Mass–Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K
Allard, Nicole F.
AffiliationUniv Arizona, Dept Astron
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PublisherIOP PUBLISHING LTD
CitationWelbanks, L., Madhusudhan, N., Allard, N. F., Hubeny, I., Spiegelman, F., & Leininger, T. (2019). Mass–Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K. The Astrophysical Journal Letters, 887(1), L20.
JournalASTROPHYSICAL JOURNAL LETTERS
RightsCopyright © 2019. The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
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AbstractAtmospheric 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 H-2 broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between similar to 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 system giant planets and similar predictions for exoplanets. On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. The H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. The differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.
NoteOpen access article
VersionFinal published version
SponsorsGates Cambridge Trust; Science and Technology Facilities Council (STFC) UK Science & Technology Facilities Council (STFC)
Except where otherwise noted, this item's license is described as Copyright © 2019. The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.