Connecting planet formation and astrochemistry (original) (raw)
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Data Archiving and Networked Services (DANS), 2020
The chemical composition of planetary atmospheres has long been thought to store information regarding where and when a planet accretes its material. Predicting this chemical composition theoretically is a crucial step in linking observational studies to the underlying physics that govern planet formation. As a follow-up to a study of hot Jupiters in our previous work, we present a population of warm Jupiters (semi-major axis between 0.5-4 AU) extracted from the same planetesimal formation population synthesis model as used in our previous work. We compute the astrochemical evolution of the protoplanetary disks included in this population to predict the carbon-to-oxygen (C/O) and nitrogen-to-oxygen (N/O) ratio evolution of the disk gas, ice, and refractory sources, the accretion of which greatly impacts the resulting C/O and N/O in the atmosphere of giant planets. We confirm that the main sequence (between accreted solid mass and atmospheric C/O) we found previously is largely reproduced by the presented population of synthetic warm Jupiters. And as a result, the majority of the population fall along the empirically derived mass-metallicity relation when the natal disk has solar or lower metallicity. Planets forming from disks with high metallicity ([Fe/H] > 0.1) result in more scatter in chemical properties which could explain some of the scatter found in the mass-metallicity relation. Combining predicted C/O and N/O ratios shows that Jupiter does not fall among our population of synthetic planets, suggesting that it likely did not form in the inner 5 AU of the solar system before proceeding into a Grand Tack. This result is consistent with recent analysis of the chemical composition of Jupiter's atmosphere which suggests that it accreted most of its heavy element abundance farther than tens of AU away from the Sun. Finally we explore the impact of different carbon refractory erosion models, including the location of the carbon erosion front. Shifting the erosion front has a major impact on the resulting C/O ratio of Jupiter and Neptune-like planets, but warm Saturns see a smaller shift in C/O, since their carbon and oxygen abundances are equally impacted by gas and refractory accretion.
Monthly Notices of the Royal Astronomical Society
We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted onto planets forming by core accretion from evolving protoplanetary disks. The astrochemistry of the host disk is computed using an ionization driven, nonequilibrium chemistry network within viscously evolving disk models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted onto the protoplanet. We choose a fiducial disk model and evolve planets in 3 traps-water ice line, dead zone and heat transition. For a disk with a lifetime of 4.1 Myr we produce two Hot Jupiters (M = 1.43, 2.67 M Jupiter , r = 0.15, 0.11 AU) in the heat transition and ice line trap and one failed core (M = 0.003 M Jupiter , r = 3.7 AU) in the dead zone. These planets are found with mixing ratios for CO and H 2 O of 1.99 × 10 −4 , 5.0 × 10 −4 respectively for both Hot Jupiters. Additionally for these planets we find CO 2 and CH 4 , with mixing ratios of 1.8 × 10 −6 → 9.8 × 10 −10 and 1.1 × 10 −8 → 2.3 × 10 −10 respectively. These ranges correspond well with the mixing ratio ranges that have been inferred through the detection of emission spectra from Hot Jupiters by multiple authors. We compute a carbon-to-oxygen ratio of 0.227 for the ice line planet and 0.279 for the heat transition planet. These planets accreted their gas inside the ice line, hence the sub-solar C/O.
2016
We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted onto planets forming by core accretion from evolving protoplanetary disks. The astrochemistry of the host disk is computed using an ionization driven, non-equilibrium chemistry network within viscously evolving disk models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted onto the protoplanet. We choose a fiducial disk model and evolve planets in 3 traps - water ice line, dead zone and heat transition. For a disk with a lifetime of 4.1 Myr we produce two Hot Jupiters (M = 1.43, 2.67 M_ Jupiter, r = 0.15, 0.11 AU) in the heat transition and ice line trap and one failed core (M = 0.003 M_ Jupiter, r =3.7 AU) in the dead zone. These planets are found with mixing ratios for CO and...
Astronomy & Astrophysics, 2015
Context. Direct observations of gaseous exoplanets reveal that their gas envelope has a higher C/O ratio than that of the host star (e.g., Wasp 12-b). This has been explained by considering that the gas phase of the disc could be inhomogeneous, exceeding the stellar C/O ratio in regions where these planets formed; but few studies have considered the drift of the gas and planet migration. Aims. We aim to derive the gas composition in planets through planet formation to evaluate if the formation of giant planets with an enriched C/O ratio is possible. The study focusses on the effects of different processes on the C/O ratio, such as the disc evolution, the drift of gas, and planet migration. Methods. We used our previous models for computing the chemical composition, together with a planet formation model, to which we added the composition and drift of the gas phase of the disc, which is composed of the main volatile species H2O, CO, CO2, NH3, N2, CH3OH, CH4, and H2S, H2 and He. The study focusses on the region where ice lines are present and influence the C/O ratio of the planets. Results. Modelling shows that the condensation of volatile species as a function of radial distance allows for C/O enrichment in specific parts of the protoplanetary disc of up to four times the solar value. This leads to the formation of planets that can be enriched in C/O in their envelope up to three times the solar value. Planet migration, gas phase evolution and disc irradiation enables the evolution of the initial C/O ratio that decreases in the outer part of the disc and increases in the inner part of the disc. The total C/O ratio of the planets is governed by the contribution of ices accreted, suggesting that high C/O ratios measured in planetary atmospheres are indicative of a lack of exchange of material between the core of a planet and its envelope or an observational bias. It also suggests that the observed C/O ratio is not representative of the total C/O ratio of the planet.
