Tracing Carbon From the Interior of Stars to Surface of Planets

Following the Interstellar History of Carbon: From the Interiors of Stars to the Surfaces of Planets

Authors:

Zuirys et al

Abstract:

The chemical history of carbon is traced from its origin in stellar nucleosynthesis to its delivery to planet surfaces. The molecular carriers of this element are examined at each stage in the cycling of interstellar organic material and their eventual incorporation into solar system bodies. The connection between the various interstellar carbon reservoirs is also examined. Carbon has two stellar sources: supernova explosions and mass loss from evolved stars. In the latter case, the carbon is dredged up from the interior and then ejected into a circumstellar envelope, where a rich and unusual C-based chemistry occurs. This molecular material is eventually released into the general interstellar medium through planetary nebulae. It is first incorporated into diffuse clouds, where carbon is found in polyatomic molecules such as H2CO, HCN, HNC, c-C3H2, and even C60+. These objects then collapse into dense clouds, the sites of star and planet formation. Such clouds foster an active organic chemistry, producing compounds with a wide range of functional groups with both gas-phase and surface mechanisms. As stars and planets form, the chemical composition is altered by increasing stellar radiation, as well as possibly by reactions in the presolar nebula. Some molecular, carbon-rich material remains pristine, however, encapsulated in comets, meteorites, and interplanetary dust particles, and is delivered to planet surfaces.

Metal enrichment leads to low atmospheric C/O ratios in transiting giant exoplanets

Metal enrichment leads to low atmospheric C/O ratios in transiting giant exoplanets

Authors:

Espinoza et al

Abstract:

We predict the carbon-to-oxygen (C/O) ratios in the hydrogen-helium envelope and atmospheres of a sample of nearly 50 relatively cool (Teq< 1000 K) transiting gas giant planets. The method involves planetary envelope metallicity estimates that use the structure models of Thorngren et al. (2016) and the disk and planetary accretion model of \"Oberg et al. (2011). We find that nearly all of these planets are strongly metal-enriched which, coupled with the fact that solid material is the main deliverer of metals in the protoplanetary disk, implies that the substellar C/O ratios of their accreted solid material dominate compared to the enhanced C/O ratio of their accreted gaseous component. We predict that these planets will have atmospheres that are typically reduced in their C/O compared to parent star values independent of the assessed formation locations, with C/O

Volatile carbon locking and release in protoplanetary disks

Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546

Authors:

Kama et al

Abstract:

The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. We combine our recent detections of C0 in these disks with observations of other carbon reservoirs (CO, C+, C2H) and gas mass and warm gas tracers (HD, O0), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of ≤0.1M⊙. The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is at most moderately depleted compared to the interstellar medium, with [C]/[H]gas=(0.1−1.5)×10−4. HD 100546 itself is a λBo\"{o}tis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H]gas=(0.2−5.0)×10−6 and C/O greater than 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H]gas in the outer disk of TW Hya and allows to make predictions for other systems.

Carbon to oxygen ratios in extrasolar planetesimals

Carbon to oxygen ratios in extrasolar planetesimals

Authors:

Wilson et al

Abstract:

Observations of small extrasolar planets with a wide range of densities imply a variety of planetary compositions and structures. Currently, the only technique to measure the bulk composition of extrasolar planetary systems is the analysis of planetary debris accreting onto white dwarfs, analogous to abundance studies of meteorites. We present measurements of the carbon and oxygen abundances in the debris of planetesimals at ten white dwarfs observed with the Hubble Space Telescope, along with C/O ratios of debris in six systems with previously reported abundances. We find no evidence for carbon-rich planetesimals, with C/O less than 0.8 by number in all 16 systems. Our results place an upper limit on the occurrence of carbon-rich systems at less than 17 percent with a 2 sigma confidence level. The range of C/O of the planetesimals is consistent with that found in the Solar System, and appears to follow a bimodal distribution: a group similar to the CI chondrites, with log(less than C/O greater than)=-0.92, and oxygen-rich objects with C/O less than or equal to that of the bulk Earth. The latter group may have a higher mass fraction of water than the Earth, increasing their relative oxygen abundance.

