Detecting Water in the Atmosphere of HR 8799 c with L-band High-dispersion Spectroscopy Aided by Adaptive Optics

High-dispersion spectroscopy of brown dwarfs and exoplanets enables exciting science cases, e.g., mapping surface inhomogeneity and measuring spin rate. Here, we present L-band observations of HR 8799 c using Keck NIRSPEC (R = 15,000) in adaptive optics (AO) mode (NIRSPAO). We search for molecular species (H2O and CH4) in the atmosphere of HR 8799 c with a template-matching method, which involves cross-correlation between reduced spectra and a template spectrum. We detect H2O but not CH4, which suggests disequilibrium chemistry in the atmosphere of HR 8799 c, and this is consistent with previous findings. We conduct planet signal injection simulations to estimate the sensitivity of our AO-aided high-dispersion spectroscopy observations. We conclude that 10−4 contrast can be reached in the L band. The sensitivity is mainly limited by the accuracy of line list used in modeling spectra and detector noise. The latter will be alleviated by the NIRSPEC upgrade.