Connecting planet formation and astrochemistry: Refractory carbon depletion leading to super-stellar C/O in giant planetary atmospheres

[Abridged] Combining a time-dependent astrochemical model with a model of planet formation and migration, we compute the carbon-to-oxygen ratio (C/O) of a range of planetary embryos starting their formation in the inner solar system (1-3 AU). The volatile and ice abundance of relevant carbon and oxygen bearing molecular species are determined through a complex chemical kinetic code which includes both gas and grain surface chemistry. This is combined with a model for the abundance of the refractory dust grains to compute the total carbon and oxygen abundance in the protoplanetary disk available for incorporation into a planetary atmosphere. We include the effects of the refractory carbon depletion that has been observed in our solar system, and posit two models that would put this missing carbon back into the gas phase. This excess gaseous carbon then becomes important in determining the final planetary C/O because the gas disk now becomes more carbon rich relative to oxygen (high gaseous C/O). One model, where the carbon excess is maintained throughout the lifetime of the disk results in Hot Jupiters that have super-stellar C/O. The other model deposits the excess carbon early in the disk life and allows it to advect with the bulk gas. In this model the excess carbon disappears into the host star within 0.8 Myr, returning the gas disk to its original (sub-stellar) C/O, so the Hot Jupiters all exclusively have sub-stellar C/O. This shows that while the solids will tend to be oxygen rich, Hot Jupiters can have super-stellar C/O if a carbon excess can be maintained by some chemical processing of the dust grains. Whether the carbon and oxygen content of the atmosphere was accreted primarily by gas or solid accretion is heavily dependent on the mass of the atmosphere and where in the disk the growing planet accreted.Comment: 13 pages, 7 figures, resubmitted to A&A after referee's comment

Deep Search For Molecular Oxygen in TW Hya

The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H$_2$O). Yet, in planet-forming disks around young stars, gas-phase CO and H$_2$O are less abundant relative to their ISM values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O$_2$) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue $^{16}$O$^{18}$O ($N_J=2_1-0_1$ line at 233.946 GHz) in the nearby protoplanetary disk around TW Hya. We used imaging techniques and matched filtering to search for weak emission but do not detect $^{16}$O$^{18}$O. Based on our results, we calculate upper limits on the gas-phase O$_2$ abundance in TW Hya of $(6.4-70)\times10^{-7}$ relative to H, which is $2-3$ orders of magnitude below solar oxygen abundance. We conclude that gas-phase O$_2$ is not a major oxygen-carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO$_2$, were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C$^{15}$N $N_J = 2_{5/2}-1_{3/2}$ hyperfine transitions, $F = 3 - 2$ and $F = 2 - 1$, at 219.9 GHz, which we found via matched filtering and confirm through imaging.Comment: 10 pages, 6 figures, Accepted for publication in Ap

Molecular abundances and C/O ratios in chemically evolving planet-forming disk midplanes

Context. Exoplanet atmospheres are thought be built up from accretion of gas as well as pebbles and planetesimals in the midplanes of planet-forming disks. The chemical composition of this material is usually assumed to be unchanged during the disk lifetime. However, chemistry can alter the relative abundances of molecules in this planet-building material. Aims. We aim to assess the impact of disk chemistry during the era of planet formation. This is done by investigating the chemical changes to volatile gases and ices in a protoplanetary disk midplane out to 30 AU for up to 7 Myr, considering a variety of different conditions, including a physical midplane structure that is evolving in time, and also considering two disks with different masses. Methods. An extensive kinetic chemistry gas-grain reaction network was utilised to evolve the abundances of chemical species over time. Two disk midplane ionisation levels (low and high) were explored, as well as two different makeups of the initial abundances (“inheritance” or “reset”). Results. Given a high level of ionisation, chemical evolution in protoplanetary disk midplanes becomes significant after a few times 105 yr, and is still ongoing by 7 Myr between the H2O and the O2 icelines. Inside the H2O iceline, and in the outer, colder regions of the disk midplane outside the O2 iceline, the relative abundances of the species reach (close to) steady state by 7 Myr. Importantly, the changes in the abundances of the major elemental carbon and oxygen-bearing molecules imply that the traditional “stepfunction” for the C/O ratios in gas and ice in the disk midplane (as defined by sharp changes at icelines of H2O, CO2 and CO) evolves over time, and cannot be assumed fixed, with the C/O ratio in the gas even becoming smaller than the C/O ratio in the ice. In addition, at lower temperatures (<29 K), gaseous CO colliding with the grains gets converted into CO2 and other more complex ices, lowering the CO gas abundance between the O2 and CO thermal icelines. This effect can mimic a CO iceline at a higher temperature than suggested by its binding energy. Conclusions. Chemistry in the disk midplane is ionisation-driven, and evolves over time. This affects which molecules go into forming planets and their atmospheres. In order to reliably predict the atmospheric compositions of forming planets, as well as to relate observed atmospheric C/O ratios of exoplanets to where and how the atmospheres have formed in a disk midplane, chemical evolution needs to be considered and implemented into planet formation models

Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?

