Connection between planetary He I λ10 830 Å absorption and extreme-ultraviolet emission of planet-host stars

Sanz-Forcada, J.; López-Puertas, M.; Lampón, M.; Czesla, S.; Nortmann, L.; Caballero, J. A.; Zapatero Osorio, M. R.; Amado, P. J.; Murgas, F.; Orell-Miquel, J.; Pallé, E.; Quirrenbach, A.; Reiners, A.; Ribas, I.; Sánchez-López, A.; Solano, E.
Bibliographical reference

Astronomy and Astrophysics

Advertised on:
1
2025
Number of authors
16
IAC number of authors
3
Citations
26
Refereed citations
18
Description
Context. The detection of the He I λ10 830 Å triplet in exoplanet atmospheres has opened a new window for probing planetary properties, including atmospheric escape. Unlike Lyman α, the triplet is significantly less affected by interstellar medium (ISM) absorption. Sufficient X-ray and extreme ultraviolet (XUV) stellar irradiation may trigger the formation of the He I triplet via photoionization and posterior recombination processes in the planet atmospheres. Only a weak trend between stellar XUV emission and the planetary He I strength has been observed so far. Aims. We aim to confirm this mechanism for producing near-infrared He I absorption in exoplanetary atmospheres by examining a substantial sample of planetary systems. Methods. We obtained homogeneous measurements of the planetary He I line equivalent width and consistently computed the stellar XUV ionizing irradiation. Our first step was to derive new coronal models for the planet-host stars. We used updated data from the X-exoplanets database, archival X-ray spectra of M-type stars (including AU Mic and Proxima Centauri), and new XMM-Newton X-ray data recently obtained for the CARMENES project. These data were complemented at longer wavelengths with publicly available HST, FUSE, and EUVE spectra. A total of 75 stars are carefully analyzed to obtain a new calibration between X-ray and extreme ultraviolet (EUV) emission. Results. Two distinct relationships between stellar X-ray emission (5–100 Å) and EUVH (100–920 Å) or EUVHe (100–504 Å) radiation are obtained to scale the emission from late-type (F to M) stellar coronae. A total of 48 systems with reported planetary He I λ 10 830 Å studies, including 21 positive detections and 27 upper limits, exhibit a robust relationship between the strength of the planetary He I feature and the ionizing XUVHe received by the planet, corrected by stellar and planetary radii, as well as the planet's gravitational potential. Some outliers could be explained by a different atmospheric composition or the lack of planetary gaseous atmospheres. This relation may serve as a guide to predict the detectability of the He I λ 10 830 Å absorption in exoplanet atmospheres.
Related projects
Projects' name image
Exoplanets and Astrobiology
The search for life in the universe has been driven by recent discoveries of planets around other stars (known as exoplanets), becoming one of the most active fields in modern astrophysics. The growing number of new exoplanets discovered in recent years and the recent advance on the study of their atmospheres are not only providing new valuable
Enric
Pallé Bago