homogeneously magnetized pebbles with a size of typical aggregates in the range of ~5 cm, this translates into an upper limit of ~5 × 10−5 Am2 kg−1 for the specific magnetic moment. Depending on the exact history of formation, this results in an upper limit of 4 μT for the magnitude of the magnetic field in the solar nebula during the formation of comet 67P.

We study the formation of the protoplanetary disk by the collapse of a primordial molecular cloud, and how its evolution leads to the selection of specific types of planets. We use a hydrodynamical code that accounts for the dynamics, thermodynamics, geometry, and composition of the disk to numerically model its evolution as it is fed by the infalling cloud material. As the mass accretion rate of the disk onto the star determines its growth, we can calculate the stellar characteristics by interpolating its radius, luminosity, and temperature over the stellar mass from pre-calculated stellar evolution models. The density and midplane temperature of the disk then allow us to model the interactions between the disk and potential planets and determine their migration. At the end of the collapse phase, when the disk reaches its maximum mass, it pursues its viscous spreading, similarly to the evolution from a minimum mass solar nebula (MMSN). In addition, we establish a timeline equivalence between the MMSN and a "collapse-formed disk" that would be older by about 2 Myr. We can save various types of planets from a fatal type-I inward migration: in particular, planetary embryos can avoid falling on the star by becoming trapped at the heat transition barriers and at most sublimation lines (except the silicates one). One of the novelties concerns the possible trapping of putative giant planets around a few astronomical units from the star around the end of the infall. Moreover, trapped planets may still follow the traps outward during the collapse phase and inward after it. Finally, this protoplanetary disk formation model shows the early possibilities of trapping planetary embryos at disk stages that are anterior by a few million years to the initial state of the MMSN approximation., 14 pages, 11 figures, accepted in A&A 2019

Context.Comet formation by gravitational instability requires aggregates that trigger the streaming instability and cluster in pebble-clouds. These aggregates form as mixtures of dust and ice from (sub-)micrometre-sized dust and ice grains via coagulation in the solar nebula.Aim.We investigate the growth of aggregates from (sub-)micrometre-sized dust and ice monomer grains. We are interested in the properties of these aggregates: whether they might trigger the streaming instability, how they compare to pebbles found on comets, and what the implications are for comet formation in collapsing pebble-clouds.Methods.We used Monte Carlo simulations to study the growth of aggregates through coagulation locally in the comet-forming region at 30 au. We used a collision model that can accommodate sticking, bouncing, fragmentation, and porosity of dust- and ice-mixed aggregates. We compared our results to measurements of pebbles on comet 67P/Churyumov-Gerasimenko.Results.We find that aggregate growth becomes limited by radial drift towards the Sun for 1μm sized monomers and by bouncing collisions for 0.1μm sized monomers before the aggregates reach a Stokes number that would trigger the streaming instability (Stmin). We argue that in a bouncing-dominated system, aggregates can reach Stminthrough compression in bouncing collisions if compression is faster than radial drift. In the comet-forming region (~30 au), aggregates with Stminhave volume-filling factors of ~10−2and radii of a few millimetres. These sizes are comparable to the sizes of pebbles found on comet 67P/Churyumov-Gerasimenko. The porosity of the aggregates formed in the solar nebula would imply that comets formed in pebble-clouds with masses equivalent to planetesimals of the order of 100 km in diameter.

Aims. 60% of the A star members of the 12 Myr oldβ Pictoris moving group (BPMG) show significant excess emissi on in the mid-infrared, several million years after the proto-plane tary disk is thought to disperse. Theoretical models suggest this peak may coincide with the formation of Pluto-sized planetesimals in the disk, stirring smaller bodies into collisional destru ction. Here we present resolved mid-infrared imaging of the disk ofη Tel (A0V in the BPMG) and consider its implications for the state of planet formation in this system. Methods.η Tel was observed at 11.7 and 18.3� m using T-ReCS on Gemini South. The resulting images were compared to simple disk models to constrain the radial distribution of the emit ting material. Results. The emission observed at 18.3� m is shown to be significantly extended beyond the PSF along a p osition angle 8 ◦ . This is the first time dust emission has been resolved around η Tel. Modelling indicates that the extension arises from an edge-on disk of radius 0.5 arcsec (∼ 24 AU). Combining the spatial constraints from the imaging with those from the spectral energy distribution shows that >50% of the 18� m emission comes from an unresolved dust component at∼4 AU. Conclusions. The radial structure of theη Tel debris disk is reminiscent of the Solar System, suggesting that this is a young Solar System analogue. For an age of 12Myr, both the radius and dust level of the extended cooler component are consistent with selfstirring models for a protoplanetary disk of 0.7 times minimum mass solar nebula. The origin of the hot dust component may arise in an asteroid belt undergoing collisional destruction, or in massive collisions in ongoing terrestrial planet formatio n.

Aims. The aim is to check whether the Nice model – which refers to an initially compact configuration of the giant planets that lasted for about 700 Myr and eventually became unstable when Jupiter and Saturn crossed their 2:1 mean motion resonance – is consistent with what we know of the Oort cloud, and to provide an explanation for the mass and structure of the Oort cloud. Methods. A two-stage formation scenario of the Oort cloud is proposed. The starting condition is the Nice model of planet migration, which contains a disc outside Neptune with a mass of 35 M⊕. From the formation efficiency I calculate the amount of mass deposited in the outer Oort cloud (comets with semi-major axis a > 20 000 AU; the inner Oort cloud has comets with semi-major axis a < 20 000 AU) and the innermost Oort cloud (a

We report the results of a mm-wave molecular line survey of the nearby (D ~ 70 pc), 12 Myr-old system V4046 Sgr -- a tight (9 R_sun separation), short-period (2.42 day) binary with nearly equal component masses of ~0.9 M_sun -- conducted with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We detected rotational transitions of 12CO 13CO, HCN, CN, and HCO+. The double-peaked CO line profiles of V4046 Sgr are well fit by a model invoking a Keplerian disk with outer radius of ~250 AU that is viewed at an inclination i = 35 degrees. We infer minimum disk gas and dust masses of ~13 and ~20 Earth masses from the V4046 Sgr CO line and submm continuum fluxes, respectively. The actual disk gas mass could be much larger if the gas-phase CO is highly depleted and/or 13CO is very optically thick. The overall similarity of the circumbinary disk of V4046 Sgr to the disk orbiting the single, ~8 Myr-old star TW Hya -- a star/disk system often regarded as representative of the early solar nebula -- indicates that gas giant planets are likely commonplace among close binary star systems. Given the relatively advanced age and proximity of V4046 Sgr, these results provide strong motivation for future high-resolution imaging designed to ascertain whether a planetary system now orbits its twin suns., Comment: 5 pages, 3 figures; to appear in Astronomy & Astrophysics

