TY  - JOUR
A1  - Poppenhäger, Katja
T1  - Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances
T2  - Monthly notices of the Royal Astronomical Society
N2  - Transit observations in the helium triplet around 10 830 Angstrom are a successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionization and recombination of helium in the exoplanetary atmosphere. This ionization is caused by stellar photons at extreme ultraviolet (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 ionization 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 ionization. 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.
KW  - planets and satellites: atmospheres
KW  - stars: abundances
KW  - stars: coronae
KW  - stars: late-type
KW  - ultraviolet: stars
KW  - X-rays: stars
Y1  - 2022
UR  - https://publishup.uni-potsdam.de/frontdoor/index/index/docId/61786
SN  - 0035-8711
SN  - 1365-2966
VL  - 512
IS  - 2
SP  - 1751
EP  - 1764
PB  - Oxford Univ. Press
CY  - Oxford
ER  -