2020A&A...641A.161Z

Context.It has been suggested that the helium absorption line at 10830Å that originates from the metastable triplet state 2³S is an excellent probe for the extended atmospheres of hot Jupiters and their hydrodynamic escape processes.It has recently been detected in the transmission spectra of a handful of planets. The isotropic reemission will lead to helium airglow that may be observable at other orbital phases.
Aims. We investigate the detectability of HeI emission at 10830Å in the atmospheres of exoplanets using high-resolution spectroscopy. This would provide insights into the properties of the upper atmospheres of close-in gas giants.
Methods. We estimated the expected strength of HeI emission in hot Jupiters based on their transmission signal. We searched for the HeI 10 830 Å emission feature in τ Boo b in three nights of high-resolution spectra taken by CARMENES at the 3.5m Calar Alto telescope. The spectra from each night were corrected for telluric absorption, sky emission lines, and stellar features, and were shifted to the planetary rest frame to search for the emission.
Results. The HeI emission is not detected in τ Boo b at a 5σ contrast limit of 4x10^–4 for emission line widths of >20km/s. This is about a factor 8 above the expected emission level (assuming a typical HeI transit absorption of 1% for hot Jupiters). This suggests that targeting the HeI emission with well-designed observations using upcoming instruments such as VLT/CRIRES+ and E-ELT/HIRES is possible.