Franck Hersant

A TWO-DIMENSIONAL MODEL FOR THE PRIMORDIAL NEBULA CONSTRAINED BY D/H MEASUREMENTS IN THE SOLAR SYSTEM: IMPLICATIONS FOR THE FORMATION OF GIANT PLANETS

Using the density and temperature profiles resulting from a two-dimensional turbulent model of the solar nebula as well as an appropriate law for the time variation of the disk accretion rate, we integrate the equation of diffusion that rules the evolution of the D/H ratio in H2O and HCN throughout the nebula. By fitting D/H measured in LL3 meteorites and comets or inferred in proto-Uranian and proto-Neptunian ices, we constrain the parameters of the model, namely, the initial accretion rate (0), the initial radius of the turbulent disk RD, and the α-coefficient of turbulent viscosity, and we find 2 × 10-6 < (0) < 10-5 M☉ yr-1, 12.8 < RD < 39 AU, and 0.006 < α < 0.04. Under the assumption that cometary cores are homogeneous, the microscopic icy grains that subsequently formed cometesimals were produced in the Uranus-Neptune region and no later than 3.5 × 105 yr. The epochs of the formation of Jupiter and Saturn cannot be lower than 0.7 and 5.7 Myr, respectively, after the formation of the Sun. Uranus and Neptune were completed after the dissipation of the nebula. The enrichment in volatiles with respect to the solar abundance measured by the Galileo probe in Jupiter may result from the trapping of these gases in the form of clathrate hydrates in the feeding zone of the forming planet.

ENRICHMENTS IN VOLATILES IN JUPITER: A NEW INTERPRETATION OF THE GALILEO MEASUREMENTS

Using an evolutionary model of the solar nebula, we fit all enrichments in volatiles with respect to the solar abundance measured in Jupiter by the Galileo probe. We argue that volatiles were trapped in the form of solid clathrate hydrates in the cooling feeding zone of Jupiter while the gas mass of the nebula was continuously decreasing with time. Enrichments in Jupiter are those acquired in planetesimals at the time of the hydrodynamic collapse of the feeding zone. The O/H ratio in Jupiter is predicted to be at least 8 times solar.

From Prestellar to Protostellar Cores. II. Time Dependence and Deuterium Fractionation

We investigate the molecular evolution and D/H abundance ratios that develop as star formation proceeds from a dense molecular cloud core to a protostellar core, by solving a gas-grain reaction network applied to a one-dimensional radiative hydrodynamic model with infalling fluid parcels. Spatial distributions of gas and ice-mantle species are calculated at the first-core stage, and at times after the birth of a protostar. Gas-phase methanol and methane are more abundant than CO at radii r 100 AU in the first-core stage, but gradually decrease with time, while abundances of larger organic species increase. The warm-up phase, when complex organic molecules are efficiently formed, is longer-lived for those fluid parcels infalling at later stages. The formation of unsaturated carbon chains (warm carbon-chain chemistry) is also more effective in later stages; C+, which reacts with CH4 to form carbon chains, increases in abundance as the envelope density decreases. The large organic molecules and carbon chains are strongly deuterated, mainly due to high D/H ratios in the parent molecules, determined in the cold phase. We also extend our model to simulate simply the chemistry in circumstellar disks, by suspending the one-dimensional infall of a fluid parcel at constant disk radii. The species CH3OCH3 and HCOOCH3 increase in abundance in 104-105 yr at the fixed warm temperature; both also have high D/H ratios.

A chemical model for the atmosphere of hot Jupiters

Our purpose is to release a chemical network, and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD209458b, HD189733b) with a model that includes photolyses and vertical mixing and we produce synthetic spectra. The chemical scheme is derived from applied combustion models that have been methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters. We compare the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species and with previously published photochemical models. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever non-equilibrium processes take place. The deviations from the equilibrium, and thus the sensitivity to the network, are more important for HD189733b, as we assume a cooler atmosphere than for HD209458b. We found that the abundances of NH3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH3 at 10.5

An interpretation of the nitrogen deficiency in comets

\mu

Hot super-Earths and giant planet cores from different migration histories

m is sensitive to these abundance variations and thus to the chemical scheme. Due to the influence of the kinetics, we recommend the use of a validated scheme to model the chemistry of exoplanet atmospheres. Our network is robust for temperatures within 300-2500K and pressures from 10mbar up to a few hundreds of bars, for species made of C,H,O,N. It is validated for species up to 2 carbon atoms and for the main nitrogen species.

WATER IN PROTOPLANETARY DISKS: DEUTERATION AND TURBULENT MIXING

Abstract We propose an interpretation of the composition of volatiles observed in comets based on their trapping in the form of clathrate hydrates in the solar nebula. The formation of clathrates is calculated from the statistical thermodynamics of Lunine and Stevenson (1985 , Astrophys. J. Suppl. 58, 493–531), and occurs in an evolutionary turbulent solar nebula described by the model of Hersant et al. (2001 , Astrophys. J. 554, 391–407). It is assumed that clathrate hydrates were incorporated into the icy grains that formed cometesimals. The strong depletion of the N2 molecule with respect to CO observed in some comets is explained by the fact that CO forms clathrate hydrates much more easily than does N2. The efficiency of this depletion, as well as the amount of trapped CO, depends upon the amount of water ice available in the region where the clathration took place. This might explain the diversity of CO abundances observed in comets. The same theory, applied to the trapping of volatiles around 5 AU, explains the enrichments in Ar, Kr, Xe, C, and N with respect to the solar abundance measured in the deep troposphere of Jupiter Gautier et al 2001a , Gautier et al 2001b .

Pseudo 2D chemical model of hot-Jupiter atmospheres: application to HD 209458b and HD 189733b

Planetary embryos embedded in gaseous protoplanetary disks undergo Type I orbital migration. Migration can be inward or outward depending on the local disk properties but, in general, only planets more massive than several

Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model

M_\oplus

Chemistry in disks - VIII. The CS molecule as an analytic tracer of turbulence in disks

can migrate outward. Here we propose that an embryos migration history determines whether it becomes a hot super-Earth or the core of a giant planet. Systems of hot super-Earths (or mini-Neptunes) form when embryos migrate inward and pile up at the inner edge of the disk. Giant planet cores form when inward-migrating embryos become massive enough to switch direction and migrate outward. We present simulations of this process using a modified N-body code, starting from a swarm of planetary embryos. Systems of hot super-Earths form in resonant chains with the innermost planet at or interior to the disk inner edge. Resonant chains are disrupted by late dynamical instabilities triggered by the dispersal of the gaseous disk. Giant planet cores migrate outward toward zero-torque zones, which move inward and eventually disappear as the disk disperses. Giant planet cores migrate inward with these zones and are stranded at ~1-5 AU. Our model reproduces several properties of the observed extra-solar planet populations. The frequency of giant planet cores increases strongly when the mass in solids is increased, consistent with the observed giant exoplanet - stellar metallicity correlation. The frequency of hot super-Earths is not a function of stellar metallicity, also in agreement with observations. Our simulations can reproduce the broad characteristics of the observed super-Earth population.