2025MNRAS.542..927S

Hydrodynamic outflows, such as those observed escaping close-in gas giant planets, are not isothermal in structure. Their highly ionized nature allows them to cool adiabatically at distances beyond several planetary radii. The contrast between the hottest gas temperatures at around 10 000 K and the coldest at around 1000 K triggers an excess population of the observable helium triplet. This excess is caused by the suppression of collisional de-excitation from the triplet state at cool temperatures. Using radiation-hydrodynamic simulations, we show that this helium triplet excess may explain the excess broadening seen in HD 189733b's observed transmission spectrum, demonstrating adiabatic cooling of its outflow, confirming its hydrodynamic nature on scales of several planetary radii. However, further observations are required to confirm this conclusion. Furthermore, we explore a range of electron transitions for neutral helium, which were not considered in the previous literature. We find that the He$2^1$S state is unavailable as a potential reservoir for He$2^3$S electrons. Additionally, the de-excitation to the ground state must be considered for stellar spectra later than K2 in predicting the correct helium triplet population. Importantly, since triplet helium inherits momentum from ionized helium as it is generated by recombination, it is significantly less prone to fractionation than ground-state neutral helium. However, at separations of ${\gtrsim} 0.05$ au, ionization at the flow base and drag on helium weaken, leading to significant fractionation of the then mostly neutral helium. This, in turn, can cause a suppression of the helium transit depth, even though the helium line width remains large.