Aims. Considering a model of an initial disk of planetesimals that consists of 10 038 test particles, we simulate the formation of distant-comet reservoirs for the first 1 Gyr. Since only the outer part of the Oort cloud can be formed within this period, we analyse the efficiency of the formation process and describe approximately the structure of the part formed. Methods. The dynamical evolution of the particles is followed by numerical integration of their orbits. We consider the perturbations by four giant planets on their current orbits and with their current masses, in addition to perturbations by the Galactic tide and passing stars. Results. In our simulation, the population size of the outer Oort cloud reaches its maximum value at about 210 Myr. After a subsequent, rapid decrease, it becomes almost stable (with only a moderate decrease) from about 500 Myr. At 1 Gyr, the population size decreases to about 40% of its maximum value. The efficiency of the formation is low. Only about 0.3% of the particles studied still reside in the outer Oort cloud after 1 Gyr. The space density of particles in the comet cloud, beyond the heliocentric distance, r ,o f 25 000 AU is proportional to r −s ,w heres = 4.08 ± 0.34. From about 50 Myr to the end of the simulation, the orbits of the Oort cloud comets are not distributed randomly, but high galactic inclinations of the orbital planes are strongly dominant. Among all of the outer perturbers considered, this is most likely caused by the dominant, disk component of the Galactic tide.

Aims. We attempt to ellucidate the structure and chemical composition of the carbon bulk detected in cometary Stardust particles. We determine if the carbon material observed spectroscopically is of true cometary origin and whether or not it was formed by direct UV-photoprocessing of icy grain mantles in the local dense cloud and/or the solar nebula. Methods. We acquire infrared spectroscopy of ten Stardust cometary particles from track 35 and the aerogel inside and outside the particle track. Using infrared and Raman spectroscopy, the dominant carbon component in cometary Stardust particles was compared to IDPs and organics made from UV-photoprocessing of interstellar/circumstellar ice analogs in the laboratory. The Raman spectra of Stardust particles used in this comparison are adapted from the literature. Results. As indicated in previous works, it is found that the collecting aerogel medium, processed during particle impact, poses serious problems for the infrared analysis of the Stardust cometary particles reported in this paper. We identify the structure of the carbon bulk of the organic material retrieved from the aerogel with a form of (hydrogenated) amorphous carbon. It is found that this material is not a direct product of ice photoprocessing.

International audience; Aims:The aim of this paper is to investigate the surface composition of the Transneptunian Object (TNO) Orcus. Methods: High quality observations have been carried out with the new instrument SINFONI at the Very Large Telescope (VLT) of ESO. Results: Crystalline water ice, and possibly ammonia ice, have been found from spectroscopic observations of the TNO Orcus between 1.4 and 2.4 mum. Conclusions: The existence of such ices on the surface of Orcus may indicate a renewal mechanism on the surface and geological activity. The presence of ammonia on the surface of Orcus, if confirmed, could have important implications for the composition of the primitive solar nebula and the formation of the TNO population.

We study the interaction of a low-mass planet with a protoplanetary disk with a realistic treatment of the energy balance by doing radiation-hydrodynamical simulations. We look at accretion and migration rates and compare them to isothermal studies. We used a three-dimensional version of the hydrodynamical method RODEO, together with radiative transport in the flux-limited diffusion approach. The accretion rate, as well as the torque on the planet, depend critically on the ability of the disk to cool efficiently. For densities appropriate to 5 AU in the solar nebula, the accretion rate drops by more than an order of magnitude compared to isothermal models, while at the same time the torque on the planet is positive, indicating outward migration. It is necessary to lower the density by a factor of 2 to recover inward migration and more than 2 orders of magnitude to recover the usual Type I migration. The torque appears to be proportional to the radial entropy gradient in the unperturbed disk. These findings are critical for the survival of protoplanets, and they should ultimately find their way into population synthesis models., Comment: Accepted for publication in Astronomy and Astrophysics

Aims. In this paper, we analyze the collisional and dynamical evolution of the population of L 4 Jovian Trojans. Methods. To do this, we test different collisional parameters and include a dynamical treatment, taking into account the stability and instability zones of the L 4 Trojan swarm. This procedure allows us to estimate the size distribution of the L 4 Trojans, to study their mean collisional lifetimes, to analyze the formation of families, to obtain ejection rates of Trojan fragments and to discuss their possible contribution to the current populations of Centaurs and Jupiter-family comets. Results. Our estimates of the L 4 Trojan cumulative size distribution show waves that propagate from diameters of ~0.1 to ~80 km around the values derived from optical surveys. On the other hand, the mean collisional lifetimes obtained from our simulations indicate that the large Trojan asteroids have likely survived without being catastrophically fragmented over the age of the Solar System. With regards to the Trojan removal, we calculate a maximum ejection rate of Trojan fragments from L 4 of ~50 objects larger than 1 km of diameter per Myr, which results to be significantly smaller than values previously published. Such estimates allow us to infer that the contribution of the Trojan asteroids to the current populations of Centaurs and Jupiter-family comets is negligible. In addition, our results are in agreement with the formation of few Trojan families in the L 4 swarm. On the other hand, we infer that the current orbital distribution of the Trojan asteroids does not offer a strong constraint on the dynamical origin of this population.

Aims. An anomalous effect in the abundances of oxygen isotopes in the most refractory calcium-aluminum-rich inclusions (CAIs) was discovered some thirty years ago. The origin of these oxygen isotopic anomalies has hitherto remained unexplained. The origin is neither nuclear, nor has the recent photochemical self-shielding explanation been proven to be valid. We discuss a possible chemical mechanism to resolve these observed effects. Methods. By uniting the most recent laboratory observations of nanoclusters of silicates in beams and the first principles theoretical studies of their structure and properties with a major dynamical constraint recently described as the surface non-RRKM effect during SiO 2 formation on the growing grain, we show that the origin of the anomalous isotopic effect in high-temperature minerals in CAIS is chemical and strictly mass-independent. Results. We report that the surface non-RRKM effect would represent a major process in the formation of our own solar system and observable protoplanetary accretion disks, and the mass-independent isotope effects are directly associated with the formation of primary grains in the high temperature nebular environment. We expect that this chemical reaction mechanistic approach combined with future time-resolved studies on the kinetics of growth of silicates and a precise knowledge of the oxygen isotopic abundances of the sun would provide a very detailed understanding of the origins of formation of our solar system.

