Physical Understanding of Helium Absorption: Effects of Metal Cooling on Atmospheric Escape

Atmospheric escape driven by extreme ultraviolet (EUV) radiation plays an important role in the long-term evolution of close-in exoplanets. Intense stellar irradiation can heat the upper atmospheres and drive substantial hydrodynamic mass loss of close-in exoplanets.
Recent spectroscopic observations have revealed the presence of helium triplet absorption at 10830 Å  in more than 20 close-in exoplanets. This growing body of detections underscores the importance of accurately modeling the upper atmosphere, especially in terms of its composition, thermal structure, and the processes governing mass loss. In particular, the presence of metal species—those heavier than hydrogen and helium—has been confirmed in several exoplanetary atmospheres, with some exhibiting super-solar metallicities. However, the influence of these metal species on atmospheric escape dynamics and on spectroscopic observables, such as helium absorption, remains insufficiently understood.

In this study,  we derive a semi-analytic formula for estimating the equivalent width of helium triplet absorption, assuming an isothermal temperature profile. This formula provides a useful tool for interpreting observational data without the need for full hydrodynamic modeling. We also develop one-dimensional radiation-hydrodynamics simulations that self-consistently incorporate the effects of metal line cooling on the upper atmosphere. By accounting for the radiative cooling contributed by atomic and ionic metal species, our model captures the resulting changes in temperature, density, and velocity profiles throughout the outflowing atmosphere. 

Our simulation results demonstrate that increasing atmospheric metallicity significantly enhances radiative cooling efficiency in the upper atmosphere, leading to lower equilibrium temperatures and, consequently, reduced thermal mass-loss rates. Interestingly, despite this suppression of escape rates, we find that the equivalent width of the helium triplet absorption line remains largely insensitive to changes in metallicity. This behavior arises because lower temperatures promote a higher population of helium atoms in the metastable triplet state, compensating for the reduced column density due to weaker outflows.

Overall, our work offers a comprehensive framework for understanding the coupled thermal, chemical, and dynamical processes that govern atmospheric escape in metal-enriched exoplanetary atmospheres. By understanding how metal species affect both the physical structure and the observational signatures of escaping atmospheres, our findings provide critical insights for interpreting existing observations and guiding future observational campaigns targeting helium absorption in exoplanets.