Probing the Atmospheres of Young Long-Period Sub-Neptune Progenitors with ELT/ANDES

Abstract

High-resolution cross-correlation spectroscopy (HRCCS) has become a powerful ground-based technique for detecting and characterizing exoplanet atmospheres. While highly successful for ultra-hot and hot Jupiters, next-generation facilities such as ELT/ANDES will observe smaller and longer-period planets, including young sub-Neptunes and their progenitors. We investigate whether HRCCS with ELT/ANDES can robustly recover orbital parameters and atmospheric signals for the long-period sub-Neptunes V1298 Tau b and TOI-451 c. In long-period systems, the slow Doppler drift during a single night limits separation between planetary and telluric signals, increasing the risk of signal loss during detrending. We therefore quantify the impact of including out-of-transit exposures on signal recovery and parameter estimation. We simulate YJH-band transmission observations using the \texttt{Ratri} pipeline and analyze them with the HRCCS detrending and cross-correlation framework \texttt{Upamana}. For V1298 Tau b, injected atmospheric models are consistent with HST, Spitzer, and JWST constraints. For TOI-451 c, we explore sub-solar to super-solar C/O ratios to test compositional sensitivity. Incorporating out-of-transit exposures significantly improves detectability, provided detrending effects are consistently propagated to the template spectra prior to cross-correlation. Without this step, orbital parameters can deviate from injected values and detection significance decreases. For V1298 Tau b, $>4\sigma$ detections of H$_2$O, H$_2$S, and CO are achievable at $\lesssim$10 hours (minimum 2 nights, cloud-free scenario). For TOI-451 c, distinguishing sub-solar and solar from super-solar C/O requires $\sim$17 hours (minimum 4 nights). HRCCS with ELT/ANDES will therefore be a key tool for atmospheric characterization of young, long-period sub-Neptunes in the ELT era.