We examine the production of methane via Fischer-Tropsch catalysis in an evolving turbulent model of the Jovian subnebula and its implications for the composition of satellitesimals produced in situ. We show that there is a catalytically-active region in the Jovian subnebula from 65 Jupiter radii that moves inwards with time. The pressure range in this region is about 10-4 to 10-3 bar and implies that, if transport processes and the cooling of the subnebula are not considered, CO and CO2 are entirely converted into CH4 via Fischer-Tropsch catalysis in about 101 –102 and 103 –104 years, respectively. On the other hand, the comparison of the chemical conversion times with the viscous timescale of the subdisk in the catalytically-active region implies that only CO can be fully converted into CH4 , the conversion of CO2 thus being restricted to a limited production of CH4 . Moreover, the time required by the Jovian subnebula to cool down from the optimal temperature for Fischer-Tropsch catalysis to the condensation temperature of ices is at least two orders of magnitude higher than the viscous timescale. This implies that any CH4 produced in the catalytically-active zone will be accreted onto Jupiter long before being incorporated into the forming ices. We then conclude that in an evolving turbulent subnebula, even if Fischer-Tropsch catalysis is active, it has no influence on the composition of the forming satellitesimals that will ultimately take part in the formation of regular icy satellites, in opposition to what has been expected from stationary models.

We study the effects of Jupiter mass growth in order to permanently capture prograde satellites. Adopting the restricted three-body problem, Sun-Jupiter-Particle, we performed numerical simulations backward in time while considering the decrease in Jupiter’s mass. We considered the particle’s initial conditions to be prograde, at pericenter, in the region 100R ≤ a ≤ 400R and 0 ≤ e ≤ 0.5. The results give Jupiter’s mass at the moment when the particle escapes from the planet. Such values give an indication of the conditions that are necessary for capture. An analysis of these results shows that prograde satellite capture is more complex than a retrograde one. It occurs in a two-step process. First, when the particles get inside about 0.85RHill (Hills’ radius), they become weakly bound to Jupiter. Then, they keep migrating toward the planet with a strong decrease in eccentricity, while the planet is growing. The radial oscillation of the particles reduces significantly when they reach a radial distance that is less than about 0.45RHill from the planet. Three-dimensional simulations for the known prograde satellites of Jupiter were performed. The results indicate that Leda, Himalia, Lysithea, and Elara could have been permanently captured when Jupiter had between 50% and 60% of its present mass.

We have developed an evolutionary turbulent model of the Jovian subnebula consistent with the extended core accretion formation models of Jupiter described by Alibert et al. (2005b) and derived from Alibert et al. (2004,2005a). This model takes into account the vertical structure of the subnebula, as well as the evolution of the surface density as given by an $\alpha$-disk model and is used to calculate the thermodynamical conditions in the subdisk, for different values of the viscosity parameter. We show that the Jovian subnebula evolves in two different phases during its lifetime. In the first phase, the subnebula is fed through its outer edge by the solar nebula as long as it has not been dissipated. In the second phase, the solar nebula has disappeared and the Jovian subdisk expands and gradually clears with time as Jupiter accretes the remaining material. We also demonstrate that early generations of satellites formed during the beginning of the first phase of the subnebula cannot survive in this environment and fall onto the proto-Jupiter. As a result, these bodies may contribute to the enrichment of Jupiter in heavy elements. Moreover, migration calculations in the Jovian subnebula allow us to follow the evolution of the ices/rocks ratios in the proto-satellites as a function of their migration pathways. By a tempting to reproduce the distance distribution of the Galilean satellites, as well as their ices/rocks ratios, we obtain some constraints on the viscosity parameter of the Jovian subnebula., Comment: Accepted in Astronomy and Astrohpysics

We experimentally investigated tribocharging in bouncing dust grain collisions under conditions which are relevant for the early solar nebula. We used silica, diamond, graphite, and silicon carbide grains and cm-sized targets of aluminum, iron, and graphite. We varied particle size around 1 µm, collision velocity between 38 to 106 m s −1 , and target temperature between 240 K and 360 K. Generally, the grains acquire something in the order of hundreds or thousands of negative ele- mentary charges, which is at the upper end of the range previously discussed in astrophysical research. Mainly, a temperature difference caused by the dissipation of collisional energy or by initial temperatures determines the amount of charge transfer. It suggests a thermally diffusive charge transfer process of negative charge carriers. Nevertheless, also other processes of sec- ondary importance were effective. Material-dependent contact charging is ruled out as the dominant charge transfer process in our experiments. For most particle-target combinations, the charge transfer can be described by an empirical function of collision energy which we propose for future astrophysical applications, especially for applications on the sticking efficiency in the preplanetary dust aggregation process, on the particle motion in the solar nebula, and on the possibility of nebular lightning.

A high accuracy 2D hydrodynamical code has been developed to simulate the flow of gas and solid particles in protoplanetary disks. Gas is considered as a compressible fluid while solid particles, fully coupled to the gas by aerodynamical forces, are treated as a pressure-free diluted second phase. The solid particles lose energy and angular momentum which are transfered to the gas. As a result particles migrate inward toward the star and gas moves outward. High accuracy is necessary to account for the coupling. Boundary conditions must account for the inward/outward motions of the two phases. The code has been tested on one and two dimensional situations. The numerical results were compared with analytical solutions in three different cases: i) the disk is composed of a single gas component; ii) solid particles migrate in a steady flow of gas; iii) gas and solid particles evolve simultaneously. The code can easily reproduce known analytical solutions and is a powerful tool to study planetary formation at the decoupling stage. For example, the evolution of an over-density in the radial distribution of solids is found to differ significantly from the case where no back reaction of the particles onto the gas is assumed. Inside the bump, solid particles have a drift velocity approximately 16 times smaller than outside which significantly increases the residence time of the particles in the nebula. This opens some interesting perspectives to solve the timescale problem for the formation of planetesimals.

The presence of crystalline dust materials in the outer, cold regions of protoplanetary accretion discs requires conditions for their formation, which are typical for the inner, warm regions of the disc. This suggests the existence of a mechanism that allows an efficient, outward-directed radial transport of material in accretion discs. Higher order analytical calculations, as well as numerical simulations reveal meridional flow structures in α -discs, which exhibit outflow of matter in regions near the disc midplane and may play a significant role in radial mixing in protoplanetary accretion discs. We present an analytical, isothermal model for the large-scale meridional flow pattern in an α -disc, verifying the approximations which it is based on by means of a 2-D numerical computation. The solution for the flow structure obtained is used for calculating the transport of a tracer in accretion discs in combination with diffusional mixing. The impact of radial flows on mixing of tracers is compared to the height-averaged inflow solution of the standard one-zone approximation.

We study the influence of mass transport processes on the chemical evolution in a protoplanetary accretion disk. Local transport processes by advection as well as global transport processes by diffusion are taken into account. Concerning the multi-component system only diffusion in the vertical direction was taken into account. Depending on the transport properties, different schemes are explored to couple/decouple the physical and chemical evolution. Our model is based on a simplified description of hydrodynamics in terms of a steady 1+1-D-α-disk model and includes the kinetics of an extended chemical network of about 250 species. We restrict our calculations to the inner planet formation zone within a distance to the central star of 10 AU. Vertical mixing does change the global chemical evolution as it is demonstrated in detail through a discussion of the chemistry of sulphur-bearing molecules. In addition, the influence of the local disk structure on the chemical evolution is analysed. Our results demonstrate that the chemical evolution is influenced globally by mass transport processes. However, in addition to mass transport processes, information about the local conditions, which determine the kinetics, is still needed in order to analyze the chemical evolution.