Carbon Enhanced Metal Poor Stars Might Have Hosted Diamond Worlds

CEMP stars: possible hosts to carbon planets in the early universe

Authors:

Mashian et al

Abstract:

We explore the possibility of planet formation in the carbon-rich protoplanetary disks of carbon-enhanced metal-poor (CEMP) stars, possible relics of the early Universe. The chemically anomalous abundance patterns ([C/Fe] ≥ 0.7) in this subset of low-mass stars suggest pollution by primordial core-collapsing supernovae (SNe) ejecta that are particularly rich in carbon dust grains. By comparing the dust-settling timescale in the protoplanetary disks of CEMP stars to the expected disk lifetime (assuming dissipation via photoevaporation), we determine the maximum distance rmax from the host CEMP star at which carbon-rich planetesimal formation is possible, as a function of the host star's [C/H] abundance. We then use our linear relation between rmax and [C/H], along with the theoretical mass-radius relation derived for a solid, pure carbon planet, to characterize potential planetary transits across host CEMP stars. Given that the related transits are detectable with current and upcoming space-based transit surveys, we suggest initiating an observational program to search for carbon planets around CEMP stars in hopes of shedding light on the question of how early planetary systems may have formed after the Big Bang.

First Detections of [CI] and Constraints on the Carbon Abundance

Observations and modelling of CO and [CI] in disks. First detections of [CI] and constraints on the carbon abundance

Authors:

Kama et al

Abstract:

The gas-solid budget of carbon in protoplanetary disks is related to the composition of the cores and atmospheres of the planets forming in them. The key gas-phase carbon carriers CO, C0 and C+ can now be observed in disks. The gas-phase carbon abundance in disks has not yet been well characterized, we aim to obtain new constraints on the [C]/[H] ratio in a sample of disks, and to get an overview of the strength of [CI] and warm CO emission.

We carried out a survey of the CO6--5 and [CI]1--0 and 2--1 lines towards 37 disks with APEX, and supplemented it with [CII] data from the literature. The data are interpreted using a grid of models produced with the DALI code. We also investigate how well the gas-phase carbon abundance can be determined in light of parameter uncertainties.

The CO6--5 line is detected in 13 out of 33 sources, the [CI]1--0 in 6 out of 12, and the [CI]2--1 in 1 out of 33. With deep integrations, the first unambiguous detections of [CI]~1--0 in disks are obtained, in TW~Hya and HD~100546.

Gas-phase carbon abundance reductions of a factor 5--10 or more can be identified robustly based on CO and [CI] detections. The atomic carbon detection in TW~Hya confirms a factor 100 reduction of [C]/[H]gas in that disk, while the data are consistent with an ISM-like carbon abundance for HD~100546. In addition, BP~Tau, T~Cha, HD~139614, HD~141569, and HD~100453 are either carbon-depleted or gas-poor disks. The low [CI]~2--1 detection rates in the survey mostly reflect insufficient sensitivity to detect T~Tauri disks. The Herbig~Ae/Be disks with CO and [CII] upper limits below the models are debris disk like systems. A roughly order of magnitude increase in sensitivity compared to our survey is required to obtain useful constraints on the gas-phase [C]/[H] ratio in most of the targeted systems.

SuperEarth 55 Cancri e's Atmosphere is Carbon and Hydrogen Rich, may Have Hydrogen cyanide

Detection of an atmosphere around the super-Earth 55 Cancri e

Authors:

Tsiaras et al

Abstract:

