Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances

TL;DR

This paper links stellar coronal iron abundances to EUV flux and helium absorption in exoplanet atmospheres, improving understanding of atmospheric mass loss and refining target selection for helium transit observations.

Contribution

It introduces a new scaling law connecting stellar coronal iron abundance with EUV flux, explaining variations in helium absorption in exoplanet atmospheres.

Findings

Stellar coronal iron abundance affects EUV flux relevant for helium ionisation.

Measuring iron to oxygen ratio from X-ray spectra is feasible and consistent.

Accounting for iron abundance reduces scatter in EUV-helium absorption relationships.

Abstract

Transit observations in the helium triplet around 10830 Angstrom are a successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionisation and recombination of helium in the exoplanetary atmosphere. This ionisation is caused by stellar photons at extreme ultra-violet (EUV) wavelengths; however, no currently active telescopes can observe this part of the stellar spectrum. The relevant part of the stellar EUV spectrum consists of individual emission lines, many of them being formed by iron at coronal temperatures. The stellar iron abundance in the corona is often observed to be depleted for high-activity low-mass stars due to the first ionisation potential (FIP) effect. I show that stars with high versus low coronal iron abundances follow different scaling laws that tie together their X-ray emission and the narrow-band EUV flux that causes helium ionisation. I also show that the stellar iron to oxygen abundance ratio in the corona can be measured reasonably well from X-ray CCD spectra, yielding similar results to high-resolution X-ray observations. Taking coronal iron abundance into account, the currently observed large scatter in the relationship of EUV irradiation with exoplanetary helium transit depths can be reduced, improving the target selection criteria for exoplanet transmission spectroscopy. In particular, previously puzzling non-detections of helium for Neptunic exoplanets are now in line with expectations from the revised scaling laws.