Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances

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.

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

“…This is because K stars have relatively high EUV fluxes, which populate the metastable state via ground-state ionization and subsequent recombination, and relatively low mid-UV fluxes, which ionize and depopulate the metastable state. A large fraction of the EUV flux also comes from coronal iron lines, and Poppenhaeger (2022) highlighted that this consideration also favors K stars for helium studies. The stellar spectral type gives only an approximate expectation for the high-energy spectrum of the star however, and the discovery of a strong helium signature for HAT-P-32b (a gas-giant planet orbiting an F star) is a reminder that other stars with favorable spectral energy distributions (SEDs) can also exhibit strong signals regardless of their spectral type (Czesla et al 2021).…”

“…In order to create a significant population of metastable helium, there must be enough EUV photons to ionize ground-state helium in the outflow, which then recombines into the metastable state (Oklopčić & Hirata 2018;Oklopčić 2019). As demonstrated by Poppenhaeger (2022), many of the stellar EUV photons that ultimately ionize helium come from coronal iron lines. Iron has a relatively low first ionization potential (FIP), and in very active, low-mass stars, the abundance of species with low FIPs often appear to be diminished (termed the inverse first ionization potential effect, or iFIP; Brinkman et al 2001;Güdel et al 2001;Wood et al 2018).…”

“…WASP-52 and WASP-80 are by far the most active stars in our sample, so they may be affected by this phenomenon. Our assumed proxy star for these two planets, Eridani, does not exhibit the iFIP effect and has strong coronal iron lines (Poppenhaeger 2022); this might cause us to overestimate their metastable helium populations in our models.…”

Searching for Helium in the Extended Atmosphere of the Hot Jupiter WASP-48b

“…Though X-ray spectra are needed to validate coronal iron abundances, placing stars in the "high" versus "low" category can be estimated based on spectral type. Poppenhaeger (2022) establishes that all M0 stars and later should be in the low abundance category, as should hot stars and old stars, which are both expected to have low coronal temperatures. We follow this protocol and assign a high or low coronal iron abundance to the stars in our sample.…”

“…Many papers have used X-ray luminosity either as a proxy or as a scaling factor for EUV luminosity (Sanz-Forcada et al 2011). A recent paper by Poppenhaeger (2022) made the assertion that EUV luminosity can be derived from X-ray luminosity, but instead of following one scaling law, it should follow two, based on iron abundances in the stellar corona. This is because stellar EUV emission is dominated largely by line emissions from bound-bound transitions, many of which are formed by iron in the corona.…”