Modeling the Dynamics and Thermochemistry for the Outer Atmospheres of the Ultra-hot Jupiter WASP-121b

TL;DR

This work presents a GPU-accelerated, three-dimensional simulation framework that couples non-LTE thermochemistry, ray-tracing radiative transfer, and hydrodynamics to model the atmosphere of the ultra-hot Jupiter WASP-121b from the surface to extended outflows. The fiducial simulation reveals a supersonic, spiral-arm outflow shaped by stellar gravity and orbital motion, with distinct tracers (Fe, Na, H$\alpha$, He$^*$) mapping different atmospheric reservoirs and kinematics. A parametric study shows how high-energy irradiation and stellar wind alter outflow density, ionization, and observable absorption features, illuminating the complex interplay between irradiation, winds, and chemistry in UHJs. The results demonstrate that multi-species transmission spectroscopy, interpreted through state-of-the-art 3D simulations, can constrain the thermochemical and dynamical structure of exoplanetary upper atmospheres and their interactions with the host star. This framework paves the way for integrating JWST and ELT data to characterize wind–planet coupling and dynamical instabilities in UHJs.

Abstract

We present three-dimensional simulations of the ultra-hot Jupiter (UHJ) WASP-121b from the planetary surface to extended outflows, coupling hydrodynamics with consistent non-equilibrium thermochemistry, ray-tracing radiative transfer, and hydrodynamics using the GPU-accelerated Kratos framework. The fiducial model exhibits several atmospheric layers, including the lower atmospheres controlled by day-night circulation, and transonic photoevaporative outflows at higher altitudes shaped into two spiral arms by the stellar gravity and orbital motion effects. Different species could trace different regions: Fe probes rotation-dominated inner layers, Na maps dense spiral arms where recombination balances photoionization, and H$α$ and He $10830~{\rm A}$ features trace progressively more extended, ionized gas. With spiral arm velocities reaching $\sim 40~{\rm km\ s}^{-1}$ projected along the line of sight, this morphology explains observed high-velocity Na and H$α$ absorption features without requiring significant super-rotation jet streams. Parametric studies reveal complex dependencies on stellar irradiation: enhanced FUV intensifies outflows and extends spiral arms spatially and kinematically, while EUV and X-ray expands spiral structures into attenuated, ionized regions. Stellar wind confinement compresses the dayside outflow and enhances metastable helium absorption. This work demonstrates that current and future transmission spectral observations that probe multiple species can provide important constraints on astrophysical environments of UHJs by comparing state-of-the-art simulations.

Figures (12)

• Figure 1: Equatorial slices of key hydrodynamic and thermochemical quantities from the fiducial simulation of WASP-121b. The panels show (from left to right, top to bottom): gas density $\rho$, temperature $T$, line-of-sight velocity in the laboratory frame $v_{\rm los}$, and the number densities of Fe, Na, H$^{2s}$ (metastable neutral hydrogen on the $2s$ state, the H$\alpha$ absorber), free electrons ($e^-$), He$^+$, and $\mathrm{He}^*$ (metastable neutral helium). The snapshot is the temporal average over the final 10 hours over the $300$ total simulated hours, after evolving the system through a quasi-steady state (§\ref{['sec:method-chem']}). The spatial coordinates are in units of Earth radii ($R_\oplus$), and the central circle marks the nominal planetary radius ($R_{\rm p}$). Equipotential contours are shown in gray dotted lines, streamlines are indicated by white lines, and the sonic critical lines are overlaid as dashed magenta lines. The planet orbital motion is downwards, and the host star is to the right.
• Figure 2: Colormapped volume rendering for the 10-based logarithms of number densities (in ${\rm cm}^{-3}$) for four key tracing species (top left: Fe; top right: Na; bottom left: H$\alpha$; bottom right: $\mathrm{He}^*$). Note that the dynamical ranges of colormaps are different. The host star is on the far side from the reader, and the planet moves from left to right. The rendering boxes have the same sizes ($280~R_\oplus$ along every dimension). The high-abundance region of $\mathrm{He}^*$ in the planet shadow is clearly seen as a cylinder.
• Figure 3: Schematic illustration of the transit geometry and the origin of asymmetric velocity shifts. The planet (black solid circle) moves from top to bottom. The leading (redhifted) and trailing (blueshifted) spiral arms are shaped by both the spilling through the Lagrangian points and the Coriolis force (§\ref{['sec:morphology']}). The LoS during ingress and egress samples different projections of the arm velocities, leading to the observed asymmetry in the transmission spectrum.
• Figure 4: Instantaneous velocity fields in streamlines and arrows, and hydrodynamic quantities in colormaps (tangential wind speed in the top row, pressure in the middle row, and temperature in the bottowm row), showing two representative equipotential surfaces from the fiducial simulation. Left column presents the surface near the planetary surface ($\Delta r_0 = 0.1~R_\oplus$ above the substellar radius), illustrating the substellar anticyclone and the antistellar cyclone, along with prograde super-rotation near the equator. Right column presents the equipotential surface at $\Delta r_0 = 5~R_\oplus$, where the retrograde winds are predominantly deflected by the Coriolis force on the outflow.
• Figure 5: Extinction intensities (quantified by $1-\mathrm{e}^{-\tau}$) at three different velocities and orbit phases (denoted at the top of each panel) for four key tracers from the fiducial simulation (denoted at the colobar in each row). White dashed circles indicate the projection of the host star, in which the extinction by planetary atmospheres are calculated. The asymmetric velocity shifts and phase-dependent absorption depths trace the geometry and kinematics of the spiral arms and inner atmospheric layers.