Cold dayside winds shape large leading streams in evaporating exoplanet atmospheres

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
² Center for Astrophysics, Harvard & Smithsonian 60 Garden Street, MS-16, Cambridge, MA 02138, USA
³ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, Santa Cruz, CA 95064, USA

^★ Corresponding author; f.nail@uva.nl

Received: 24 October 2024
Accepted: 10 February 2025

Abstract

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. Using 3D hydrodynamic simulations, we identified the key factors influencing the formation of a dense leading outflow stream and characterized its morphology. Our models suggest that such a geometry of escaped material is caused by a relatively cold outflow with a high mass-loss rate, launched preferentially from the planet’s dayside. From the simulations we calculated 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.