ON THE COMPOSITION OF YOUNG, DIRECTLY IMAGED GIANT PLANETS

Abstract

The past decade has seen significant progress on the direct detection and characterization of young, self-luminous giant planets at wide orbital separations from their host stars. Some of these planets show evidence for disequilibrium processes like transport-induced quenching in their atmospheres; photochemistry may also be important, despite the large orbital distances. These disequilibrium chemical processes can alter the expected composition, spectral behavior, thermal structure, and cooling history of the planets, and can potentially confuse determinations of bulk elemental ratios, which provide important insights into planet-formation mechanisms. Using a thermo/photochemical kinetics and transport model, we investigate the extent to which disequilibrium chemistry affects the composition and spectra of directly imaged giant exoplanets. Results for specific "young Jupiters" such as HR 8799 b and 51 Eri b are presented, as are general trends as a function of planetary effective temperature, surface gravity, incident ultraviolet flux, and strength of deep atmospheric convection. We find that quenching is very important on young Jupiters, leading to CO/CH₄ and N₂/NH₃ ratios much greater than, and H₂O mixing ratios a factor of a few less than, chemical-equilibrium predictions. Photochemistry can also be important on such planets, with CO₂ and HCN being key photochemical products. Carbon dioxide becomes a major constituent when stratospheric temperatures are low and recycling of water via the H₂ + OH reaction becomes kinetically stifled. Young Jupiters with effective temperatures ≲700 K are in a particularly interesting photochemical regime that differs from both transiting hot Jupiters and our own solar-system giant planets.

Keywords: planetary systems; planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: individual (51 Erib, HR 8799b, HR 8799c); stars: individual (51 Eri, HR 8799).

Fig. 1.

Theoretical temperature profiles for generic directly imaged planets from the radiative-convective equilibrium model of Marley et al. (2012), as a function of effective temperature T_eff for an assumed surface gravity (in cm s⁻²) of log(g) = 3.5 (colored solid lines) and log(g) = 4.0 (colored dashed lines) and assumed solar composition atmosphere in chemical equilibrium. Profiles are shown every 100 K from T_eff = 600 to 1400 K. The gray dot-dashed lines show the condensation curves for some important atmospheric cloud-forming species (as labeled) for an assumed solar-composition atmosphere. The thicker dotted black lines represent the boundaries where CH₄ and CO have equal abundances and where N₂ and NH₃ have equal abundance in chemical equilibrium for solar-composition models. Methane and ammonia dominate to the lower left of these curves, while CO and N₂ dominate to the upper right. Note that all the profiles remain within the CO-dominated regime at depth, whereas all except for the hottest planets transition to the CH₄-dominated regime at higher altitudes. A color version of this figure is available in the online journal.

Fig. 2.

Eddy diffusion coefficient profiles (colored solid lines) adopted in our thermo/photochemical kinetics and transport models. The K_zz profiles are assumed to vary as 10⁵ (300/P_mbar)^0.5 cm² s⁻¹ in the radiative region, with different models having different cutoff values (K_deep) at depth. Profiles derived for Jupiter (Moses et al. 2005) and the hot Jupiter HD 209458b (Parmentier et al. 2013) are shown for comparison (dashed lines). A color version of this figure is available in the online journal.

Fig. 3.

The ultraviolet stellar irradiance adopted in the models: (Top) The irradiance of 51 Eri (blue) and HR 8799 (red) as received at 1 AU, in comparison with that the Sun (black); (Bottom) the irradiance at the top of the planet’s atmosphere for 51 Eri b (blue) and HR 8799 b (red) in comparison with Jupiter (black). Note from the top panel that both 51 Eri and HR 8799 are brighter than the Sun in the ultraviolet, but 51 Eri b and HR 8799 b are farther away from their host stars than Jupiter, so in terms of the H Lyman alpha flux received, which drives much of the interesting photochemistry, Jupiter receives a flux intermediate between 51 Eri b and HR 8799 b (bottom panel). A color version of this figure is available in the online journal.

Fig. 4.

The vertical mixing-ratio profiles of CH₄ (purple) and CO (green) for planets with a surface gravity log(g) = 4 (cgs), a moderate eddy mixing K_deep = 10⁷ cm² s⁻¹, and T_eff = 600 K (Left), 800 K (Middle), 1000 K (Right). Results for chemical equilibrium are shown with dashed lines, and results from our thermo/photochemical kinetics and transport model are shown as solid lines. Note that CH₄ dominates in the observable portion of the atmosphere in chemical equilibrium, whereas CO dominates in the disequilibrium models. The CH₄/CO ratio is strongly dependent on temperature for both types of chemistry, with a higher CH₄/CO ratio being favored for cooler planets. A color version of this figure is available in the online journal.

