Hot planetary winds near a star: dynamics, wind-wind interactions, and observational signatures

TL;DR

This study uses hydrodynamic simulations to analyze the dynamics, interactions, and observational signatures of evaporative winds from hot exoplanets, revealing complex flow patterns and variability influenced by stellar and planetary forces.

Contribution

It provides detailed hydrodynamic modeling of planetary wind interactions with stellar winds, including anisotropic effects and synthetic observational predictions.

Findings

Planetary winds are strongly distorted by tidal and Coriolis forces.

Significant backflow of atmospheric material onto the planet occurs due to temperature anisotropy.

Interactions produce observable absorption features with orbit-to-orbit variability.

Abstract

Signatures of "evaporative" winds from exo-planets on short (hot) orbits around their host star have been observed in a number of systems. In this paper we present global AMR simulations that track the launching of the winds, their expansion through the circumstellar environment, and their interaction with a stellar wind. We focus on purely hydrodynamic flows including the anisotropy of the wind launching and explore the orbital/fluid dynamics of the resulting flows in detail. In particular we find that a combination of the tidal and Coriolis forces strongly distorts the planetary "Parker" wind creating "up-orbit" and "down-orbit" streams. We characterize the flows in terms of their orbital elements which change depending on their launch position on the planet. We find that the anisotropy in the atmospheric temperature leads to significant backflow on to the planet. The planetary wind interacts strongly with the stellar wind creating instabilities that cause eventual deposition of planetary gas onto the star. We present synthetic observations of both transit and absorption line-structure for our simulations. For our initial conditions, we find that the orbiting wind material produces absorption signatures at significant distances from the planet and substantial orbit to orbit variability. Ly-{\alpha} absorption shows red and blueshifted features out to 70 km/s. Finally, using semi-analytic models we constrain the effect of radiation pressure, given the approximation of uniform stellar absorption.