A formally motivated retrieval framework applied to the high resolution transmission spectrum of HD 189733 b

Abstract

Ground-based high-resolution spectra provide a powerful tool for characterising exoplanet atmospheres. However, they are greatly hampered by the dominating telluric and stellar lines, which need to be removed prior to any analysis. Such removal techniques ("preparing pipelines") deform the spectrum, hence a key point is to account for this process in the forward models used in retrievals. We develop a formal derivation on how to prepare froward models for retrievals, in the case where the telluric and instrumental deformations can be represented as a matrix multiplied element-wise with the data. We also introduce the notion of "Bias Pipeline Metric" (BPM), that can be used to compare the bias potential of preparing pipelines. We use the resulting framework to retrieve simulated observations of 1-D and 3-D exoplanet atmospheres and to re-analyse high-resolution ($\mathcal{R} \approx 80\,400$) near infrared (0.96--1.71 $μ$m) CARMENES transit data of HD~189733~b. We compare these results with those obtained from a CCF analysis. With our fiducial retrieval, we find a blueshift of the absorption features of $-5.51^{+0.66}_{-0.53}$ km$\cdot$s$^{-1}$. In addition, we retrieve a H$_2$O $\log_{10}$(VMR) of $-2.39^{+0.12}_{-0.16}$ and a temperature of $660^{+6}_{-11}$ K. We are also able to put upper limits for the abundances of CH$_4$, CO, H$_2$S, HCN and NH$_3$, consistent with a sub-solar metallicity atmosphere enriched in H$_2$O. We retrieve a broadened line shape, consistent with rotation- and wind-induced line broadening. Finally, we find a lower limit for the pressure of an opaque cloud consistent with a clear atmosphere, and find no evidence for hazes.