Detecting Water In the atmosphere of HR 8799 c with L-band High Dispersion Spectroscopy Aided By Adaptive Optics

TL;DR

This study uses high dispersion L-band spectroscopy with adaptive optics to detect water vapor in the atmosphere of exoplanet HR 8799 c, revealing disequilibrium chemistry and demonstrating the method's sensitivity limits.

Contribution

First application of AO-assisted high dispersion spectroscopy in L-band to characterize exoplanet atmospheres, detecting water vapor and assessing sensitivity constraints.

Findings

Detected H₂O in HR 8799 c's atmosphere.

No detection of CH₄ suggests disequilibrium chemistry.

Achieved 10⁻⁴ contrast sensitivity in L-band.

Abstract

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 (H$_2$O and CH$_4$) in the atmosphere of HR 8799 c with a template matching method, which involves cross correlation between reduced spectrum and a template spectrum. We detect H$_2$O but not CH$_4$, 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 $L$ 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.