The solar nebula model described by Dubrulle (1993) and Drouart et al. (1999) is used to predict the D/H ratio in H2O ice of both Jovian and Saturnian regular satellite systems. These calculations take into account recent scenarios of formation of Jupiter and Saturn's large regular icy satellites developed from evolutionary turbulent models of the surrounding subnebulae. These scenarios argue that the Saturnian and Jovian subdisks were cold enough to prevent the vaporization of volatile species trapped in solids which produced the regular icy satellites. The hydrated solids formed in the feeding zones of Jupiter and Saturn during the cooling of the solar nebula. Hence, the D/H ratio in H2O ice of hydrated solids is likely the result of the isotopic exchange between HD and HDO which occurred in the solar nebula prior to the condensation of water. In these conditions, the D/H ratio in the regular icy satellites of Jupiter and Saturn is estimated to be between 3.8 and 4.7, and between 4.8 and 6.8 times the protosolar value, respectively. Such estimates, compared with subsequent in situ measurements of the D/H ratio in H2 Oi ce of the Jovian and Saturnian regular satellite systems, should bring new constraints on the proposed scenarios of formation. This is the case with the Huygens probe mission which has the capacity of measuring the D/H ratio in the water ice which may exist on the surface of Titan.

The outer regions of protoplanetary accretion discs are formed by material from the parent molecular cloud of the freshly forming stars. The interstellar dust in this material is a mixture of species which does not correspond to any kind of chemical equilibrium state between the solid and gaseous phases. Mass accretion carries this material into the warm inner disc zones where chemical and physical processes are activated which convert the non-equilibrium solid-gas mixture into a chemical equilibrium mixture. Part of the equilibrated material is then mixed outwards by turbulent diffusion and large-scale circulation currents. This work specifically considers the evolution of the main dust components, viz. from the interstellar mixture of amorphous Mg-Fe-silicates, into a chemical equilibrium mixture of crystalline Mg-silicates, and iron. The basic set of equations for calculating the evolution of a mixture of silicates and iron is derived. Model calculations based on stationary, one-zone α-discs are combined with the advection-diffusion-reaction equations for the dust evolution to study the interstellar to equilibrium dust conversion and the radial mixing of equilibrated dust into the outer disc regions. This determines the mixture of the main dust components which form the mineral inventory of planetesimals. It is found that the results of the model calculation for the resulting mineral mixture are in rough agreement with the composition of matrix material of primitive meteorites and dust in cometary nuclei.

We study the thermodynamical conditions existing in the Uranian subnebula from which the regular satellites were presumably formed, assuming it was produced by an earth-sized body impact on proto-Uranus (Stevenson 1984; Slattery et al. 1992). Two evolutionary turbulent models of the Uranian subnebula are constructed. Both are derived from the solar nebula analytical model of Dubrulle (1993) and Drouart et al. (1999). Each model provides a distinct chemical composition which depends on the assumed origin of the subdisk's material (proto-Uranus or impactor ejected material). Moreover, the evolution of the chemistry of C and N compounds is examined in order to assess the nature of major volatiles trapped into the ices of regular satellites. The temporal evolution of the D/H ratio in water is also explored in the Uranian subdisk, as a function of the gas phase composition. Such an analysis may provide constraints on the origin of the material which led to the formation of the regular satellite system.

Protoplanetary disks as birth places of planets as well as of their host stars bear dierent element mixtures owing to the dierent chemical compositions of the environments where they are born. The chemical composition aects the structure and evolution of the disks, particularly the composition and abundance of the dust. In this work we perform one-zone model calculations of vertically selfgravitating protoplanetary accretion disks with the -prescription for the viscosity with dierent element mixtures. The models consider the chemical equilibrium condensation of the most important dust species in the disk as well as annealing of interstellar silicate dust and combustion of carbon dust. Also a new inner boundary condition is introduced which avoids the unphysical decline of the surface density of the frequently adopted no-torque ( =0) condition. The main result of the model calculations is that with decreasing metallicity the disks become less opaque and hence colder as a consequence of the reduced dust-to-gas ratio. Further we give a rough estimate for the critical value of the metallicity below which the formation of terrestrial planets is inhibited.

Current models of the formation of comets in the Solar System assume that the most likely formation site for these pristine objects is in the Uranus-Neptune zone or just beyond with subsequent dynamical ejection by the growing protoplanets to distant orbits to form the Oort cloud. However, the composition of the recently disintegrated comet C/1999 S4 LINEAR suggests that it was most likely formed in the Jupiter-Saturn region (Mumma et al. 2001b; Kawakita et al. 2001). In this paper we argue that cometesimals could easily appear inside exterior resonances resulting from gas drag, between the orbits of Jupiter and Saturn and beyond Saturn, during the formation of these giant planets. Then, the trapped material could evolve into actual comets by means of collisional coagulation, followed by gravitational instability of a layer of macroscopic bodies or two-body accretion. Properties of these objects would be rather different from those found in classical comets formed beyond Neptune as a result of different physicochemical environment. Our results also suggest that temporarily stable cometary belts may have existed in the Jupiter-Saturn region. We also discuss the implications of this scenario on the existence of a bimodal Centaur population.

We present a statistical study of the post-formation migration of giant planets in a range of initial disk conditions. For given initial conditions we model the evolution of giant planet orbits under the influence of disk, stellar, and mass loss torques. We determine the mass and semi-major axis distribution of surviving planets after disk dissipation, for various disk masses, lifetimes, viscosities, and initial planet masses. The majority of planets migrate too fast and are destroyed via mass transfer onto the central star. Most surviving planets have relatively large orbital semi-major axes of several AU or larger. We conclude that the extrasolar planets observed to date, particularly those with small semi-major axes, represent only a small fraction (~25% to 33%) of a larger cohort of giant planets around solar-type stars, and many undetected giant planets must exist at large (>1-2 AU) distances from their parent stars. As sensitivity and completion of the observed sample increases with time, this distant majority population of giant planets should be revealed. We find that the current distribution of extrasolar giant planet masses implies that high mass (more than 1-2 Jupiter masses) giant planet formation must be relatively rare. Finally, our simulations imply that the efficiency of giant planet formation must be high: at least 10% and perhaps as many as 80% of solar-type stars possess giant planets during their pre-main sequence phase. These predictions, including those for pre-main sequence stars, are testable with the next generation of ground- and space-based planet detection techniques, 25 pages, 5 figures. Double-space, single-column format to show long equations. Accepted for publication in A&A

