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

Duann, Y. ; Lai, S. H. ; Hoeijmakers, H. J. LU orcid ; Johansen, A. LU ; Lin, C. L. LU ; Huang, L. C. ; Chang, Y. Y. ; Sreejith, A. G. ; France, K. and Chang, L. C. , et al. (2025) In Astronomy and Astrophysics 703.
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... (More)

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.

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publishing date
type
Contribution to journal
publication status
published
subject
keywords
planet-star interactions, planets and satellites: atmospheres, planets and satellites: gaseous planets, planets and satellites: magnetic fields, ultraviolet: planetary systems
in
Astronomy and Astrophysics
volume
703
article number
A24
publisher
EDP Sciences
external identifiers
  • scopus:105021311458
ISSN
0004-6361
DOI
10.1051/0004-6361/202556404
language
English
LU publication?
yes
additional info
Publisher Copyright: © The Authors 2025.
id
70190520-576a-4f30-96d1-89512a7b1e4a
date added to LUP
2025-12-11 15:22:23
date last changed
2025-12-11 15:23:38
@article{70190520-576a-4f30-96d1-89512a7b1e4a,
  abstract     = {{<p>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<sup>−1</sup> and a sufficient upper atmospheric density (~4.59 × 10<sup>−11</sup> kg m<sup>−3</sup>), 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.</p>}},
  author       = {{Duann, Y. and Lai, S. H. and Hoeijmakers, H. J. and Johansen, A. and Lin, C. L. and Huang, L. C. and Chang, Y. Y. and Sreejith, A. G. and France, K. and Chang, L. C. and Ip, W. H.}},
  issn         = {{0004-6361}},
  keywords     = {{planet-star interactions; planets and satellites: atmospheres; planets and satellites: gaseous planets; planets and satellites: magnetic fields; ultraviolet: planetary systems}},
  language     = {{eng}},
  month        = {{11}},
  publisher    = {{EDP Sciences}},
  series       = {{Astronomy and Astrophysics}},
  title        = {{Magnetohydrodynamic simulation assessment of a potential near-ultraviolet early ingress in WASP-189b}},
  url          = {{http://dx.doi.org/10.1051/0004-6361/202556404}},
  doi          = {{10.1051/0004-6361/202556404}},
  volume       = {{703}},
  year         = {{2025}},
}