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.

accepted for publication in MNRAS. 17 pages, 9 figures

References in corpus (20)

• A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
• The fast spin-rotation of a young extrasolar planet
• Lyman-$\alpha$ Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b
• Hubble PanCET: An extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470 b
• Discovery of water at high spectral resolution in the atmosphere of 51 Peg b
• High-resolution confirmation of an extended helium atmosphere around WASP-107b
• The shocking transit of WASP-12b: Modelling the observed early ingress in the near ultraviolet
• The corona of HD 189733 and its X-ray activity
• A He I upper atmosphere around the warm Neptune GJ 3470b
• From dense hot Jupiter to low-density Neptune: The discovery of WASP-127b, WASP-136b and WASP-138b
• The GAPS programme at TNG XXII. The GIARPS view of the extended helium atmosphere of HD189733 b accounting for stellar activity
• Active cool stars and He I 10830 Å: the coronal connection
• Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b
• The FIP and Inverse FIP Effects in Solar Flares
• Search for helium in the upper atmosphere of the hot Jupiter WASP-127 b using Gemini/Phoenix
• Coronal properties of the EQ Peg binary system
• X-ray emission from the super-Earth host GJ 1214
• The XUV irradiation and likely atmospheric escape of the super-Earth $\pi$ Men c
• Coronal X-ray emission and planetary irradiation in HD 209458
• Probing the atmosphere of HD189733b with the Na I and K I lines

Cited by in corpus (14)

• The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation
• Keck/NIRSPEC studies of He I in the atmospheres of two inflated hot gas giants orbiting K dwarfs: WASP-52b and WASP-177b
• Homogeneous search for helium in the atmosphere of 11 gas giant exoplanets with SPIRou
• Connection between planetary He I $\lambda$10830 Å absorption and extreme-ultraviolet emission of planet-host stars
• More Evidence for Variable Helium Absorption from HD 189733b
• On the origin of the non-detection of metastable HeI in the upper atmosphere of the hot Jupiter WASP-80b
• Planet(esimal)s Around Stars with TESS (PAST) III: A Search for Triplet He I in the Atmospheres of Two 200 Myr-old Planets
• Non-detection of Helium in the Hot Jupiter WASP-48b
• Observing atmospheric escape in sub-Jovian worlds with JWST
• Evolution of helium triplet transits of close-in gas giants orbiting K-dwarfs
• NIGHT: a compact, near-infrared, high-resolution spectrograph to survey helium in exoplanet systems
• sunset: A database of synthetic atmospheric-escape transmission spectra for nearly every transiting planet
• Three young planets around the K-dwarf K2-198: High-energy environment, evaporation history and expected future
• On the center-to-limb variations of the He I 10 830 Å triplet