We have observed 9 rotational lines of methanol in comet Hale-Bopp (1995 O1) using the 45-m radio telescope at Nobeyama Radio Observatory in the frequency range 36-104 GHz. The observed transitions have upper state rotational energy levels of Eu= 7-159 K. Assuming the Haser model for density distribution, we estimated the excitation temperature and the production rate to be Tex = 81 8Ka ndQ(CH3OH)= (1:2 0:1) 10 29 molecule s 1 , respectively. We also observed the HCN J = 1 0 line at 88 GHz to derive the HCN production rate. We compared the production rate of CH3OH with that of HCN and that of other molecules whose production rates have been determined previously. The abundance ratio Q(CH3OH)/Q(X) is estimated to be in the range 3-121, where X denotes HCN, HNC, H2CO, CS and CH3CN. When we com- pare these abundance ratios with those in interstellar clouds: the bipolar flow L1157 B1/B2, the star-forming region SgrB2(M) and the dark cloud TMC-1, they are close to the values in L1157 B1/B2 or in SgrB2(M). We found that the abundance ratio Q(CH3OH)/Q(H2O) was similar to solid methanol ratio relative to water ice in interstellar medium. These results support the hypothesis that cometary nuclei are formed from dust grains outside of the primordial solar nebula so that the grain composition of the nebula remains in cometary nuclei.

The oxidation of carbon dust and its conversion into CO and some hydrocarbon compounds as intermediate products is studied for protoplanetary accretion discs. The disc model is based on the one-zone-disc approximation. The radial mixing of the combustion products into the cold outer disc region by turbulent diusion is considered in the transport-diusion-reaction equations for the chemistry. It is shown that considerable amounts of CH4 and C2H2 formed as by-products of carbon oxidation are mixed into cold disc regions beyond a distance of 10 AU where they can be included into the ice mixture of cometary nuclei formed in that region. This may explain the high abundance of these gases recently observed in comets Hyakutake (C/1996 B2) and Hale-Bopp (C/1995 O1). This mixing of hydrocarbons from carbon oxidation cannot, however, explain the high abundance of some other molecular species like C2H6 or CH3OH, which have to be formed by dierent processes.

This work investigates the annealing of silicate dust, the combustion of carbon dust and radial mixing of both dust species within protoplanetary disks. For this purpose the diffusion-transport-reaction equations of both dust species (including annealing of silicate and carbon combustion) are simultaneously solved with the equations for the global evolution of an α -disk within an one-zone, time-dependent numerical model. The protostar-disk system is assumed to be in a quiescent stage which corresponds with the class II phase of evolution of star-disk systems. The results suggest that the diffusive transport spreads the dust globally throughout the disk, and therefore provides an explanation for the existence of crystalline silicate and methane within the primordial bodies of the solar system.

The distributions of molecules in the inner regions of a protostellar disk are presented. These were calculated using an uncoupled chemical/dynamical model, with a numerical integration of the vertical disk struc- ture. A comparison between models with and without the eects of X-ray ionisation is made, and molecules are identied which are good tracers of the ionisation level in this part of the disk, notably CN and C2H. In the re- gion considered in this paper (r 10 AU), the chemistry is dominated by the thermal desorption of species from grains. This shows that a critically important detail in this region of the disk, as far as molecular distributions are concerned, is the temperature prole. We nd that not all of the gaseous material is frozen onto grain surfaces at 10 AU, and we identify species, including some organic molecules, which should exist in observable quantities in the inner regions of protostellar disks.

A high fraction of carbon bound in solid carbonaceous material is observed to exist in bodies formed in the cold outskirts of the solar nebula, while bodies in the terrestrial planets region contain nearly none. We study the fate of the carbonaceous material during the spiral-in of matter as the sun accretes matter from the solar nebula. From observational data on the composition of the dust component in comets and interplanetary dust particles, and from data on pyrolysis experiments, we construct a model for the composition of the pristine carbonaceous material in the outer parts of the solar nebula. We study the pyrolysis of the refractory and volatile organic component and the concomitant release of high-molecular-weight hydrocarbons under quiescent conditions of disk evolution where matter migrates inwards. We also study the decomposition and oxidation of the carbonaceous material during violent flash heating events, which are thought to be responsible for the formation of chondrules. It is found that the complex hydrocarbon components are removed from the solid disk matter at temperatures between 250 and 400 K, while the amorphous carbon component survives up to 1200 K. Without efficient carbon destruction during flash-heating associated with chondrule formation the carbon abundance of terrestrial planets, except for Mercury, would be not as low as it is found in cosmochemical studies. Chondrule formation seems to be a process that is crucial for the carbon-poor composition of the material of terrestrial planets., Comment: 17 pages, 7 figures, accepted by Astronomy & Astrophysics

The interplay between radial mixing process in protoplanetary accretion discs with processes leading to destruction or modication of the extinction properties of abundant dust species has signicant consequences for the properties of the disk. This paper studies the consequences of annealing amorphous silicate dust at T 800 K, of combustion of the carbon dust component at about T 1000 K and of mixing the products into cold outer disc regions out to 10 AU and beyond. A model calculation in the one-zone approximation for stationary Keplerian -disks around a solar-like protostar is combined with a solution of the equations for annealing of silicate dust grains, for carbon dust oxidation, and a solution of the diusion equations for radial mixing of the dust components in the disc by turbulent flows. It is shown that annealing of amorphous silicate dust reduces the mass extinction coecient of the disc matter by more than an order of magnitude in the warm disc zone. Radial mixing of the freshly produced crystalline silicate dust into outer disc regions reduces the opacity of the disc material also in cold disc regions where annealing is not possible. Mixing of carbon dust free material from the zone of carbon combustion into outer disc regions also leads to a considerable reduction of the opacity of the disc material. Radial mixing processes then modify the dust composition of the outer disc regions and by means of the dependence of the disk properties (midplane temperature Tc ,v iscosity , . ..) on the opacity also modify the structure and evolution of a protoplanetary disc. It is shown that turbulent mixing processes in the protoplanetary accretion disc of a Solar System like system during its evolution prior to the onset of the formation of planetary bodies carry material from inner disc regions r< 1 AU outwards to at least 10. . . 20 AU. This oers a simple explanation of the ndings that a signicant fraction of the cometary silicate dust grains are crystallised and that the matrix material of primitive meteorites contains thermally processed crystalline dust material.

