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

TL;DR

This study investigates whether magnetospheric interactions can explain the proposed near-ultraviolet early ingress of WASP-189b. By analyzing three CUTE NUV transits and performing 2D MHD simulations that map the fast-mode Mach number $M_{ m F}$ as a function of stellar-wind speed $v_{ m sw}$ and local density, the authors assess bow shocks and density pileups ahead of the planet. They find that a dense, magnetosheath pileup can form within about $5\,R_{ m p}$ under moderate wind speeds, potentially producing detectable NUV absorption when the post-shock gas cools efficiently and a wind-transition occurs, whereas classical bow shocks require $M_{ m F}>1$ and are less likely under typical wind conditions. The results suggest that the observed EI feature could arise from wind-driven magnetic pileup rather than a standard bow shock, emphasizing the importance of simultaneous UV and X-ray wind diagnostics to constrain planetary magnetic fields and interaction regimes in UHJs.

Abstract

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. 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. We analyzed three NUV transits of WASP-189b 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. During Visit 3, we identified an approximately 31.5-minute phase offset that is consistent with an early ingress. Our MHD simulations indicate that with a wind speed of 573 km s-1 and an upper atmospheric density of about 4.6e-11 kg m-3, a higher-density zone due to compression can form ahead of the planet within five planetary radii where the fast-mode Mach number falls below ~0.56, even without a MBS. Shock cooling and crossing time estimates suggest that such a pileup could produce detectable NUV absorption. Our results indicate that while MBS formation is feasible for WASP-189b, low stellar-wind speeds favor NUV-detectable magnetic pileups over classical bow shocks and enhance the potential detectability of early-ingress signatures.

Figures (5)

• Figure 1: CUTE NUV transit and spectral analysis for WASP-189b. Top: White-light curves for three visits (V1, V2, and V3; coloured points) and all data combined, with best-fit transit models (dashed lines) and uncertainties (grey shading). The magenta star marks Event 662 in V3. Middle: Residuals relative to best-fit models for each visit and all data, highlighting deviations in V3 (orange). Bottom: Spectral map for V3: flux as a function of wavelength and phase, normalised to the continuum/ The colour scale shows fractional deviations. Anomalous features after egress (Event 662) indicate transient variability.
• Figure 2: Fast-mode magnetosonic Mach number under WASP-189b-like stellar wind conditions. The colour map shows $M_\mathrm{F} - 1$ as a function of $\log_{10}(\rho)$ and $v_{\mathrm{sw}}$, for a stellar magnetic field of 79.05 G. White contours mark the $M_\mathrm{F} = 1$ boundary. Vertical magenta lines indicate estimated planetary upper atmosphere and stellar wind densities; horizontal dashed lines denote wind speeds of $1$–$3v_\mathrm{esc}$. Regions where $M_\mathrm{F} > 1$ are conducive to bow shock formation.
• Figure 3: 2D MHD simulations of bow shock formation for WASP-189b under varying stellar wind conditions. Six combinations of stellar wind speed ($v_\mathrm{sw}=1$–$3v_\mathrm{esc}$) and plasma density ($\rho_\mathrm{atm}$ or $\rho_\mathrm{sw}$), with a stellar magnetic field of 79.05 G, are shown. Top: Magnetic field strength (contours) and wind streamlines (yellow arrows); black circle marks the magnetopause stand-off distance ($R_\mathrm{mp}=1.09$–$1.58~R_\mathrm{p}$). Bottom: Fast-mode wavefronts and shock cones (brown), showing bow shock formation only for $M_\mathrm{F}>1$. Panels (a) and (b) demonstrate that high $v_\mathrm{sw}$ combined with high $\rho$ is required for a distinct bow shock.
• Figure 4: Phase-resolved spectral analysis of WASP-189 b around the Mg II region (2765.6-2834.6 Å). Each panel shows the flux variation (counts, not normalised) as a function of wavelength for different observational visits (V1, V2, and V3) and phase bins (Ph). The symbol # indicates the number of samples in each bin, while St and Ed denote the start and end phases, respectively. The grey markers represent the mean flux profile across multiple observations, with error bars indicating the standard deviation. Three specific events (557, 641, and 662) that deviate from the mean trend are highlighted in orange. The dashed red line denotes the median continuum level for each visit. These results provide insights into the temporal and phase-dependent variations in the observed spectral features, offering constraints on the atmospheric and astrophysical processes affecting WASP-189 b.
• Figure 5: Per-visit comparison between the nominal transit model and the phase-shifted EI model. Each row corresponds to one WASP-189b transit (V1-V3). The main panels display the observed white light curves (cyan for all visits, black for the highlighted visit) together with the best-fitting models: the nominal planet-only transit (solid blue) and the planet+EI model (dashed orange). The insets show the corresponding residuals, $\Delta F$, relative to each model. The EI model is constructed as the pointwise minimum of the base transit and a phase-shifted replica, providing a diagnostic test for the presence of an earlier ingress.