Toward a unified framework for helium observations and interpretation of atmospheric escape

Abstract

Atmospheric escape is considered a key process in shaping exoplanet demographics and evolution. The near-infrared metastable helium triplet (HeI at ~10833{\AA}) is one of the most powerful tracers of hydrodynamical outflows. In recent years, a large variety of instruments have been used to detect and characterize expanding upper atmospheres. High-resolution (HR) spectrographs, which can spectrally resolve the lines of the triplet, have been mostly used to probe the dynamics of atmospheric escape. More recently, JWST (low-resolution, LR) detected extended outflows that were missed by previous ground-based observations due to night-length constraints. Since the observed transmission spectrum is computed from the ratio of stellar spectra degraded by instrumental convolution, we demonstrate that directly convolving a theoretical transmission spectrum with the instrument's profile leads to inference biases as resolution decreases. This conclusion is particularly important for atmospheric escape studies but also holds in the more general context of LR atmospheric retrievals. Following the proper comparison methodology, we then investigated the complementarity of HR and LR observations for atmospheric escape measurements, comparing the capability of three instruments (NIRPS, JWST/NIRSpec, and JWST/NIRISS) to detect and characterize outflows. We find that NIRPS and JWST/NIRISS are mostly sensitive to the same helium signatures, while JWST/NIRSpec improves the detection limit of excess absorption for faint targets. Studying different outflow configurations, we show that LR space-based measurements cannot be used to infer the dynamics of the upper atmosphere close to the planet, and are best employed to measure the spatial extent of outflowing tails. We thus highlight the complementarity between HR and LR observations of atmospheric escape, the latter improving baseline reconstruction.