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

¹ Globe Institute–Center for Star and Planet Formation, University of Copenhagen, Copenhagen, Denmark
² Department of Space Science and Engineering, National Central University, Taoyuan, Taiwan
³ Center for Astronautical Physics and Engineering, National Central University, Taoyuan, Taiwan
⁴ Department of Physics-Lund Observatory, Lund University, Lund, Sweden
⁵ Institute of Astronomy, National Central University, Taoyuan, Taiwan
⁶ Steward Observatory, The University of Arizona, Tucson, AZ, USA
⁷ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China
⁸ Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA
⁹ Space Research Institute, Austrian Academy of Sciences, Graz, Austria

^★ Corresponding authors: yi.duann@sund.ku.dk; loren@g.ncu.edu.tw

Received: 14 July 2025
Accepted: 17 September 2025

Abstract

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.