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Title: CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b

Abstract

Motivated by recent spectroscopic evidence for carbon-rich atmospheres on some transiting exoplanets, we investigate the influence of the C/O ratio on the chemistry, composition, and spectra of extrasolar giant planets both from a thermochemical equilibrium perspective and from consideration of disequilibrium processes like photochemistry and transport-induced quenching. We find that although CO is predicted to be a major atmospheric constituent on hot Jupiters for all C/O ratios, other oxygen-bearing molecules like H{sub 2}O and CO{sub 2} are much more abundant when C/O < 1, whereas CH{sub 4}, HCN, and C{sub 2}H{sub 2} gain significantly in abundance when C/O > 1. Other notable species like N{sub 2} and NH{sub 3} that do not contain carbon or oxygen are relatively unaffected by the C/O ratio. Disequilibrium processes tend to enhance the abundance of CH{sub 4}, NH{sub 3}, HCN, and C{sub 2}H{sub 2} over a wide range of C/O ratios. We compare the results of our models with secondary-eclipse photometric data from the Spitzer Space Telescope and conclude that (1) disequilibrium models with C/O {approx} 1 are consistent with spectra of WASP-12b, XO-1b, and CoRoT-2b, confirming the possible carbon-rich nature of these planets; (2) spectra from HD 189733b are consistent with C/O {approx}<more » 1, but as the assumed metallicity is increased above solar, the required C/O ratio must increase toward 1 to prevent too much H{sub 2}O absorption; (3) species like HCN can have a significant influence on spectral behavior in the 3.6 and 8.0 {mu}m Spitzer channels, potentially providing even more opacity than CH{sub 4} when C/O > 1; and (4) the very high CO{sub 2} abundance inferred for HD 189733b from near-infrared observations cannot be explained through equilibrium or disequilibrium chemistry in a hydrogen-dominated atmosphere. We discuss possible formation mechanisms for carbon-rich hot Jupiters, including scenarios in which the accretion of CO-rich, H{sub 2}O-poor gas dominates the atmospheric envelope, and scenarios in which the planets accrete carbon-rich solids while migrating through disk regions inward of the snow line. The C/O ratio and bulk atmospheric metallicity provide important clues regarding the formation and evolution of the giant planets.« less

Authors:
 [1];  [2];  [3]
  1. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States)
  2. Department of Physics and Department of Astronomy, Yale University, New Haven, CT 06520-8101 (United States)
  3. Southwest Research Institute, Boulder, CO 80302 (United States)
Publication Date:
OSTI Identifier:
22167218
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 763; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION SPECTRA; ABUNDANCE; AMMONIA; ASTRONOMY; ASTROPHYSICS; CARBON; CARBON DIOXIDE; CARBON MONOXIDE; ECLIPSE; HYDROCYANIC ACID; HYDROGEN; METHANE; MOLECULES; OXYGEN; PHOTOCHEMISTRY; PLANETARY ATMOSPHERES; PLANETS; SATELLITE ATMOSPHERES; TELESCOPES; WATER

Citation Formats

Moses, J. I., Madhusudhan, N., Visscher, C., and Freedman, R. S., E-mail: jmoses@spacescience.org. CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b. United States: N. p., 2013. Web. doi:10.1088/0004-637X/763/1/25.
Moses, J. I., Madhusudhan, N., Visscher, C., & Freedman, R. S., E-mail: jmoses@spacescience.org. CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b. United States. https://doi.org/10.1088/0004-637X/763/1/25
Moses, J. I., Madhusudhan, N., Visscher, C., and Freedman, R. S., E-mail: jmoses@spacescience.org. Sun . "CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b". United States. https://doi.org/10.1088/0004-637X/763/1/25.
@article{osti_22167218,
title = {CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, CoRoT-2b, XO-1b, AND HD 189733b},
author = {Moses, J. I. and Madhusudhan, N. and Visscher, C. and Freedman, R. S., E-mail: jmoses@spacescience.org},
abstractNote = {Motivated by recent spectroscopic evidence for carbon-rich atmospheres on some transiting exoplanets, we investigate the influence of the C/O ratio on the chemistry, composition, and spectra of extrasolar giant planets both from a thermochemical equilibrium perspective and from consideration of disequilibrium processes like photochemistry and transport-induced quenching. We find that although CO is predicted to be a major atmospheric constituent on hot Jupiters for all C/O ratios, other oxygen-bearing molecules like H{sub 2}O and CO{sub 2} are much more abundant when C/O < 1, whereas CH{sub 4}, HCN, and C{sub 2}H{sub 2} gain significantly in abundance when C/O > 1. Other notable species like N{sub 2} and NH{sub 3} that do not contain carbon or oxygen are relatively unaffected by the C/O ratio. Disequilibrium processes tend to enhance the abundance of CH{sub 4}, NH{sub 3}, HCN, and C{sub 2}H{sub 2} over a wide range of C/O ratios. We compare the results of our models with secondary-eclipse photometric data from the Spitzer Space Telescope and conclude that (1) disequilibrium models with C/O {approx} 1 are consistent with spectra of WASP-12b, XO-1b, and CoRoT-2b, confirming the possible carbon-rich nature of these planets; (2) spectra from HD 189733b are consistent with C/O {approx}< 1, but as the assumed metallicity is increased above solar, the required C/O ratio must increase toward 1 to prevent too much H{sub 2}O absorption; (3) species like HCN can have a significant influence on spectral behavior in the 3.6 and 8.0 {mu}m Spitzer channels, potentially providing even more opacity than CH{sub 4} when C/O > 1; and (4) the very high CO{sub 2} abundance inferred for HD 189733b from near-infrared observations cannot be explained through equilibrium or disequilibrium chemistry in a hydrogen-dominated atmosphere. We discuss possible formation mechanisms for carbon-rich hot Jupiters, including scenarios in which the accretion of CO-rich, H{sub 2}O-poor gas dominates the atmospheric envelope, and scenarios in which the planets accrete carbon-rich solids while migrating through disk regions inward of the snow line. The C/O ratio and bulk atmospheric metallicity provide important clues regarding the formation and evolution of the giant planets.},
doi = {10.1088/0004-637X/763/1/25},
url = {https://www.osti.gov/biblio/22167218}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 763,
place = {United States},
year = {2013},
month = {1}
}