Detection of water absorption in the day side atmosphere of HD 189733 b using ground-based high-resolution spectroscopy at 3.2 μm

Abstract

We report a 4.8{$σ$} detection of water absorption features in the day side spectrum of the hot Jupiter HD 189733 b. We used high-resolution (R {\tilde} 100 000) spectra taken at 3.2 {$μ$}m with CRIRES on the VLT to trace the radial-velocity shift of the water features in the planet's day side atmosphere during 5 h of its 2.2 d orbit as it approached secondary eclipse. Despite considerable telluric contamination in this wavelength regime, we detect the signal within our uncertainties at the expected combination of systemic velocity (V\_sys=-3\^{}$\{$+5$\}$\_$\{$-6$\}$ km s$^{-1}$) and planet orbital velocity (K\_p=154\^{}$\{$+14$\}$\_$\{$-10$\}$ km s$^{-1}$), and determine a H$_{2}$O line contrast ratio of (1.3 {\plusmn} 0.2) {\times} 10$^{-3}$ with respect to the stellar continuum. We find no evidence of significant absorption or emission from other carbon-bearing molecules, such as methane, although we do note a marginal increase in the significance of our detection to 5.1{$σ$} with the inclusion of carbon dioxide in our template spectrum. This result demonstrates that ground-based, high-resolution spectroscopy is suited to finding not just simple molecules like CO, but also to more complex molecules like H$_{2}$O even in highly telluric contaminated regions of the Earth's transmission spectrum. It is a powerful tool that can be used for conducting an immediate census of the carbon- and oxygen-bearing molecules in the atmospheres of giant planets, and will potentially allow the formation and migration history of these planets to be constrained by the measurement of their atmospheric C/O ratios.