Fig. 5.

Quenched mixing ratios of CH₄ (top) and CO (bottom) for models with surface gravities of g = 10^3.5 (left) and 10⁴ cm s⁻² (right) as a function of T_eff and K_deep. High CH₄ abundances and low CO abundances are favored by small T_eff, small K_deep, and large g, although the CO abundance is relatively insensitive to these factors over the range of models investigated. A color version of this figure is available in the online journal.

Fig. 6.

Vertical profiles of several important species in our thermo/photochemical kinetics and transport models (solid colored lines) and in chemical equilibrium (dashed gray and black lines) for a planet with T_eff = 1000 K and g = 10⁴ cm s⁻², at a distance of 68 AU from a star with properties like HR 8799 (Fig. 3), as a function of K_deep (see the legend in the top left panel, and the K_zz profiles shown in Fig. 2). Note that the atmosphere is far out of equilibrium for all the eddy diffusion coefficient profiles considered. The quenched CH₄ mixing ratio increases with decreasing K_deep. The mixing ratios of methane photochemical products such as C₂H₂, C₂H₆, and H also increase with decreasing K_deep. Water quenches at the same time as CO and CH₄, remaining in disequilibrium in the photosphere. Species like HCN and CO₂ are affected both by photochemistry and by quenching of the major carbon, oxygen, and nitrogen species. A color version of this figure is available in the online journal.

Fig. 7.

The vertical mixing-ratio profiles of several atmospheric species as a function of orbital distance for a planet with T_eff =1000 K, g = 10⁴ cm s⁻², and K_deep = 10⁷ cm² s⁻¹, that is being irradiated by an HR 8799-like star at a distance of 10 AU (dashed lines), 32 AU (solid lines), and 100 AU (dotted lines). The greater UV flux received by the closest-in planet leads to increased destruction of photochemically active “parent” molecules such as CH₄, NH₃, H₂O, CO, and N₂, and increased production of photochemical “daughter” products such as HCN, CO₂, complex hydrocarbons, complex nitriles, and atomic species and small radicals. A color version of this figure is available in the online journal.

Fig. 8.

The vertical mixing-ratio profiles of several atmospheric species as a function of T_eff for a planet with g = 10^3.5 cm s⁻² and K_deep = 10⁶ cm² s⁻¹, that is being irradiated by an HR 8799-like star at a distance of 68 AU (dashed lines), for T_eff = 1200 K (dotted lines), 900 K (dashed lines), and 600 K (solid lines). Most disequilibrium photochemical products are synthesized more effectively in low-T_eff atmospheres, but some photochemical products (most notably HCN and C₂H₂) become more abundant at higher T_eff. A color version of this figure is available in the online journal.

Fig. 9.

Integrated column abundance of CO₂ (top left), HCN (top right), C₂H₆ (bottom left), and C₂H₂ (bottom right) above 1 mbar as a function of T_eff and K_deep for planets with a surface gravity of g = 10^3.5 located at 68 AU from a star with the properties of HR 8799. Photochemistry dominates in this region of the atmosphere, and different species exhibit a complicated sensitivity to both T_eff and K_deep. A color version of this figure is available in the online journal.

Fig. 10.

Synthetic spectra from our photochemical models of generic young Jupiters orbiting at 68 AU from a star with the properties of HR 8799, with planetary properties of log(g) = 4.0 cgs, R = 1.2R_J, a global gray absorbing cloud (no patchiness), at a distance of 39 pc from Earth, for (Left) T_eff = 600 K and K_deep = 10⁵ cm² s⁻¹ and (Right) T_eff = 1000 K and K_deep = 10⁷ cm² s⁻¹. The cloud base is assumed to be located at the pressure where the MgSiO₃ condensation curve crosses the temperature profile, and the cloud is assumed to extend to the top of the atmosphere, with the opacity adjusted such that the optical depth is unity between 1 and 10⁻⁴ bars. The plots show how various photochemical products affect the spectra, through the removal of CO₂ (blue), HCN (red), and C₂H₂ (green) from the spectral calculations. Of these photochemical products, only CO₂ affects the spectra significantly at near-IR wavelengths. A color version of this figure is presented in the online journal.

Fig. 11.

Synthetic spectra from our photochemical models of generic young Jupiters orbiting 68 AU from a star with the properties of HR 8799, with surface gravities g = 3200 cm s⁻², eddy K_deep = 10⁶ cm² s⁻¹, and effective temperatures T_eff = 600 K (blue), 900 K (orange), and 1200 K (red). These models correspond to the ones shown in Fig. 8. For the purpose of the spectral calculations, we have assumed that the planets have radii = 1.0R_J, are located 39 pc from Earth, and possess uniform gray absorbing clouds with optical depths of one between the base of the MgSiO₃ condensation region and the top of the atmosphere. Note that the absorption in most of the molecular bands (e.g., H₂O, CH₄, NH₃, and CO₂) increases as T_eff decreases (cf. Fig. 8). A color version of this figure is available in the online journal.