We have developed and applied a model designed to track simultaneously the evolution of gas and solids in protoplanetary disks from an early stage, when all solids are in the dust form, to the stage when most solids are in the form of a planetesimal swarm. The model is computationally ecient and allows for a global, comprehensive approach to the evolution of solid particles due to gas-solid coupling, coagulation, sedimentation, and evaporation/condensation. The co-evolution of gas and solids is calculated for 10 7 yr for several evolution regimes and starting from a comprehensive domain of initial conditions. The output of a single evolutionary run is a spatial distribution of mass locked in a planetesimal swarm. Because swarm's mass distribution is related to the architecture of a nascent planetary system, diversity of swarms is taken as a proxy for a diversity of planetary systems. We have found that disks with low values of specic angular momentum are bled out of solids and do not form planetary systems. Disks with high and intermediate values of specic angular momentum form diverse planetary systems. Solar-like planetary systems form from disks with initial masses0.02 M and angular momenta3 10 52 gc m 2 s 1 . Planets more massive than Jupiter can form at locations as close as 1 AU from

Recent works have shown that in a disc of planetesimals perturbed by a proto-Jupiter, the coupling between embryo's perturbations and inelastic collisions heats up the disc over several astronomical units in a few 10 5 years. Using a simulation of a disc made of hard-spheres suering inelastic collisions, we performed long-term numerical integrations to determine if the energy dissipated in collisions may nally damp eccentricities and inclinations induced by the proto-Jupiter. It is shown that the coupling between mean-motion resonances and collisions induces dierent damping regimes as a function of the protoplanet's mass. A 15 M proto-Jupiter is more ecient than a 300 M one to stir the disc over longer timescales, due to the non-emptying of isolated rst order mean-motion resonances.

A simple numerical model has been developed to study the evolution of a disc of planetesimals under mutual inelastic collisions in the potential eld of a central body and of an embedded giant-planet embryo. Masses for the latter range from 0.5 to 300 Earth masses. A mass of 15 M is typical of the solid-core model for the formation of giant planets. The initially cold disc consists of a few thousand particles. Those initially present between one and three Hill radii from the perturber's orbit are transferred to very eccentric orbits causing violent collisions throughout the disc. The perturbation propagates far from the perturber, like a heat transfer: a 15 M perturber orbiting at 5.2 a.u. heats up the disc from 2.3 to at least 11 a.u. from the central body in a few 10 5 to 10 6 years. Relative velocities are typically increased by a factor of 10 to 100. The extent of the heated region increases with the protoplanet's mass while the propagation timescale decreases. The resulting radial mixing has potential applications for the origin of the Asteroid Belt, in particular for the radial distribution of the asteroid's spectroscopic families.

We present two methods to determine corotation equilibrium points for the elliptical, restricted three body problem with a dissipative force. We show that our first order method is valid for high planetary eccentricity and determine corotation points for interior resonances, an improvement in respect with previous analytical first order methods, however it fails to determine the equilibrium points for the case of the extended corotation resonances. We develop another generic method which determines the solutions for this case. This method gives the corotation points with extreme accuracy for all cases (exterior, interior and extended). We compare our results with numerical integrations of the complete N -body equations in order to determine the accuracy of the points determined by both methods.

We investigate the driving of orbital eccentricity of giant protoplanets and brown dwarfs through disc-companion tidal interactions by means of two dimensional numerical simulations. We consider disc models that are thought to be typical of protostellar discs during the planet forming epoch, with characteristic surface densities similar to standard minimum mass solar nebula models. We consider companions, ranging in mass between 1 and 30 Jupiter masses $M_{\rm J},$ that are initially embedded within the discs on circular orbits about a central solar mass. We find that a transition in orbital behaviour occurs at a mass in the range 10-20$ M_{\rm J}.$ For low mass planetary companions, we find that the orbit remains essentially circular. However, for companion masses $\gtrsim 20 M_{\rm J},$ we find that non steady behaviour of the orbit occurs, characterised by a growth in eccentricity to values of $0.1 \lesssim e \lesssim 0.25$. Analysis of the disc response to the presence of a perturbing companion indicates that for the higher masses, the inner parts of the disc that lie exterior to the companion orbit become eccentric through an instability driven by the coupling of an initially small disc eccentricity to the companion's tidal potential. This coupling leads to the excitation of an $m=2$ spiral wave at the 1:3 outer eccentric Lindblad resonance, which transports angular momentum outwards, leading to a growth of the disc eccentricity. The interaction of the companion with this eccentric disc, and the driving produced by direct resonant wave excitation at the 1:3 resonance, can lead to the growth of orbital eccentricity, with the driving provided by the eccentric disc being the stronger. Eccentricity growth occurs when the tidally induced gap width is such that eccentricity damping caused by corotating Lindblad resonances is inoperative. These simulations indicate that for standard disc models, gaps become wide enough for the 1:3 resonance to dominate, such that the transition from circular orbits can occur, only for masses in the brown dwarf range. However, the transition mass might be reduced into the range for extrasolar planets if the disc viscosity is significantly lower enabling wider gaps to occur for these masses. Another possibility is that an eccentric disc is produced by an alternative mechanism, such as viscous overstability resulting in a slowly precessing non axisymmetric mass distribution. A large eccentricity in a planet orbit contained within an inner cavity might then be produced.

Infrared spectra of dust in cometary comae provide a way to identify its silicate constituents, and this is crucial for correctly understanding the condition under which our planetary system is formed. Recent studies assign a newly detected peak at a wavelength of 9.3 µm to pyroxenes and regard them as the most abundant silicate minerals in comets. Here we dispense with this pyroxene hypothesis to numerically reproduce the infrared features of cometary dust in the framework of our interstellar dust models. Presolar interstellar dust in a comet is modeled as fluffy aggregates consisting of submicrometer-sized organic grains with an amorphous-silicate core that undergoes nonthermal crystallization in a coma. We assert that forsterite (Mg2SiO4) is the carrier of all the observed features, including the 9.3 µm peak and that the major phase of iron is sulfides rather than iron-rich silicates.

The collisional and sticking properties of sub-mm-sized aggregates composed of protoplanetary dust analogue material are measured, including the statistical threshold velocity between sticking and bouncing, their surface energy and tensile strength within aggregate clusters. We performed an experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue materials were micrometre-sized monodisperse and polydisperse SiO2 particles prepared into aggregates with sizes around 120 $\mu$m and 330 $\mu$m, respectively and volume filling factors around 0.37. During the experimental run of 150 s under reduced gravity conditions, the sticking of aggregates and the formation and fragmentation of clusters of up to a few millimetres in size was observed. The sticking probability of the sub-mm-sized dust aggregates could be derived for velocities decreasing from 22 to 3 cm/s. The transition from bouncing to sticking collisions happened at 12.7 cm/s for the smaller aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the larger aggregates composed of mono- and polydisperse dust particles, respectively. Using the pull-off force of sub-mm-sized dust aggregates from the clusters, the surface energy of the aggregates composed of monodisperse dust was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for the micrometre-sized monomer particles and is in good agreement with previous measurements for silica particles. The tensile strengths of these aggregates within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and large dust aggregates, respectively. These values are in good agreement with recent tensile strength measurements for mm-sized silica aggregates. Using our data on the sticking-bouncing threshold, estimates of the maximum aggregate size can be given. For a minimum mass solar nebula model, aggregates can reach sizes of 1 cm.

