High-resolution transmission spectroscopy of KELT-18b. Metastable helium detection at 1083 nm

Abstract

Exoplanets orbiting near their host stars experience intense irradiation, which can drive the expansion and escape of their upper atmospheres. The near-infrared metastable helium triplet at 1083.3 nm has emerged as a powerful diagnostic to probe these mass-loss processes. We present the first high-resolution transmission spectroscopy of the ultrahot Jupiter KELT-18b, probing its escaping atmosphere by analyzing a single transit obtained with the CARMENES spectrograph. Transmission spectroscopy and Doppler tomography were utilized to isolate the planetary signal. To mitigate contamination from the strong telluric OH lines overlapping the region of interest, two distinct data analysis procedures were applied: a standard Molecfit and OH-masking approach, as well as a principal component analysis (PCA)-based method. By combining the two complementary approaches, we successfully detect a He,I triplet absorption feature of 1.10 ± 0.22% (5.1σ). The signal exhibits a velocity shift consistent with zero and is attributed to the exoplanetary atmosphere. An injection test reveals that the PCA-based method significantly suppresses the retrieved signal. Additionally, using the ATES and TPM codes, we modeled the atmospheric photoevaporation and the helium triplet profile for different He/H ratios and stellar extreme ultraviolet (XUV) fluxes, accounting for instrument and planetary rotational broadening. From our simulations we derive a mass-loss rate and an evaporation efficiency for KELT-18b of 10.47 łeq 10 ( ) łeq 11.68 and 0.153 łeq η łeq 0.321, showing that the atmosphere of KELT-18b is undergoing photoevaporation. Furthermore, our simulations show good agreement with the observed transmission spectrum, allowing us to exclude very low He/H ratios at a 3σ confidence level for XUV fluxes up to approximately twice the nominal reconstructed value. M Our result expands the known parameter space of exoplanets with confirmed helium absorption, establishing KELT-18b as the most massive planet orbiting the hottest host star with such a detection. Furthermore, when employing PCA, we emphasize the need to perform injection tests to investigate whether any planetary signal is removed, not only near quadrature but also during transits.