Vibrationally excited H2 mutes the He i triplet line at 1.08 {\mu}m on warm exo-Neptunes

Abstract

Context. Neptune-sized exoplanets or exo-Neptunes are fundamental in the description of exoplanet diversity. Their evolution is sculpted by atmospheric escape, often traced by absorption in the H i Lyman-{\alpha} line at 1,216 {\AA} and the He i triplet line at 1.08 {\mu}m. On warm exo-Neptunes HAT-P-11 b, GJ 3470 b and GJ 436 b, H i Lyman-{\alpha} absorption causes extreme in-transit obscuration of their host stars. This suggests that He i triplet line absorption will also be strong, yet it has only been identified on two of these planets. Aims. We explore previously unaccounted for processes that might attenuate the He i triplet line on warm exo-Neptunes. In particular, we assess the role of vibrationally excited H2 to remove the He+ ion that acts as precursor of the absorbing He(2$^3$S ). Methods. We formed thermal rate coefficients for this chemical process, leveraging the available theoretical and experimental data. The process becomes notably fast at the temperatures expected in the atmospheric layers probed by the He i triplet line. Results. Our simulations show that the proposed process severely mutes the line on GJ 3470 b and causes the nondetection on GJ 436 b. The overall efficiency of this mechanism is connected to where in the atmosphere the H2-to-H transition occurs and, ultimately, to the amount of high-energy radiation received by the planet. The process will be more significant on small exoplanets than on hotter or more massive ones, as for the latter the H2-to-H transition generally occurs deeper in the atmosphere. Conclusions. Weak He i triplet line absorption need not imply the lack of a primordial, H2-He-dominated atmosphere, an idea to bear in mind when interpreting the observations of other small exoplanets.