Search for He I airglow emission from the hot Jupiter τ Boo b

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
e-mail: yzhang@strw.leidenuniv.nl
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill, Coventry, CV4 7AL, UK
⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 13 May 2020
Accepted: 24 July 2020

Abstract

Context. It has been suggested that the helium absorption line at 10 830 Å 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 He I emission at 10 830 Å 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 He I emission in hot Jupiters based on their transmission signal. We searched for the He I 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 He I emission is not detected in τ Boo b at a 5σ contrast limit of 4 × 10⁻⁴ for emission line widths of >20 km s⁻¹. This is about a factor 8 above the expected emission level (assuming a typical He I transit absorption of 1% for hot Jupiters). This suggests that targeting the He I emission with well-designed observations using upcoming instruments such as VLT/CRIRES+ and E-ELT/HIRES is possible.