Water Dissociation and Rotational Broadening in the Atmosphere of KELT-20 b from High-resolution Spectroscopy
Abstract We present atmospheric retrievals from Keck/KPIC Phase II observations of the ultrahot Jupiter (UHJ) KELT-20/MASCARA-2 b. Previous free retrievals of molecular abundances for UHJs have been impacted by significant model biases due to variations in vertical abundance profiles, which we address by including molecular dissociation into our retrieval framework as an additional free parameter. We measure the abundance of CO ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">MMR</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) and obtain a lower limit on the abundance of H 2 O ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mi>MMR</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>5</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , >−3.0 at 95% confidence) in the atmosphere of KELT-20 b. These abundances yield an atmospheric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (C/O < 0.9 at 95% confidence) and suggest a metallicity approximately solar to 10 × solar. H 2 O is dissociated at pressures below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> bar, roughly consistent with predictions from chemical equilibrium models, and suggesting that the retrieved composition is not a result of assumptions about the vertical mixing profiles. We also constrain the rotational velocity of KELT-20 b to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>7.5</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.7</mml:mn> </mml:math> km s −1 , suggesting the presence of a jet comparable to the sound speed in the direction of the planet’s rotation, assuming the actual rotation of the planet is tidally locked.
Authors
- Katelyn Horstman (ORCID: https://orcid.org/0000-0001-9708-8667)
- Andrew Skemer (ORCID: https://orcid.org/0000-0001-6098-3924)
- Emily C. Martin (ORCID: https://orcid.org/0000-0002-0618-5128)
- Taylor Venenciano (ORCID: https://orcid.org/0000-0003-0122-8915)
- Shubh Agrawal (ORCID: https://orcid.org/0000-0003-2429-5811)
- Nemanja Jovanović (ORCID: https://orcid.org/0000-0001-5213-6207)
- Jacques-Robert Delorme (ORCID: https://orcid.org/0000-0001-8953-1008)
- Tobias Schofield
- Nicole L. Wallack (ORCID: https://orcid.org/0000-0003-0354-0187)
- Jacklyn Pezzato
- Luke Finnerty (ORCID: https://orcid.org/0000-0002-1392-0768)
- Evan Morris (ORCID: https://orcid.org/0000-0003-3165-0922)
- Randall Bartos
- J. Kent Wallace (ORCID: https://orcid.org/0000-0001-5299-6899)
- Ashley Baker (ORCID: https://orcid.org/0000-0002-6525-7013)
- Jean-Baptiste Ruffio (ORCID: https://orcid.org/0000-0003-2233-4821)
- Julie Inglis (ORCID: https://orcid.org/0000-0001-9164-7966)
- Joshua Liberman (ORCID: https://orcid.org/0000-0002-4934-3042)
- Dimitri Mawet (ORCID: https://orcid.org/0000-0002-8895-4735)
- Sylvain Cetre
- Ji Wang (ORCID: https://orcid.org/0000-0002-4361-8885)
- Chih-Chun Hsu (ORCID: https://orcid.org/0000-0002-5370-7494)
- Daniel Echeverri (ORCID: https://orcid.org/0000-0002-1583-2040)
- Yinzi Xin (ORCID: https://orcid.org/0000-0002-6171-9081)
- Geoffrey A. Blake (ORCID: https://orcid.org/0000-0003-0787-1610)
- Ronald López (ORCID: https://orcid.org/0000-0002-2019-4995)
- Ben Sappey (ORCID: https://orcid.org/0000-0003-1399-3593)
- Benjamin Calvin (ORCID: https://orcid.org/0000-0003-4737-5486)
- Greg Doppmann
- Michael P. Fitzgerald (ORCID: https://orcid.org/0000-0002-0176-8973)
- Jerry W. Xuan (ORCID: https://orcid.org/0000-0002-6618-1137)
- Jason Wang (ORCID: https://orcid.org/0000-0003-0774-6502)
Institutions
- Northwestern University (US)
- California Institute of Technology (US)
- Jet Propulsion Laboratory (US)
- University of Arizona (US)
- University of California, Los Angeles (US)
- Pomona College (US)
- University of California, Santa Cruz (US)
- Carnegie Institution for Science (US)
- W.M. Keck Observatory (US)
- University of California San Diego (US)
- University of Pennsylvania (US)
Publication Details
- Journal
- The Astronomical Journal
- Published
- 2025-05-23
- DOI
- https://doi.org/10.3847/1538-3881/adce02
- Citations
- 9
- Primary Topic
- Atmospheric Ozone and Climate
- Type
- article
- Field-Weighted Citation Impact
- 11.71