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Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. I. WASP-69b, Dimensionality, X-Ray and UV Flux Level, Spectral Types, and Flares

Wang, Lile and Dai, Fei (2021) Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. I. WASP-69b, Dimensionality, X-Ray and UV Flux Level, Spectral Types, and Flares. Astrophysical Journal, 914 (2). Art. No. 98. ISSN 0004-637X. doi:10.3847/1538-4357/abf1ee.

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The metastable helium (He*) lines near 10830 Å are ideal probes of atmospheric erosion—a common phenomenon of close-in exoplanet evolution. A handful of exoplanet observations yielded well-resolved He* absorption features in transits, yet they were mostly analyzed with 1D isothermal models prescribing mass-loss rates. This work devises 3D hydrodynamics coevolved with ray-tracing radiative transfer and nonequilibrium thermochemistry. Starting from the observed stellar/planetary properties with reasonable assumptions about the host's high-energy irradiation, we predict from first principles the mass-loss rate, the temperature and ionization profiles, and 3D outflow kinematics. Our simulations well reproduce the observed He* line profiles and light curves of WASP-69b. We further investigate the dependence of He* observables on simulation conditions and host radiation. The key findings are as follows: (1) Simulations reveal a photoevaporative outflow (~0.55 M_⊕ Gyr⁻¹ ≃ 10¹¹ g s⁻¹) for WASP-69b without a prominent comet-like tail, consistent with the symmetric transit shape. (2) 3D simulations are mandatory for hydrodynamic features, including Coriolis force, advection, and kinematic line broadening. (3) EUV (>13.6 eV) photons dominate photoevaporative outflows and populate He* via recombination; FUV is also detrimental by destroying He*; X-ray plays a secondary role. (4) K stars hit the sweet spot of EUV/far-UV balance for He* line observation, while G and M stars are also worthy targets. (5) Stellar flares create characteristic responses in the He* line profiles.

Item Type:Article
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URLURL TypeDescription Paper
Wang, Lile0000-0002-6540-7042
Dai, Fei0000-0002-8958-0683
Alternate Title:Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry I: WASP-69b, Dimensionality, XUV Flux Level, Spectral Types, and Flares
Additional Information:© 2021. The American Astronomical Society. Received 2020 December 18; revised 2021 March 2; accepted 2021 March 23; published 2021 June 21. This work is supported by the Center for Computational Astrophysics of the Flatiron Institute and the Division of Geological and Planetary Sciences of the California Institute of Technology. L.W. acknowledges the computing resources provided by the Simons Foundation and the San Diego Supercomputer Center. We thank our colleagues (in alphabetical order) Philip Armitage, Zhuo Chen, Jeremy Goodman, Xiao Hu, Heather Knutson, Mordecai Mac-Low, Jessica Spake, Kengo Tomida, Songhu Wang, Andrew Youdin, and Michael Zhang, for helpful discussions and comments. We especially thank Shreyas Vissapragada for detailed suggestions and discussions. We particularly thank the anonymous referee for the constructive comments and suggestions.
Funding AgencyGrant Number
Flatiron InstituteUNSPECIFIED
Caltech Division of Geological and Planetary SciencesUNSPECIFIED
Subject Keywords:Exoplanet atmospheres; Exoplanet evolution; Exoplanet astronomy; Hydrodynamical simulations; Astronomical simulations; Hydrodynamics; Astrochemistry
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet atmospheres (487); Exoplanet evolution (491); Exoplanet astronomy (486); Hydrodynamical simulations (767); Astronomical simulations (1857); Hydrodynamics (1963); Astrochemistry (75)
Record Number:CaltechAUTHORS:20210628-191053856
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Official Citation:Lile Wang and Fei Dai 2021 ApJ 914 98
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109632
Deposited By: George Porter
Deposited On:28 Jun 2021 22:22
Last Modified:16 Nov 2021 19:37

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