Monthly Notices of the Royal Astronomical Society
We present the next step in a series of papers devoted to connecting the composition of the atmospheres of forming planets with the chemistry of their natal evolving protoplanetary discs. The model presented here computes the coupled chemical and dust evolution of the disc and the formation of three planets per disc model. Our three canonical planet traps produce a Jupiter near 1 AU, a Hot Jupiter and a Super-Earth. We study the dependence of the final orbital radius, mass, and atmospheric chemistry of planets forming in disc models with initial disc masses that vary by 0.02 M above and below our fiducial model (M disc,0 = 0.1 M). We compute C/O and C/N for the atmospheres formed in our three models and find that C/O planet ∼ C/O disc , which does not vary strongly between different planets formed in our model. The nitrogen content of atmospheres can vary in planets that grow in different disc models. These differences are related to the formation history of the planet, the time and location that the planet accretes its atmosphere, and are encoded in the bulk abundance of NH 3. These results suggest that future observations of atmospheric NH 3 and an estimation of the planetary C/O and C/N can inform the formation history of particular planetary systems.
2017
We present the next step in a series of papers devoted to connecting the composition of the atmospheres of forming planets with the chemistry of their natal evolving protoplanetary disks. The model presented here computes the coupled chemical and dust evolution of the disk and the formation of three planets per disk model. Our three canonical planet traps produce a Jupiter near 1 AU, a Hot Jupiter and a Super-Earth. We study the dependency of the final orbital radius, mass, and atmospheric chemistry of planets forming in disk models with initial disk masses that vary by 0.02 M_ above and below our fiducial model (M_disk,0 = 0.1 M_). We compute C/O and C/N for the atmospheres formed in our 3 models and find that C/O_ planet∼ C/O_ disk, which does not vary strongly between different planets formed in our model. The nitrogen content of atmospheres can vary in planets that grow in different disk models. These differences are related to the formation history of the planet, the time and l...
The Astrophysical Journal, 2013
The super-Earth exoplanet 55 Cnc e, the smallest member of a five-planet system, has recently been observed to transit its host star. The radius estimates from transit observations, coupled with spectroscopic determinations of mass, provide constraints on its interior composition. The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio, which to first order can be estimated from the host stars. Results from a recent spectroscopic study analyzing the 6300Å [O I] line and two C I lines suggest that 55 Cnc has a carbon-rich composition (C/O=1.12±0.09). However oxygen abundances derived using the 6300Å [O I] line are highly sensitive to a Ni I blend, particularly in metal-rich stars such as 55 Cnc ([Fe/H]=0.34±0.18). Here, we further investigate 55 Cnc's composition by deriving the carbon and oxygen abundances from these and additional C and O absorption features. We find that the measured C/O ratio depends on the oxygen lines used. The C/O ratio that we derive based on the 6300Å [O I] line alone is consistent with the previous value. Yet, our investigation of additional abundance indicators results in a mean C/O ratio of 0.78±0.08. The lower C/O ratio of 55 Cnc determined here may place this system at the sensitive boundary between protoplanetary disk compositions giving rise to planets with high (>0.8) versus low (<0.8) C/O ratios. This study illustrates the caution that must applied when determining planet host star C/O ratios, particularly in cool, metal-rich stars. Recently Madhusudhan et al. (2012) suggest an alternative and carbon-rich composition of 55 Cnc e, garnering the super-Earth popular attention as "the diamond planet." Measurements of the carbon and oxygen abundances from two C I lines (5052Å, 5135Å) and one forbidden [O I] line (6300Å) indicate a C/O 1 ratio of 1.12±0.19 (Delgado Mena et al. 2010), i.e., a highly carbon-rich star compared to the solar C/O∼0.50 (Asplund et al. 2005). If the disk shared the host star's composition, and the host star is carbon-rich, then the planetesimals accreted during the formation of 55 Cnc e were likely Fe-and C-rich (Bond et al. 2010; Madhusudhan et al. 2012). To investigate the composition of the possibly carbon-rich exoplanet, Madhusudhan et al. (2012) consider two families of carbon-rich interior models of 55 Cnc e, consisting of layers, from inner to outer, of Fe-SiC-C and Fe-MgSiO 3 -C. Included in their carbon equation of state (EOS) are the graphite EOS at low pressures, the phase transition to diamond between 10 GPa<P<1000 GPa, and the Thomas-Fermi-Dirac EOS at high pressures. Madhusudhan et al. (2012) find a wide range of compositions are possible, including extreme combinations like (Fe, SiC, C) = (33%, 0%, 67%)