Before the discovery of extrasolar planets, super-Earths belonged in the realm of science fiction. However, they appear to constitute the most common planetary type in our galaxy. We know very little about these planets beyond very basic planetary and orbital parameters. The WFC3 camera onboard the HST has enabled the spectroscopic observations of the atmospheres of two super-Earths, GJ1214b and HD97658b, with unprecedented precision; but the published spectra of these two objects are featureless, suggesting an atmosphere covered by thick clouds or made of molecular species much heavier than hydrogen. We report here the analysis of the observations performed with the WFC3 of a third, very hot, super-Earth, 55 Cancri e. Given the brightness of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length and a very high scanning speed. These observational parameters are coupled with the geometrical distortions of the instrument, so we have developed a specialized pipeline to de-correlate the signal from the systematics. We measure the transit depth per wavelength channel with an average relative uncertainty of 21 ppm and find a spectral modulation of about 100 ppm. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is hydrogen-rich. Our fully Bayesian spectral retrieval code, TauREx, has identified HCN to be one of the possible trace gases in the atmosphere. While additional observations in a broader wavelength range will be needed to confirm the HCN detection, we discuss here the implications of such result. We adopt a chemical scheme developed with combustion specialists and validated by a wide range of experiments. Our chemical model indicates that a relatively high mixing ratio of HCN would reveal a high C/O ratio, suggesting the atmosphere of 55 Cancri e is a carbon-rich environment.

Some Lambda Boo Stars are Eating Their hot Jupiters

Lambda Boo Abundance Patterns: Accretion from Orbiting Sources

Authors:

Jura et al

Abstract:

The abundance anomalies in lambda Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically-inferred bounds for interstellar accretion. Therefore, a lambda Boo star's thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt Objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some lambda Boo stars accrete from the winds of hot Jupiters.

Elemental Ratios in Stars vs ExoPlanets

Elemental ratios in stars vs planets

Authors:

Thiabaud et al

Abstract:

Context.

The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same.

Aims.

Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O.

Methods.

A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances.

Results.

We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.

The Exoplanet Atmospheric Chemistry of Carbon & Oxygen

Atmospheric Chemistry for Astrophysicists: A Self-consistent Formalism and Analytical Solutions for Arbitrary C/O

Authors:

Heng et al

Abstract:

We present a self-consistent formalism for computing and understanding the atmospheric chemistry of exoplanets. Starting from the first law of thermodynamics, we demonstrate that the van't Hoff equation (which describes the equilibrium constant), Arrhenius equation (which describes the rate coefficients) and procedures associated with the Gibbs free energy (minimisation, rescaling) have a common physical and mathematical origin. We correct an ambiguity associated with the equilibrium constant, which is used to relate the forward and reverse rate coefficients, and rigorously derive its two definitions. By necessity, one of the equilibrium constants must be dimensionless and equate to an exponential function involving the Gibbs free energy, while the other is a ratio of rate coefficients and must therefore possess physical units. To avoid confusion, we simply term them the dimensionless and dimensional equilibrium constants. We demonstrate that the Arrhenius equation takes on a functional form that is more general than previously thought without recourse to tagging on ad hoc functional forms. Our formulation of the evolution equations for chemical kinetics correctly enforces the book-keeping of elemental abundances, reproduces chemical equilibrium in the steady-state limit and is able to explain why photochemistry is an intrinsically disequilibrium effect. Finally, we derive analytical models of chemical systems with only hydrogen and with carbon, hydrogen and oxygen. For the latter, we include acetylene and are able to reproduce several key trends, versus temperature and carbon-to-oxygen ratio, published in the literature. The rich variety of behavior that mixing ratios exhibit as a function of the carbon-to-oxygen ratio is merely the outcome of stoichiometric book-keeping and not the direct consequence of temperature or pressure variations.

Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters' Atmospheres

Strict Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters from Self-Consistent Atmospheric Retrieval

Author:

Benneke

Abstract:

