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Cold Day-side Winds Shape Large Leading Streams in Evaporating Exoplanet Atmospheres

By Keith Cowing
Status Report
astro-ph.EP
October 29, 2024
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Cold Day-side Winds Shape Large Leading Streams in Evaporating Exoplanet Atmospheres
Metastable helium number density derived from the planetary wind, contrasting hot and cold planetary winds alongside isotropic and day side dominant outflow configurations. The star is positioned at the center, with the planet located in the −x direction, orbiting the star counterclokwise. The black circles indicate their extent, while the red circle represents the Hill radius RH = 2.6 Rp of the planet. Black, gray, and white contours illustrate the cumulative optical depth of τ = 10, 1, and 0.1, respectively, of the metastable helium line towards the observer in the radial direction. The dotted lines mark the orbital phase angles φ. Only the scenarios involving a cold planetary wind result in a widely extended, high-density outflow. Specifically, a cold, day-side dominated outflow (S1) produces a more prominent leading stream compared to the trailing one. — astro-ph.EP

Recent observations of planetary atmospheres in HAT-P-32 b and HAT-P-67 b reveal extensive outflows reaching up to hundreds of planetary radii. The helium 1083 nm light curves for these planets, captured across their full orbits, show notable asymmetries: both planets display more pronounced pre-transit than post-transit absorptions, with HAT-P-67 b being the more extreme case of that geometry.

Using three-dimensional (3D) hydrodynamic simulations, we identify key factors influencing the formation of a dense leading outflow stream and characterize its morphology. Our models suggest that such a geometry of escaped material is caused by a relatively cold outflow of high mass-loss rate, launched preferentially from the planet’s day side. From the simulations we calculate synthetic He I 1083 nm spectra that show large absorption depths and irregular line profiles due to complex gas kinematics.

We find that the measurements of the He I 1083 nm equivalent width and the velocity shift relative to the planet’s rest frame, observed over a significant portion of the planet’s orbital phase, can provide important constraints on the outflow properties and its interaction with the stellar wind.

F. Nail, M. MacLeod, A. Oklopčić, M. Gully-Santiago, C.V. Morley, Z. Zhang

Comments: 10 pages, 6 figures, https://doi.org/10.5281/zenodo.13988501
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2410.19381 [astro-ph.EP] (or arXiv:2410.19381v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.19381
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Submission history
From: Fabienne Nail
[v1] Fri, 25 Oct 2024 08:32:20 UTC (4,608 KB)
https://arxiv.org/abs/2410.19381
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