Yapeng Zhang1, I.A.G. Snellen1, P. Mollière1,2, F. J. Alonso-Floriano1, R. K. Webb 3, M. Brogi3,4, A Wyttenbach1,5
1 Leiden Observatory, 2 Max-Planck-Institut für Astronomie, 3 University of Warwick, 4 INAF - Osservatorio Astrofisico di Torino, 5 Université Grenoble Alpes
The helium absorption line at 10830 Å has been suggested as an excellent probe for the extended atmospheres of hot Jupiters, and has recently been detected in the transmission spectra of a handful of planets. The isotropic re-emission will lead to helium airglow that may be observable at other orbital phases. The goal of this work is to investigate the detectability of He I emission at 10830 Å in the atmospheres of exoplanets using high-resolution spectroscopy. We estimate the expected strength of He I emission in hot Jupiters based on their transmission signal. We search for the He I 10830 Å emission feature in τ Boo b in three nights of high-resolution spectra taken by CARMENES. The He I emission is not detected in τ Boo b, reaching a 5σ contrast limit of 4×10-4 for emission line widths of >20 km s-1, which is not sensitive enough to make a detection yet. 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.
Figure 1: Illustration of He I emission from an exosphere of a close-in planet.
The helium absorption line at 10830 Å originating from the metastable triplet state 23S, opens a new window for porbing the extended atmospheres of gas giants from the ground . The absorption feature has been detcted in the transmission spectra of a handful of planets (see Figure 4). As helium atoms at 23S state absorb photons to reach 23P state, the opposite transition will happen concurrently, with 10830 Å photons re-emitted in random directions, leading to He I airglow emission. The study of this emission allows for better understanding of the radiation fields and level populations under non-local thermodynamic equilibrium in the upper atmospheres of exoplanets. In addition, it provides a way of probing the extended atmospheres of non-transiting close-in gas giants, which have not been investigated before.
Based on the absorption level measured in transmission spectroscopy (Tλ), we estimate the amount of emission expected from the planetary atmospheres:
which may be reached by combining multiple nights of observations.
We search for the He I 10830 Å emission feature in τ Boo b in five nights of high-resolution spectra taken by CARMENES at the 3.5m Calar Alto telescope. The spectra in each night were corrected for telluric absorption, sky emission lines and stellar features, to obtain the residual spectra (see Figure 2), and then shifted to the planetary rest frame and combined to search for the emission.
Figure 2: The residual spectral series from night 1 to 5 with the y-axis representing time binned to 0.006 in orbital phase. The slanted dashed lines in red trace the expected planetary helium signal, shifting in time due to the change in the radial component of the orbital velocity of the planet.
 Oklopčić, A. & Hirata, C. M. 2018, ApJ, 855, L11, ADS
No statistically significant signal from the planetary He I 10830 Å line was found. We reach a 5σ contrast limit of 4×10-4 for emission line widths of >20 km s-1, which is a factor of ~8 above the expected level of emission.
Figure 3: The 5σ detection limit of the He I airglow emission as a function of the line width of the potential signal using 3 nights of observations of τ Boo b (solid black line), and corresponding equivalent widths of the detection limit (solid red curve).
Figure 4: The V magnitude of host stars plotted against the estimated airglow emission signal for exoplanets with He I absorption detections (in red dots) or upper limits (in blue triangles). The black dashed line shows the detection limit of our analysis.