Vibrationally excited H2 mutes the He i triplet line at 1.08 μm on warm exo-Neptunes

TL;DR

This work addresses why the HeI $1.08~\mu$m triplet, a key tracer of atmospheric escape, is often weak or undetected on warm exo-Neptunes. By incorporating vibrationally excited H2 chemistry into a 1D hydrodynamic escape framework, the authors derive LTE rate coefficients for the reaction $R_2: He^+ + H2(v) -> He + H^+ + H$ using $v$-resolved data and three scenarios (theory-based and two experimental bounds). They find that vibrational excitation can dramatically reduce He^+$ and He(2^3S) abundances in the probed layers, leading to a severely muted or nondetected HeI line on planets like GJ 3470 b and GJ 436 b, with a more modest effect on HAT-P-11 b. The results imply that weak HeI absorption does not necessarily indicate a non-primordial, H2–He-dominated atmosphere for small exoplanets and highlight the critical need for accurate $k_{R_2,LTE}$ data and coordinated XUV/FUV observations to interpret HeI signals and constrain atmospheric composition and stellar activity.

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-α line at 1,216 Å and the He i triplet line at 1.08 μm. On warm exo-Neptunes HAT-P-11 b, GJ 3470 b and GJ 436 b, H i Lyman-α 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.

Figures (7)

• Figure 1: Profiles of temperature and neutral densities (top), ion densities (middle) and the He(2$^3S$) metastable state density (bottom) for the three warm exo-Neptunes. Solid, dotted, dashed and dotted-dashed curves refer to calculations based on $k_{{{\rm{R_2}},v=0}}$, $k_{\rm{R_2, LTE}}^{\rm{the}}$, $k_{\rm{R_2, LTE}}^{\rm{exp,min}}$ and $k_{\rm{R_2, LTE}}^{\rm{exp,max}}$, respectively. The gray boxes in the bottom panel bracket the range of opaque radii calculated here (Table \ref{['req_table']}).
• Figure 2: Same as Fig. \ref{['bigpanel_fig']}, for the virtual sub-Neptune motivated by GJ 436 b described in the text. The He i triplet line is one of the few atmospheric features detectable on sub-Neptunes with current technology ahreretal2025. The limited set of simulations presented in this figure confirm that H$_2$ is likely to survive to far distances on them, which has a significant effect on the He i triplet line strength.
• Figure 3: Excess absorption predicted for a GJ 436 b-like exoplanet under a range of XUV fluxes. On its current orbit, GJ 436 b receives a XUV flux of about 278 erg cm$^{-2}$s$^{-1}$ (Table \ref{['SED_table']}). Simulations based on the various rate coefficients for reaction R$_2$. For small XUV fluxes, the EA varies by up to a factor of 2 depending on the rate coefficient for reaction R$_2$ that is adopted. For strong XUV fluxes, the choice of reaction rate coefficient is, in relative terms, weaker. This figure complements the EA quoted in Table \ref{['req_table']} for HAT-P-11 b, GJ 3470 b and GJ 436 b.
• Figure 4: Thermal (Local Thermodynamic Equilibrium, LTE) rate coefficients for reaction R$_2$ implemented in our simulations (see text for details). Also, relative abundances $f_v$ for H$_2$($v$$\leq$2) are shown as the dotted lines. The experiment-based rate coefficients are consistent with the measurements at temperatures between 400 and 700 K johnsenetal1980. For reference, the low-temperature measurements schaueretal1989 recommended for astrochemical applications mcelroyetal2013 are $\sim$3$\times$10$^{-14}$ cm$^3$s$^{-1}$.
• Figure 5: LTE rate coefficients for reaction R$_2$. Top: theory-based value. Middle and Bottom: Experiment-based values; johnsenetal1980 measurements are shown for reference.