Space Weather & Heliophysics

The Space Weather Around The Exoplanet GJ 436 b. II. Stellar Wind-exoplanet Interactions

By Keith Cowing
Status Report
September 5, 2023
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The Space Weather Around The Exoplanet GJ 436 b. II. Stellar Wind-exoplanet Interactions
Three dimensional view of the magnetised stellar wind for Models I and II. The surface magnetic field from March-June 2016 reconstructed by Bellotti et al. (2023) is used at the boundary of the models. The grey streamlines are the magnetic field lines embedded in the wind. The rotation axis of the star is along the z⋆-axis. The two-dimensional cut is placed at the x⋆z⋆ plane, and for indication, the orbital distance of the planet is shown by the translucent sphere at 14.56 R⋆ ≃ 0.028 au. The contour shows the wind velocity and the Alfvén surface is shown by the translucent outer surface. Models I and II predict stellar wind mass-loss rates of 1.1 × 10−15 M⊙ yr−1 and 2.5 × 10−14 M⊙ yr−1 — astro-ph.SR .

The M dwarf star GJ 436 hosts a warm-Neptune that is losing substantial amount of atmosphere, which is then shaped by the interactions with the wind of the host star. The stellar wind is formed by particles and magnetic fields that shape the exo-space weather around the exoplanet GJ 436 b.

Here, we use the recently published magnetic map of GJ 436 to model its 3D Alfvén-wave driven wind. By comparing our results with previous transmission spectroscopic models and measurements of non-thermal velocities at the transition region of GJ 436, our models indicate that the wind of GJ 436 is powered by a smaller flux of Alfvén waves than that powering the wind of the Sun. This suggests that the canonical flux of Alfvén waves assumed in solar wind models might not be applicable to the winds of old M dwarf stars.

Compared to the solar wind, GJ 436’s wind has a weaker acceleration and an extended sub-Alfvénic region. This is important because it places the orbit of GJ 436 b inside the region dominated by the stellar magnetic field (i.e., inside the Alfvén surface).

Due to the sub-Alfvénic motion of the planet through the stellar wind, magnetohydrodynamic waves and particles released in reconnection events can travel along the magnetic field lines towards the star, which could power the anomalous ultraviolet flare distribution recently observed in the system. For an assumed planetary magnetic field of Bp≃2 G, we derive the power released by stellar wind-planet interactions as P∼1022 — 1023 erg s−1, which is consistent with the upper limit of 1026 erg s−1 derived from ultraviolet lines.

We further highlight that, because star-planet interactions depend on stellar wind properties, observations that probe these interactions and the magnetic map used in 3D stellar wind simulations should be contemporaneous for deriving realistic results.

A. A. Vidotto, V. Bourrier, R. Fares, S. Bellotti, J. F. Donati, P. Petit, G. A. J. Hussain, J. Morin

Comments: 12 pages, 7 figures, 1 table; accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2309.00324 [astro-ph.SR] (or arXiv:2309.00324v1 [astro-ph.SR] for this version)
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Submission history
From: Stefano Bellotti
[v1] Fri, 1 Sep 2023 08:15:13 UTC (4,117 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