Context. It is widely accepted that the current distribution of material in the main asteroidal belt (MB) is a product of the evolutionary history of the solar system during its whole lifetime of ~4.5 billions of years and is, consequently, a major witness of the diverse stages of this evolution. Aims. The purpose of this paper is twofold: first, we study the principal aspects of the distribution of the asteroids in proper element space, mass, and, physical composition for a complete picture of the current MB. Second, we analyze if and how these current distributions can be explained by the long-lasting dynamical effects of the planets on this region of the solar system. Methods. We studied the distribution in the proper element space for the sample that consists of about 350 000 objects whose proper orbital elements are available from the database AstDyS. We studied the distribution in size and physical composition using the most recent and large available datasets. We constructed the dynamical portrait of the MB in form of the dynamical and averaged maps via the spectral analysis method. Results. The main properties of the current distributions of MB objects are identified. A comparison of the distributions of real objects with dynamical maps allows us to detect principal mechanisms of the diffusive transportation of the objects. These mechanisms are related to mean-motion resonances (MMRs) and secular resonances (SRs), overlaying with the slow dissipative Yarkovsky/Yorp drift. Conclusions. We identify the most relevant distributions of the material in the MB and show that many of the current features of the MB can be explained by the interplay of diverse dynamical mechanisms due to the planetary perturbations over 4 Gyr with nongravitational effects, without the need of ’catastrophic’ events or ’ad hoc’ migration mechanisms during the early stages of the solar system. In this sense, the obtained distributions can provide relevant constraints for modeling the origin and evolution of the MB.

We report on the composition of the Halley-family comet (HFC) 8P/Tuttle investigated with high-dispersion near-infrared spectroscopic observations. The observations were carried out at the ESO VLT (Very Large Telescope) with the CRIRES instrument as part of a multi-wavelength observation campaign of 8P/Tuttle performed in late January and early February 2008. Radar observations suggested that 8P/Tuttle is a contact binary, and it was proposed that these components might be heterogeneous in chemistry. We determined mixing ratios of organic volatiles with respect to H2O and found that mixing ratios were consistent with previous near infrared spectroscopic observations obtained in late December 2007 and in late January 2008. It has been suggested that because 8P/Tuttle is a contact binary, it might be chemically heterogeneous. However, we find no evidence for chemical heterogeneity within the nucleus of 8P/Tuttle. We also compared the mixing ratios of organic molecules in 8P/Tuttle with those of both other HFCs and long period comets (LPCs) and found that HCN, C2H2, and C2H6 are depleted whereas CH4 and CH3OH have normal abundances. This may indicate that 8P/Tuttle was formed in a different region of the early solar nebula than other HFCs and LPCs. We estimated the conversion efficiency from C2H2 to C2H6 by hydrogen addition reactions on cold grains by employing the C2H6/(C2H6+C2H2) ratio. The C2H6/(C2H6+C2H2) ratio in 8P/Tuttle is consistent with the ratios found in other HFCs and LPCs within the error bars. We also discuss the source of C2 and CN based on our observations and conclude that the abundances of C2H2 and C2H6 are insufficient to explain the C2 abundances in comet 8P/Tuttle and that the abundance of HCN is insufficient to explain the CN abundances in the comet, so at least one additional parent is needed for each species, as pointed out in previous study. ispartof: Astronomy & Astrophysics vol:509 issue:1 status: published

The first stage of planet formation is the accumulation of dust and ice grains into mm-cm-sized pebbles. These pebbles can clump together through the streaming instability and form gravitationally bound pebble 'clouds'. Pebbles inside such a cloud will undergo mutual collisions, dissipating energy into heat. As the cloud loses energy, it gradually contracts towards solid density. We model this process and investigate two important properties of the collapse: (i) the timescale of the collapse and (ii) the temporal evolution of the pebble size distribution. Our numerical model of the pebble cloud is zero-dimensional and treats collisions with a statistical method. We find that planetesimals with radii larger than 100 km collapse on the free-fall timescale of about 25 years. Lower-mass clouds have longer pebble collision timescales and collapse much more slowly, with collapse times of a few hundred years for 10-km-scale planetesimals and a few thousand years for 1-km-scale planetesimals. The mass of the pebble cloud also determines the interior structure of the resulting planetesimal. The pebble collision speeds in low-mass clouds are below the threshold for fragmentation, forming pebble-pile planetesimals consisting of the primordial pebbles from the protoplanetary disk. Planetesimals above 100 km in radius, on the other hand, consist of mixtures of dust (pebble fragments) and pebbles which have undergone substantial collisions with dust and other pebbles. The Rosetta mission to the comet 67P/Churyumov-Gerasimenko and the New Horizons mission to Pluto will both provide valuable information about the structure of planetesimals in the Solar System. Our model predicts that 67P is a pebble-pile planetesimal consisting of primordial pebbles from the Solar Nebula, while the pebbles in the cloud which contracted to form Pluto must have been ground down substantially during the collapse., Comment: 11 pages, 7 figures, accepted for publication in A&A, Fig 4 corrected, comments welcome

International audience; Context. Little is known about the internal structure of cometary nuclei. In addition to understanding their accretion in the early solar nebula and their subsequent evolution in the solar system, we find this question to be of acute and timely interest in the case of 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) due to be visited by the Rosetta spacecraft in the second half of 2014. In particular, the successful landing of the Philae surface module depends critically upon the bulk density of the nucleus and the structure of its surface layer. Aims. In addition to fostering our general knowledge of these properties, it is important to exploit all possible information to assist in preparing the delivery of Philae. Methods. We performed an in-depth analysis of the observations done with the radar system of the Arecibo Observatory in November 1982 when comet 67P/C-G had a close encounter with Earth at a geocentric distance of 0.4 AU taking our present knowledge of the properties of its nucleus (size, rotational state) into account. Results. In the absence of a detectable radar echo, we determined a maximum radar cross section of 0.7 km(2), leading to a maximum radar albedo of 0.05. This low albedo probably results from a combination of a low radar reflectivity material and a lightly packed upper layer of the nucleus with substantial roughness (rms slope of approximate to 55 degrees), consistent with its low thermal inertia. Based on radar observations of other cometary nuclei and asteroids, it is unlikely that the albedo can be lower than 0.04 so that we were able to constrain the dielectric permittivity of the subsurface layer to a narrow range of 1.9 to 2.1. Laboratory measurements and our modeling of mixtures of ice and dust have led to a porosity in the range of approximately 55 to 65% and a density in the range of approximate to 600 approximate to 1000 kg m(-3) for the top approximate to 2.5 m layer of the nucleus. This would be the bulk density range for a homogeneous nucleus and would place the success of the landing at risk, but an inhomogeneous nucleus with an overall density below this range remains a possibility.

