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Context. The opacity due to grains in the envelope of a protoplanet gr regulates the accretion rate of gas during formation, meaning that the final bulk composition of planets with a primordial H/He envelope is a function of that opacity. Observationally, for extrasolar planets with known mass and radius it is possible to estimate the bulk composition via internal structure models. Aims. We want to study the global effects of gr as a poorly known, but important quantity on synthetic planetary populations. Methods. We first determine the reduction factor of the interstellar medium (ISM) grain opacity fopa that leads to a gas accretion timescale consistent with grain evolution models for specific cases. In the second part we compare the mass–radius relationship of low-mass planets and the heavy element content of giant planets for different values of the reduction factor with observational constraints. Results. For fopa = 1 (full ISM opacity) the synthetic super-Earth and Neptunian planets have radii that are too small (i.e., envelope masses that are too low) compared to observations because at such high opacity, they cannot e ciently accrete H/He during the formation phase. At fopa = 0:003, the value calibrated with the grain evolution models, the synthetic and actual planets occupy a similar mass–radius domain. Another observable consequence is the metal enrichment of giant planets relative to the host star, Zpl/Zstar. We find that the mean enrichment of giant planets as a function of mass M can be approximated as Zpl/Zstar = β(M=M4)α both for synthetic and actual planets. The decrease in Zpl=Zstar with mass follows α≈-0.7 independent of fopa in synthetic populations, in agreement with the value derived from observations (-0.71 ± 0.10). The absolute enrichment level decreases from 8.5 at fopa = 1 to 3.5 at fopa = 0. At fopa = 0.003, one finds β = 7:2 which is similar to the result derived from observations (6.3 ± 1.0). Conclusions. We find observational hints that the opacity in protoplanetary atmospheres is much smaller than in the ISM even if the specific value of gr cannot be constrained in this first study as gr is found by scaling the ISM opacity. Our results for the enrichment of giant planets are also important to distinguish core accretion and gravitational instability. In the simplest picture of core accretion, where first a critical core forms, and afterwards only gas is added, α≈-1. If a core accretes all planetesimals inside the feeding zone during runaway gas accretion α≈-2=3. The observational result (-0.71 ± 0.10) lies between these two values, pointing to core accretion as the likely formation mechanism. [ABSTRACT FROM AUTHOR]

ABSTRACT We studied the chemical evolution of interstellar grain mantle by varying the physical parameters of the interstellar medium (ISM). To mimic the actual interstellar condition, gas-grain interactions via accretion from the gas phase and desorption (thermal evaporation and photoevaporation) from the grain surface were considered. We found that the chemical composition of the interstellar grain mantle is highly dependent on the physical parameters associated with molecular cloud. Interstellar photons have been found to play an important role in the growth and structure of the interstellar grain mantle. We considered the effects of interstellar photons (photodissociation and photoevaporation) in our simulation under various interstellar conditions. We noticed that the effects of interstellar photons dominate around the region of lower visual extinction. These photons contribute significantly to the formation of the grain mantle. The energy of the incoming photon is attenuated by the absorption and scattering by the interstellar dust. The topmost layers are assumed to be affected mainly by the incoming radiation. We have studied the effects of photodissociation by varying the number of layers which could be affected by it. Model calculations were carried out for the static (extinction parameter is changing with the density of the cloud) as well as the time-dependent case (i.e. both extinction parameter and number density of the cloud are changing with time) and the results are discussed in detail. Different routes to the formation of water molecules have been studied and it has been noticed that production of water molecules via O3 and H2O2 contributes significantly around the dense region. At the end, various observational evidences for the condensed phase species are summarized with their physical conditions and are compared with our simulation results. [ABSTRACT FROM AUTHOR]

The evolution of grain mantles in various interstellar environments is studied. We concentrate mainly on water, methanol and carbon dioxide, which constitute nearly 90 per cent of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas-phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space that reproduces these molecules close to the observed abundances. Allowing the accretion of only H, O and CO on the grains and using the Monte Carlo method, we follow the chemical processes for a few million years. We allow the formation of multilayers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as on the average cloud density. Specifically, when the number density of accreting O is less than three times that of CO, methanol is always overproduced. Using the available reaction pathways it appears to be difficult to match the exact observed abundances of all three molecules simultaneously. Only in a narrow region of parameter space are all three molecules produced within the observed limits. Furthermore, we found that the incorporation of the freeze-outs of O and CO leads to an almost steady state on the grain surface. The mantle thickness grows anywhere between 60 and 500 layers in a period of two million years. In addition, we consider a case in which the gas number density changes with time owing to the gradual collapse of the molecular cloud and present the evolution of the composition of different species as a function of the radius of the collapsing cloud. [ABSTRACT FROM AUTHOR]