Context. The atmospheres of extrasolar planets are thought to be built largely through accretion of pebbles and planetesimals. Such pebbles are also the building blocks of comets. The chemical composition of their volatiles are usually taken to be inherited from the ices in the collapsing cloud. However, chemistry in the protoplanetary disk midplane can modify the composition of ices and gases. Aims. To investigate if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2–30 AU range. Methods. A disk model used in planet population synthesis models is adopted, providing temperature, density and ionisation rate at different radial distances in the disk midplane. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1 Myr. Both molecular (inheritance from the parent cloud) and atomic (chemical reset) initial conditions are investigated. Results. Great diversity is observed in the relative abundance ratios of the main considered species: H2O, CO, CO2, CH4, O2, NH3 and N2. The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether or not grain-surface chemistry is included. The grain temperature plays an important role, especially in the critical 20–28 K region where atomic H no longer sticks long enough to the surface to react, but atomic O does. Above 28 K, efficient grain-surface production of CO2 ice is seen, as well as O2 gas and ice under certain conditions, at the expense of H2O and CO. H2O ice is produced on grain surfaces only below 28 K. For high ionisation levels at intermediate disk radii, CH4 gas is destroyed and converted into CO and CO2 (in contrast with previous models), and similarly NH3 gas is converted into N2. At large radii around 30 AU, CH4 ice is enhanced leading to a low gaseous CO abundance. As a result, the overall C/O ratios for gas and ice change significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times 105 yr. Conclusions. Chemistry in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals. In the inner <30 AU disk, interstellar ice abundances are preserved only if the ionisation level is low, or if these species are included in larger bodies within 105 yr

Connecting Planetary Composition with Formation

The rapid advances in observations of the different populations of exoplanets, the characterization of their host stars and the links to the properties of their planetary systems, the detailed studies of protoplanetary disks, and the experimental study of the interiors and composition of the massive planets in our solar system provide a firm basis for the next big question in planet formation theory. How do the elemental and chemical compositions of planets connect with their formation? The answer to this requires that the various pieces of planet formation theory be linked together in an end-to-end picture that is capable of addressing these large data sets. In this review, we discuss the critical elements of such a picture and how they affect the chemical and elemental make up of forming planets. Important issues here include the initial state of forming and evolving disks, chemical and dust processes within them, the migration of planets and the importance of planet traps, the nature of angular momentum transport processes involving turbulence and/or MHD disk winds, planet formation theory, and advanced treatments of disk astrochemistry. All of these issues affect, and are affected by the chemistry of disks which is driven by X-ray ionization of the host stars. We discuss how these processes lead to a coherent end-to-end model and how this may address the basic question.Comment: Invited review, accepted for publication in the 'Handbook of Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018). 46 pages, 10 figure

Chemical evolution in ices on drifting, planet-forming pebbles

Context. Planets and their atmospheres are built from gas and solid material in protoplanetary disks. Recent results suggest that solid material such as pebbles may contribute significantly to building up planetary atmospheres. In order to link observed exoplanet atmospheres and their compositions to their formation histories, it is important to understand how icy pebbles may change their composition when they drift radially inwards in disks. Aims. Our goal is to model the compositional evolution of ices on pebbles as they drift in disks, and track how their chemical evolution en route changes the ice composition relative to the ice composition of the pebbles in the region where they grew from micron-sized grains. Methods. A state-of-the-art chemical kinetics code was utilised for modelling chemical evolution. This code accounts for the time-evolving sizes of the solids that drift. Chemical evolution was modelled locally for 0.1 Myr at two starting radii, with the micron-sized solids growing into pebbles simultaneously. The pebbles and local gas, isolated as a parcel, were then exposed to changing physical conditions, which was intended to mimic the pebbles drifting inwards in the disk midplane, moving to 1 AU on three different timescales. A modelling simplification was that the pebbles are not moved through, or exposed to new gas, but they stayed in the same chemical gas surroundings in all models. Results. For ice species with initial abundances relative to hydrogen of >10-5, such as H2O, CO2, CH3OH, and NH3, the abundances change by less than 20% for both radii of origin, and for the two smaller drift timescales (10kyr and 100 kyr). For less abundant ice species, and the longest drift timescale (1 Myr), the changes are larger. Pebble drift chemistry generally increases the ice abundances of CO2, HCN, and SO, at the expense of decreasing the abundances of other volatile molecules