The elemental compositions of hot Jupiters are informative relics of planet formation that can help us answer long-standing questions regarding the origin and formation of giant planets. Here, I present the main conclusions from a comprehensive atmospheric retrieval survey of eight hot Jupiters with detectable molecular absorption in their near-infrared transmission spectra. I analyze the eight transmission spectra using the newly-developed, self-consistent atmospheric retrieval framework, SCARLET. Unlike previous methods, SCARLET combines the physical and chemical consistency of complex atmospheric models with the statistical treatment of observational uncertainties known from atmospheric retrieval techniques. I find that all eight hot Jupiters consistently require carbon-to-oxygen ratios (C/O) below 0.9. The finding of C/O less than 0.9 is highly robust for HD209458b, WASP-12b, WASP-19b, HAT-P-1b, and XO-1b. For HD189733b, WASP-17b, and WASP-43b, I find that the published WFC3 transmission spectra favor C/O less than 0.9 at greater than 95% confidence. I further show that the water abundances on all eight hot Jupiters are consistent with solar composition. The relatively small depth of the detected water absorption features is due to the presence of clouds, not due to a low water abundance as previously suggested for HD209458b. The presence of a thick cloud deck is inferred for HD209458b and WASP-12b. HD189733b may host a similar cloud deck, rather than the previously suggested Rayleigh hazes, if star spots affect the observed spectrum. The approach taken in SCARLET can be regarded as a new pathway to interpreting spectral observations of planetary atmospheres. In this work, including our prior knowledge of H-C-N-O chemistry enables me to constrain the C/O ratio without detecting a single carbon-bearing molecule.

A new way to Study Carbon-rich Exoplanet Atmospheres

A new chemical scheme to study carbon-rich exoplanet atmospheres

Authors:

Venot et al

Abstract:

Atmospheres with a high C/O ratio are expected to contain an important quantity of hydrocarbons, including heavy molecules (with more than 2 carbon atoms). To study correctly these C-rich atmospheres, a chemical scheme adapted to this composition is necessary. We have implemented a chemical scheme that can describe the kinetics of species with up to 6 carbon atoms. This chemical scheme has been developed with specialists of combustion and validated through experiments on a wide range of T and P. This chemical network is available on the online database KIDA. We have created a grid of 12 models to explore different thermal profiles and C/O ratios. For each of them, we have compared the chemical composition determined with a C0-C2 chemical scheme (species with up to 2 carbon atoms) and with the C0-C6 scheme. We found no difference in the results obtained with the two schemes when photolyses are not included in the model, whatever the temperature of the atmosphere. In contrast, when there is photochemistry, differences can appear in the upper atmosphere. These differences are found for all the tested PT profiles in the case that the C/O ratio is above 1. When the C/O ratio of the atmosphere is solar, differences are only found at temperatures lower than 1000K. The differences linked to the use of different chemical schemes do not have important influence on the synthetic spectra. However, we have confirmed that C2H2 and HCN as possible tracers of warm C-rich atmospheres. The use of this new chemical scheme is mandatory to model atmospheres with a high C/O ratio and, in particular, if one is interested in studying in details the photochemistry. If one is just interested in the synthetic spectra, the use of a smaller scheme may be sufficient.

Implications for 'Diamond' SuperEarths: 2/3s of Earth's Carbon Hidden in Core

Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3

Authors:

Chen et al

Abstract:

Earth’s inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (vS) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.

Carbon's Role in Exoplanetary Geodynamics & Habitability

THE ROLE OF CARBON IN EXTRASOLAR PLANETARY GEODYNAMICS AND HABITABILITY

Authors:

Unterborn et al

Abstract:

The proportions of oxygen, carbon, and major rock-forming elements (e.g., Mg, Fe, Si) determine a planet's dominant mineralogy. Variation in a planet's mineralogy subsequently affects planetary mantle dynamics as well as any deep water or carbon cycle. Through thermodynamic models and high pressure diamond anvil cell experiments, we demonstrate that the oxidation potential of C is above that of Fe at all pressures and temperatures, indicative of 0.1-2 Earth-mass planets. This means that for a planet with (Mg+2Si+Fe+2C)/O greater than 1, excess C in the mantle will be in the form of diamond. We find that an increase in C, and thus diamond, concentration slows convection relative to a silicate-dominated planet, due to diamond's ~3 order of magnitude increase in both viscosity and thermal conductivity. We assert then that in the C-(Mg+2Si+Fe)-O system, there is a compositional range in which a planet can be habitable. Planets outside of this range will be dynamically sluggish or stagnant, thus having limited carbon or water cycles leading to surface conditions inhospitable to life as we know it.