Fig. 12.

Synthetic spectra from our photochemical models of generic young Jupiters with T_eff = 1000 K, g = 10⁴ cm s⁻², K_deep = 10⁷ cm² s⁻¹, orbiting a star with the properties of HR 8799 at 10 AU (red), 32 AU (green), and 100 AU (blue). These models correspond to the ones shown in Fig. 7. For the purpose of the spectral calculations, we have assumed that the planets have radii = 1.2R_J, are located 39 pc from Earth, and possess uniform gray absorbing clouds with optical depths of one between 1 and 10⁻⁴ mbar. Note that at this relatively high T_eff and K_deep the photochemical products have little impact on the spectrum, except for the minor increase in CO₂ absorption at 4.2–4.3 μm in the shorter-period model, due to its greater photochemical production and correspondingly larger CO₂ abundance. A color version of this figure is available in the online journal.

Fig. 13.

Chemical model for HR 8799 b assuming T_eff = 1000 K, g = 3000 cm s⁻² (assumed M = 1.9M_J), and solar metallicity, except C/O = 0.66: (Top) The temperature profile (red curve, bottom axis) from the radiative-convective equilibrium model of Marley et al. (2012) assuming the above bulk constraints, and the eddy diffusion coefficient profile (purple curve, top axis) adopted in the photochemical model; (Middle) the predicted thermochemical equilibrium mixing-ratio profiles for the major oxygen, carbon, and nitrogen species, as labeled, for the assumed pressure-temperature profile; (Bottom) mixing-ratio profiles predicted from our thermo/photochemical kinetics and transport model for the above thermal structure, K_zz profile, and assumed bulk elemental composition. The line segments in the bottom plot are the observational constraints for CH₄ (red), H₂O (blue), and CO (black) from Barman et al. (2015). A color version of this figure is available in the online journal.

Fig. 14.

(Top) Model results for HR 8799 b assuming T_eff = 1000 K, log(g) = 3.5 cgs, C/O = 0.7, a subsolar metallicity, a temperature profile from Barman et al. (2015), and K_deep = 10⁸ cm² s⁻¹ (solid curves) and 10⁹ cm² s⁻¹ (dotted curves). (Bottom two panels) HR 8799 b observations (black data points with error bars; see text) compared with synthetic spectra generated from our thermo/photochemical kinetics and transport models from the top panel, for K_deep = 10⁸ cm² s⁻¹ (green) and 10⁹ cm² s⁻¹ (red); see text and Barman et al. (2015) for details. A color version of this figure is available in the online journal.

Fig. 15.

Chemical model for 51 Eri b assuming T_eff = 700 K, log(g) = 3.5 cgs, mass = 2M_J, and solar metallicity: (Top) The temperature profile (red curve, bottom axis) from the radiativeconvective equilibrium model of Marley et al. (2012) assuming the above bulk constraints, and the eddy diffusion coefficient profile (purple curve, top axis) adopted in the photochemical model; (Middle) the predicted thermochemical equilibrium mixing-ratio profiles for the major oxygen, carbon, and nitrogen species, as labeled, for the assumed pressure-temperature profile; (Bottom) mixing-ratio profiles predicted from our thermo/photochemical kinetics and transport model for the above thermal structure, K_zz profile, and assumed bulk elemental composition. A color version of this figure is available in the online journal.

Fig. 16.

The 51 Eri b GPI observations of Macintosh et al. (2015) (gray/black data points with error bars), in comparison with synthetic spectra from photochemical models that assume T_eff =700 K, g = 3500 cm s⁻², a solar metallicity, R = 0.8R_J, and K_deep = 10⁴ (blue), 10⁴ (green), or 10⁷ cm² s⁻¹ (red). Lower K_deep values lead to larger quenched CH₄ abundances and greater absorption in the long-wavelength side of the H band. The inset shows an expanded wavelength range. As indicated by Macintosh et al. (2015), we also find that we need to invoke partial cloud cover in order to reproduce the near-infrared observations. For this particular analysis, we combined the spectrum of a cloud-free planet with one covered by a uniform global cloud, such that the “cloud fraction” was 30%. The cloudy model assumed a uniform gray absorbing cloud with a base at ~10 bar (representing Mgsilicates) and an optical depth of 1.76 between 0.1–1000 mbar (e.g., from Na₂S clouds or photochemical haze). The planet was assumed to be 29.4 pc from Earth. A color version of this figure is available in the online journal.