How Metallicity Affects Volatile Abundances: Implications for Planetary System Formation
Monthly Notices of the Royal Astronomical Society, 2020
Astronomers have confirmed the existence of several thousand extra-solar planetary systems having a wide range of orbital and compositional characteristics. A host star's metallicity, defined as the abundance of all elements heavier than helium (metals), appears to play a role in determining whether an exoplanetary system is more likely to include Jupiter-sized gas and ice giants. Here we show how molecular cloud metallicity is likely to significantly affect the initial conditions of planetary formation by affecting the abundances of volatile ices (H 2 O, CO, etc.) in parent molecular clouds. Through analytic and numerical treatments of molecular chemical lifetimes, we show that volatile elements are more likely to be found as ices in metal rich clouds compared to metal poor ones. These correlations, in turn, may impact the characteristics of planetary systems as a function of their metallicity as suggested by the systematic shifts in snowline distances as a function of metallicity. We evaluate the "wet Earth" hypothesis for the origins of Earth's water and find that elevated protoplanetary disk pressures are required to retain the required partial (∼ 2%) monolayer of water on interstellar dust grain surfaces with MRN distribution.
Mass–Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K
The Astrophysical Journal
Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H2O, Na, and K, in a sample of transiting exoplanets using the latest transmission spectra and new H2 broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between ∼300 and 2700 K. Using homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and H2O, and their detection significances, confirming 6 planets with strong Na detections, 6 with K, and 14 with H2O. We find a mass–metallicity trend of increasing H2O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H2O abundances, from mini-Neptunes to hot Jupiters, is significantly lower than the mass–metallicity relation for carbon in the solar sy...
The recent inference of a carbon-rich atmosphere, with C/O ≥ 1, in the hot Jupiter WASP-12b motivates the exotic new class of carbon-rich planets (CRPs). We report a detailed study of the atmospheric chemistry and spectroscopic signatures of carbon-rich giant planets (CRGs), the possibility of thermal inversions in their atmospheres, the compositions of icy planetesimals required for their formation via core accretion, and the apportionment of ices, rock, and volatiles in their envelopes. Our results show that CRG atmospheres probe a unique region in composition space, especially at high temperature (T ). For atmospheres with C/O ≥ 1, and T 1400 K in the observable atmosphere, most of the oxygen is bound up in CO, while H 2 O is depleted and CH 4 is enhanced by up to two or three orders of magnitude each, compared to equilibrium compositions with solar abundances (C/O = 0.54). These differences in the spectroscopically dominant species for the different C/O ratios cause equally distinct observable signatures in the spectra. As such, highly irradiated transiting giant exoplanets form ideal candidates to estimate atmospheric C/O ratios and to search for CRPs. We also find that the C/O ratio strongly affects the abundances of TiO and VO, which have been suggested to cause thermal inversions in highly irradiated hot Jupiter atmospheres. A C/O = 1 yields TiO and VO abundances of ∼100 times lower than those obtained with equilibrium chemistry assuming solar abundances, at P ∼ 1 bar. Such a depletion is adequate to rule out thermal inversions due to TiO/VO even in the most highly irradiated hot Jupiters, such as WASP-12b. We estimate the compositions of the protoplanetary disk, the planetesimals, and the envelope of WASP-12b, and the mass of ices dissolved in the envelope, based on the observed atmospheric abundances. Adopting stellar abundances (C/O = 0.44) for the primordial disk composition and low-temperature formation conditions (T 30 K) for WASP-12b leads to a C/O ratio of 0.27 in accreted planetesimals, and, consequently, in the planet's envelope. In contrast, a C/O ratio of 1 in the envelope of WASP-12b requires a substantial depletion of oxygen in the disk, i.e. by a factor of ∼ 0.41 for the same formation conditions. This scenario also satisfies the constraints on the C/H and O/H ratios reported for WASP-12b. If, alternatively, hotter conditions prevailed in a stellar composition disk such that only H 2 O is condensed, the remaining gas can potentially have a C/O ∼ 1. However, a high C/O in WASP-12b caused predominantly by gas accretion would preclude super-stellar C/H ratios which also fit the data. Subject headings: planetary systems -planets and satellites: general -planets and satellites: individual (WASP-12b)