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Anders Johansen

Anders Johansen

Professor

Anders Johansen

Magnetohydrodynamic simulation assessment of a potential near-ultraviolet early ingress in WASP-189b

Author

  • Y. Duann
  • S. H. Lai
  • H. J. Hoeijmakers
  • A. Johansen
  • C. L. Lin
  • L. C. Huang
  • Y. Y. Chang
  • A. G. Sreejith
  • K. France
  • L. C. Chang
  • W. H. Ip
Show all

Summary, in English

Context. Ultra-hot Jupiters (UHJs) in close orbits around early-type stars provide natural laboratories for studying atmospheric escape and star-planet interactions under extreme irradiation and wind conditions. The near-ultraviolet (NUV) regime is particularly sensitive to extended upper atmospheric and magnetospheric structures. Aims. We investigate whether star-planet interactions in the WASP-189 system could plausibly account for the early ingress feature suggested by NUV transit fitting models. Methods. We analysed three NUV transits of WASP-I89b observed as part of the Colorado Ultraviolet Transit Experiment (CUTE), which employs a 6U CubeSat dedicated to exoplanet spectroscopy. To explore whether the observed transit asymmetry could plausibly arise from a magnetospheric bow shock (MBS), we performed magnetohydrodynamic (MHD) simulations using representative stellar wind velocities and planetary atmospheric densities. Results. During Visit 3, we identified a ~31.5-minute phase offset that is consistent with an early ingress. Our MHD simulations indicate that, with a wind speed of 572.97 km s−1 and a sufficient upper atmospheric density (~4.59 × 10−11 kg m−3), a higher-density zone due to compression can form ahead of the planet within five planetary radii in regions where the fast-mode Mach number falls below ~0.56, even without a MBS. Shock cooling and crossing time estimates from the simulations further suggest that such a pileup could, in principle, produce detectable NUV absorption. Conclusions. Our results indicate that while MBS formation is feasible for WASP-189b, low stellar-wind speeds favour NUV-detectable magnetic pileups over classical bow shocks. Immediately after the shock formation, the post-shock plasma is too hot for strong NUV absorption, but a high-to-low wind-speed transition shortens the cooling time while preserving the compressed plasma, increasing its opacity. Pressure-balance estimates show that magnetic pressure dominates across wind regimes in the low-density case, and at low wind speeds in the high-density case, favouring pileup and reconnection near the magnetopause and enhancing the potential detectability of early-ingress signatures.

Department/s

  • Astrophysics
  • eSSENCE: The e-Science Collaboration

Publishing year

2025-11-01

Language

English

Publication/Series

Astronomy and Astrophysics

Volume

703

Document type

Journal article

Publisher

EDP Sciences

Topic

  • Astronomy, Astrophysics and Cosmology

Keywords

  • planet-star interactions
  • planets and satellites: atmospheres
  • planets and satellites: gaseous planets
  • planets and satellites: magnetic fields
  • ultraviolet: planetary systems

Status

Published

ISBN/ISSN/Other

  • ISSN: 0004-6361