Contributed Talk - Splinter Exoplanets

Thursday, 24 September 2020, 16:05   (virtual room D)

Connecting helium ionisation in exoplanet atmospheres with stellar X-ray flux

K. Poppenhaeger
Leibniz Institute for Astrophysics Potsdam

Transit observations in the helium lines near 10830 Angstrom are a new and valuable tool to study exoplanetary atmospheres. The pathway to forming the metastable state involved in the relevant lines requires ionisation and recombination of helium in the exoplanetary atmosphere. This ionisation is caused by stellar photons in the extreme UV (EUV). In this wavelength range, the stellar spectrum consists of distinct emission lines formed at specific coronal temperatures. However, there are currently no observatories that can directly measure the EUV irradiation received by exoplanets. Using archival observations from the EUVE space telescope and modern X-ray telescopes, I derive a scaling relationship between helium ionisation rate and stellar brightness in currently observationally accessible bands, such as soft X-rays. This allows a quantitative prediction of expected helium signal strength for exoplanet observations.