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

TL;DR

This study quantifies how stellar XUV activity cycles shape atmospheric escape and the HeI $1083$ nm transit signature in heavily irradiated exoplanets. Using a 1D hydrodynamic framework fed by Sun-like and $\iota$ Hor–type SEDs, it demonstrates that solar-like cycles induce substantial enhancements in mass-loss rates (factors of $\sim$1.7–2.0) and HeI transit depths, especially at larger orbital distances where the escape regime shifts toward energy-limited. In contrast, the $\iota$ Hor cycle, with its smaller EUV variability, yields modest changes in escape and helium signatures, though absolute EUV flux still elevates the observable HeI signal. The results offer a plausible explanation for conflicting HeI observations and emphasize the importance of observing planets at favorable cycle phases and, when possible, at orbital distances where mid-UV depopulation effects are minimized. Overall, the work highlights cycle-aware interpretation for HeI-based atmospheric escape studies and informs target selection and scheduling for helium transit spectroscopy.

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.

Figures (7)

• Figure 1: Upper panel: Solar SED at minimum and maximum phases of the solar activity cycle. The data at wavelengths below 189.5 nm (marked by the vertical grey line) were obtained with the SEE instrument while above with the SORCE instrument as described in section \ref{['sec:XUV_cycle_Sun']}. Lower panel: Reconstructed SED of $\iota$ Hor during a minimum and maximum phase of the activity cycle, as described in section \ref{['sec:SED_iHor']}. The shaded regions common to both panels distinguish the wavelength bins used as input in our atmospheric escape model. From left to right they are X-ray (0.517-12.4 nm), hard-EUV (10-36 nm), soft-EUV (36-92 nm) and mid-UV (91.2-320 nm).
• Figure 2: Maximum-to-minimum cycle variation in various properties of the escaping atmosphere for inner 0.025 au (circles) and outer 0.05 au (crosses) orbits around the Sun-like star. The absolute values of each parameter are listed in Table \ref{['Tab:escape_predictions_solar_all']}. The horizontal lines in the upper panel mark expected mass-loss rate variations based on the change in EUV flux for two different regimes of atmospheric escape as explained in the text.
• Figure 3: Predicted transit-phase-averaged helium triplet profiles assuming high energy fluxes consistent with maximum (red) and minimum (blue) stages during the solar activity cycle. The different rows correspond to the four different types of planets while the columns distinguish the assumed orbital distance. Note the differing y-scales for the differing planet types.
• Figure 4: Demonstrative test suppressing the mid-UV flux received by the modelled Saturn-mass planet orbiting a solar-like star during a maximum phase of its activity cycle. For the mid-UV suppressed case (solid-line) the mid-UV flux capable of depopulating helium out of the $2^3$S state has been reduced by a factor of 1000. The upper and lower panels correspond to orbital distances of 0.025 and 0.05 au, respectively. For comparison, the smaller $\ion{He}{i}$ 1083 nm profile of the original modelled signature with the true mid-UV flux is shown by the dashed profile.
• Figure 5: Predicted helium triplet profiles assuming high energy fluxes obtained from the SED of $\iota$ Hor previously presented in section \ref{['sec:SED_iHor']}. The three lighter, non-solid lines distinguish individual contributions from the three lines comprising the triplet.