Detecting isotopologues in exoplanet atmospheres using ground-based high-dispersion spectroscopy

Cross-correlation is a well-tested method for exoplanet characterization. A new, potentially powerful application is the measurement of atmospheric isotope ratios. In particular D/H can give unique insights into a planet's formation and evolution. Here we aim to study the detectability of isotopologues in the high-dispersion spectra of exoplanets, to identify the optimal wavelengths ranges, and to predict the required observational efforts with current and future ground-based instruments. High-dispersion (R=10$^5$) thermal emission (and sometimes reflection) spectra were simulated by self-consistently modeling exoplanet atmospheres over a wide range of temperatures. These were synthetically observed with telescopes equivalent to the VLT or ELT, and analyzed with cross-correlation, resulting in S/N predictions for the detection of $^{13}$CO, HDO, and CH$_3$D. For the best observable exoplanets, $^{13}$CO is in range of current telescopes. It will be most favorably detected at 2.4 microns, just longward of the spectral range probed by several high-dispersion observations in the literature. CH$_3$D can best be seen at 4.7 microns, using 40m-class telescopes for planets with $T_{\rm equ}$ below 600 K. In this case, sky emission is often dominating the noise. HDO can be targeted at 3.7 microns, where sky emission is smaller. 40m-class telescopes may detect it in planets with $T_{\rm equ}$ below 900~K, potentially even 8m-class telescopes in the case of methane quenching. If Proxima Cen b is water-rich, HDO could be detected with the ELT in 1 night in reflected light. Isotopologues will soon belong to the exoplanet characterisation tools. Measuring D/H, and ratios of other isotopes, could be a prime science case for the METIS instrument on the ELT, especially for nearby rocky and ice giant planets. This can give unique insights in their history of ice enrichment and atmospheric evaporation.