Formation of cometary O

Context. Detection of abundant O2 at 1–10% relative to H2O ice in the comae of comets 1P/Halley and 67P/Churyumov-Gerasimenko has motivated attempts to explain the origin of the high O2 ice abundance. Recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of O2 ice at the same abundance as that measured in the comet. Aims. We aim to carry out a thorough investigation to constrain the conditions under which O2 ice could have been produced through kinetic chemistry in the pre-solar nebula midplane. Methods. We have utilised an updated chemical kinetics code to evolve chemistry under pre-solar nebula midplane conditions. Four different chemical starting conditions and the effects of various chemical parameters have been tested. Results. Using the fiducial network, and for either reset conditions (atomic initial abundances) or atomic oxygen only conditions, the abundance level of O2 ice measured in the comets can be reproduced at an intermediate time, after 0.1–2 Myr of evolution, depending on ionisation level. When including O3 chemistry, the abundance of O2 ice is much lower than the cometary abundance (by several orders of magnitude). We find that H2O2 and O3 ices are abundantly produced (at around the level of O2 ice) in disagreement with their respective abundances or upper limits from observations of comet 67P. Upon closer investigation of the parameter space, and varying parameters for grain–surface chemistry, it is found that for temperatures 15–25 K, densities of 109−1010 cm−3, and a barrier for quantum tunnelling set to 2 Å, the measured level of O2 ice can be reproduced with the new chemical network, including an updated binding energy for atomic oxygen (1660 K). However, the abundances of H2O2 and O3 ices still disagree with the observations. A larger activation energy for the O + O2 → O3 reaction (Eact > 1000 K) helps to reproduce the non-detection of O3 ice in the comet, as well as reproducing the observed abundances of H2O2 and O2 ices. The only other case in which the O2 ice matches the observed abundance, and O3 and H2O2 ice are lower, is the case when starting with an appreciable amount of oxygen locked in O2. Conclusions. The parameter space investigation revealed a sweet spot for production of O2 ice at an abundance matching those in 67P and 1P, and O3 and H2O2 ice abundances matching those in 67P. This means that there is a radial region in the pre-solar nebula from 120–150 AU, within which O2 could have been produced in situ via ice chemistry on grain surfaces. However, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). Hence, because the more likely scenario starting with a percentage of elemental oxygen locked in O2 also reproduces the O2 ice abundance in 67P at early stages, this supports previous suggestions that the cometary O2 ice could have a primordial origin

Chemical evolution in planet-forming regions with growing grains

Context. Planets and their atmospheres are built from gas and solid material in protoplanetary disks. This solid material grows from smaller micron-sized grains to larger sizes in the disks during the process of planet formation. This solid growth may influence the efficiency of chemical reactions that take place on the surfaces of the grains and in turn affect the chemical evolution that the gas and solid material in the disk undergoes, with implications for the chemical composition of the planets. Aims. Our goal is to model the compositional evolution of volatile ices on grains of different sizes, assuming both time-dependent grain growth and several constant grain sizes. We also examine the dependence on the initial chemical composition. Methods. The custom Walsh chemical kinetics code was used to model the chemical evolution. This code was upgraded to account for the time-evolving sizes of solids. Chemical evolution was modelled locally at four different radii in a protoplanetary disk midplane (with associated midplane temperatures of 120, 57, 25, and 19.5 K) for up to 10 Myr. The evolution was modelled for five different constant grain sizes, and in one model, the grain size changed with time according to a grain-growth model appropriate for the disk midplane. Results. Local grain growth, with conservation of the total grain mass, and assuming spherical grains, acts to reduced the total grain-surface area that is available for ice-phase reactions. This reduces the efficiency of these reactions compared to a chemical scenario with a conventional grain-size choice of 0.1 μm. The chemical evolution modelled with grain growth leads to increased abundances of H2O ice. For carbon in the inner disk, grain growth causes CO gas to overtake CO2 ice as the dominant carrier, and in the outer disk, CH4 ice becomes the dominant carrier. Larger grain sizes cause less change the C/O ratio in the gas phase over time than when 0.1 μm sized grains are considered. Overall, a constant grain size adopted from a grain evolution model leads to an almost identical chemical evolution as a chemical evolution with evolving grain sizes. A constant grain size choice, albeit larger than 0.1 μm, may therefore be an appropriate simplification when modelling the impact of grain growth on chemical evolution