The effects of stellar activity cycles on planetary atmospheric escape and the HeI 1083nm transit signature

TL;DR

This study models how stellar activity cycles influence atmospheric escape and helium signatures in exoplanets, revealing significant variability and implications for observations during different stellar activity phases.

Contribution

It provides the first detailed analysis of how stellar activity cycles affect helium signatures and atmospheric escape in highly irradiated exoplanets, considering different star types and orbital distances.

Findings

Helium signatures are stronger during stellar activity maximum.

Planets at larger orbits show more variability in escape rates.

HeI 1083nm absorption can be unexpectedly strong at greater distances.

Abstract

The HeI 1083nm transit signature is commonly used in tracing escaping planetary atmospheres. However, it can be affected by stellar activity, complicating detections and interpretations of atmospheric escape. We model how stellar activity cycles affect the atmospheric escape and HeI 1083nm signatures of four types of highly irradiated exoplanets, at 0.025 and 0.05 au, during minimum and maximum cycle phases. We consider two stars, exhibiting different cycle behaviours: the Sun and the more active star iota Hor, for which we reconstruct its spectral energy distributions at minimum and maximum phases using X-ray observations and photospheric models. We show that over a modulated activity cycle, the release of extreme ultraviolet photons, responsible for atmospheric escape, varies substantially more than that of mid-UV photons, capable of photoionising HeI (23S). This leads to consistently stronger helium signatures during maximum phases. We show that planets at the largest orbit are more affected by cycles, showing larger variations in escape rates and absorptions between minimum and maximum. We also confirm the counter-intuitive behaviour that, despite the fall-off in escape rate with orbital distance, the HeI 1083nm absorption is not significantly weaker at further orbits, even strengthening with orbital distance for some iota Hor planets. We partially explain this behaviour with the lower mid-UV fluxes at more distant orbits, leading to less HeI (23S) photoionisations. Finally, we propose that stellar cycles could explain some of the conflicting HeI 1083nm observations of the same planet, with detections more likely during a phase of activity maximum.