HCl and HF are expected to be the main reservoirs of fluorine and chlorine wherever hydrogen is predominantly molecular. They are found to be strongly depleted in dense molecular clouds, suggesting freeze-out onto grains in such cold environments. We can then expect that HCl and HF were also the major carriers of Cl and F in the gas and icy phases of the outer solar nebula, and were incorporated into comets. We aimed to measure the HCl and HF abundances in cometary ices as they can provide insights on the halogen chemistry in the early solar nebula. We searched for the J(1-0) lines of HCl and HF at 626 and 1232 GHz, respectively, using the HIFI instrument on board the Herschel Space Observatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1 (Garradd), whereas observations of HF were conducted in comet C/2009 P1. In addition, observations of H$_2$O and H$_2^{18}$O lines were performed in C/2009 P1 to measure the H$_2$O production rate. Three lines of CH$_3$OH were serendipitously observed in the HCl receiver setting. HCl is not detected, whereas a marginal (3.6-$\sigma$) detection of HF is obtained. The upper limits for the HCl abundance relative to water are 0.011% and 0.022%, for 103P and C/2009 P1, respectively, showing that HCl is depleted with respect to the solar Cl/O abundance by a factor more than 6$^{+6}_{-3}$ in 103P, where the error is related to the uncertainty in the chlorine solar abundance. The marginal HF detection obtained in C/2009 P1 corresponds to an HF abundance relative to water of (1.8$\pm$0.5) $\times$ 10$^{-4}$, which is approximately consistent with a solar photospheric F/O abundance. The observed depletion of HCl suggests that HCl was not the main reservoir of chlorine in the regions of the solar nebula where these comets formed. HF was possibly the main fluorine compound in the gas phase of the outer solar nebula., Comment: Accepted for publication in Astronomy & Astrophysics

The solar system was most likely born in a star cluster containing at least 1000 stars. It is highly probable that this cluster environment influenced various properties of the solar system like its chemical composition, size and the orbital parameters of some of its constituting bodies. In the Milky Way, clusters with more than 2000 stars only form in two types - starburst clusters and leaky clusters - each following a unique temporal development in the mass-radius plane. The aim is here to determine the encounter probability in the range relevant to solar system formation for starburst or leaky cluster environments as a function of cluster age. N-body methods are used to investigate the cluster dynamics and the effect of gravitational interactions between cluster members on young solar-type stars surrounded by discs. Using the now available knowledge of the cluster density at a given cluster age it is demonstrated that in starburst clusters the central densities over the first 5Myr are so high (initially > 10^5 Msun pc^{-3}) that hardly any discs with solar system building potential would survive this phase. This makes a starburst clusters an unlikely environment for the formation of our solar system. Instead it is highly probable that the solar system formed in a leaky cluster (often classified as OB association). It is demonstrated that an encounter determining the characteristic properties existing in our solar systems most likely happened very early on (< 2Myr) in its formation history and that after 5Myr the likelihood of a solar-type star experiencing such an encounter in a leaky cluster is negligible even if it was still part of the bound remnant. This explains why the solar system could develop and maintain its high circularity later in its development., 11 pages, 7 figures, accepted by A&A

Modeling the formation of the ice giants Uranus and Neptune is a long-lasting problem in planetary science. Due to gas-drag, collisional damping, and resonant shepherding, the planetary embryos repel the planetesimals away from their reach and thus they stop growing (Levison et al. 2010). This problem persists independently of whether the accretion took place at the current locations of the ice giants or closer to the Sun. Instead of trying to push the runaway/oligarchic growth of planetary embryos up to 10-15 Earth masses, we envision the possibility that the planetesimal disk could generate a system of planetary embryos of only 1-3 Earth masses. Then we investigate whether these embryos could have collided with each other and grown enough to reach the masses of current Uranus and Neptune. Our results point to two major problems. First, there is typically a large difference in mass between the first and the second most massive core formed and retained beyond Saturn. Second, in many simulations the final planetary system has more than two objects beyond Saturn. The growth of a major planet from a system of embryos requires strong damping of eccentricities and inclinations from the disk of gas. But strong damping also favors embryos and cores to find a stable resonant configuration, so that systems with more than two surviving objects are found. In addition to these problems, in order to have substantial mutual accretion among embryos, it is necessary to assume that the surface density of the gas was several times higher than that of the minimum-mass solar nebula. However this contrasts with the common idea that Uranus and Neptune formed in a gas-starving disk, which is suggested by the relatively small amount of hydrogen and helium contained in the atmospheres of these planets. Only one of our simulations "by chance" successfully reproduced the structure of the outer Solar System.

Context. The Themis collisional family is one of the largest and best established families in the main belt. Composed of primitive asteroids, there is evidence that water is likely present in a large fraction of its members, either in aqueously altered silicates or in water ice reservoirs. The study of the abundance of water in the outer asteroid belt is important as it may be linked to the origin of Earth’s water. Studying the Themis family can also help to constrain the compositional and thermal environment in the region of the solar nebula where these asteroids formed. Aims. Our aim is to constrain the composition and thermal properties of the surfaces of several Themis family asteroids. Methods. We present 5–14 μm spectra of 8 Themis family asteroids observed with Spitzer: (222) Lucia, (223) Rosa, (316) Goberta, (383) Janina, (468) Lina, (492) Gismonda, (515) Athalia, and (526) Jena. We determine their diameters, geometric albedos and beaming parameters using the near-Earth asteroid thermal model. Their emissivity spectra are studied in order to determine if they exhibit an emission plateau from 9 to 12 μm which has been observed in other primitive asteroids and attributed to fine-grained silicates (the Si-O stretch fundamental). Results. The derived mean albedo of our sample of Themis family asteroids is ¯ pV = 0.07 ± 0.02, and the mean beaming parameter is ¯ η = 1.05 ± 0.10. The derived ¯ η value is close to unity, which implies that the infrared beaming is not significant, there is likely little night-side emission from the asteroids, and the thermal inertia is probably low. The emissivity spectra of at least 5 of our 8 asteroids show a 9–12 μm emission plateau with spectral contrast of ∼2–4%, similar but smaller than that observed in the spectra of Trojan asteroids and cometary dust. The plateau may be due to the surfaces having either small silicate grains embedded in a relatively transparent matrix, or from a very under-dense (fairy-castle) surface structure. Conclusions. The surfaces of a large fraction of Themis family asteroids with D ∼ 50 km are covered by a fine grained silicate mantle as observed on Trojan asteroids of similar or larger size. The lower amplitude of the silicate emission in Themis family asteroids spectra (2–4%) with respect to that of Trojan asteroids (10–15%) could be attributed to larger dust particles, a slightly denser structure, or a lower silicate dust fraction.