Ly{\alpha} Absorption at Transits of HD 209458b: A Comparative Study of Various Mechanisms Under Different Conditions

TL;DR

This study uses 2D hydrodynamic modeling to analyze Lyα absorption during HD 209458b transits, revealing that observed absorption mainly results from natural line broadening, with ENAs playing a minor role under typical stellar conditions.

Contribution

It provides a comprehensive comparison of mechanisms producing Lyα absorption, emphasizing the limited role of ENAs and the dominance of line broadening effects under various stellar activity levels.

Findings

ENAs are insufficient to explain observed absorption under Sun-like conditions.

Lower XUV flux increases planetary atom survival, enhancing absorption.

Radiation pressure has negligible impact on ENA production.

Abstract

To shed more light on the nature of the observed Ly{\alpha} absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6-8 percent can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10-15 percent absorption at the blue wing of the Lya line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.