A model is proposed for the formation of water ice mantles on grains in interstellar clouds. This occurs by direct accretion of monomers from the gas, be they formed by gas or surface reactions. The formation of the first monolayer requires a minimum extinction of interstellar radiation, sufficient to lower the grain temperature to the point where thermal evaporation of monomers is just offset by monomer accretion from the gas. This threshold is mainly determined by the adsorption energy of water molecules on the grain material; for hydrocarbon material, chemical simulation places this energy between 0.5 and 2 kcal mol−1, which sets the (true) visible extinction threshold at a few magnitudes. However, realistic distributions of matter in a cloud will usually add to this an unrelated amount of cloud core extinction, which can explain the large dispersion of observed (apparent) thresholds. Once the threshold is crossed, all available water molecules in the gas are quickly adsorbed, because the grain cools down and the adsorption energy on ice is higher than on bare grain. The relative thickness of the mantle, and, hence, the slope of depend only on the available water vapour, which is a small fraction of the oxygen abundance. Chemical simulation was also used to determine the adsorption sites and energies of O and OH on hydrocarbons and study the dynamics of formation of water molecules by surface reactions with gaseous H atoms, as well as their chances to stick in situ. [ABSTRACT FROM AUTHOR]

We have applied the srim computer code to study the sputtering of some likely astrophysical grain materials, and we have shown that selective embedding of metallic projectiles offers a partial explanation of gas-phase depletions. We show that supernova shockwaves sweep a significantly larger mass of interstellar gas per unit time than the shockwaves generated by outflows in star-forming regions. We apply our sputtering model to the bombardment levels expected in a supernova shock, and show that net embedding may dominate over net sputtering, leading to grain growth under some circumstances, particularly when the bombarding gas is enriched with metals from the supernova progenitor star. A combination of short cooling times and net embedding means that it is possible for a Type II supernova to generate more dust than it destroys, and we conclude that, in general, the sputtering process often leads to a compositional change in the grain material rather than simply to grain erosion. [ABSTRACT FROM AUTHOR]

Discusses astrophysicist Fred Hoyle's contributions to the literature on interstellar matter and star formation. Research paper on the physical aspects of accretion by stars; Theory of the fragmentation of gas clouds into galaxies and stars; Work on the composition of interstellar grains.

Dusty disks around young stars are formed out of interstellar dust that consists of amorphous, submicrometre grains. Yet the grains found in comets and meteorites, and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures in excess of 1,000 K, even though they are mixed with surrounding material that never experienced temperatures as high as that. This prompted theories of large-scale mixing capable of transporting thermally altered grains from the inner, hot part of accretion disks to outer, colder disk regions, but all have assumptions that may be problematic. Here I report that infrared radiation arising from the dusty disk can loft grains bigger than one micrometre out of the inner disk, whereupon they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density. Properties of the observed disks suggest that this process might be active in almost all young stellar objects and young brown dwarfs. [ABSTRACT FROM AUTHOR]

We present the first high-resolution sub-millimeter survey of both dust and gas for a large population of protoplanetary disks. Characterizing fundamental properties of protoplanetary disks on a statistical level is critical to understanding how disks evolve into the diverse exoplanet population. We use the Atacama Large Millimeter/Submillimeter Array (ALMA) to survey 89 protoplanetary disks around stars with in the young (1–3 Myr), nearby (150–200 pc) Lupus complex. Our observations cover the 890 μm continuum and the 13CO and C18O 3–2 lines. We use the sub-millimeter continuum to constrain to a few Martian masses (0.2–0.4 M⊕) and the CO isotopologue lines to constrain to roughly a Jupiter mass (assuming an interstellar medium (ISM)-like abundance). Of 89 sources, we detect 62 in continuum, 36 in 13CO, and 11 in C18O at significance. Stacking individually undetected sources limits their average dust mass to Lunar masses (0.03 M⊕), indicating rapid evolution once disk clearing begins. We find a positive correlation between and M*, and present the first evidence for a positive correlation between and M*, which may explain the dependence of giant planet frequency on host star mass. The mean dust mass in Lupus is 3× higher than in Upper Sco, while the dust mass distributions in Lupus and Taurus are statistically indistinguishable. Most detected disks have and gas-to-dust ratios , assuming an ISM-like abundance; unless CO is very depleted, the inferred gas depletion indicates that planet formation is well underway by a few Myr and may explain the unexpected prevalence of super-Earths in the exoplanet population. [ABSTRACT FROM AUTHOR]