While recent studies of the solar-mass protostar IRAS16293-2422 have focused on its inner arcsecond, the wealth of Herschel/HIFI data has shown that the structure of the outer envelope and of the transition region to the more diffuse ISM is not clear
ly constrained. We use rotational ground-state transitions of CH (methylidyne), as a tracer of the lower-density envelope. Assuming LTE, we perform a $chi^2$ minimization of the high spectral resolution HIFI observations of the CH transitions at ~532 and ~536 GHz in order to derive column densities in the envelope and in the foreground cloud. We obtain column densities of (7.7$pm$0.2)$times10^{13}$ cm$^{-2}$ and (1.5$pm$0.3)$times10^{13}$ cm$^{-2}$, respectively. The chemical modeling predicts column densities of (0.5-2)$times10^{13}$ cm$^{-2}$ in the envelope (depending on the cosmic-ray ionization rate), and 5$times10^{11}$ to 2.5$times10^{14}$ cm$^{-2}$ in the foreground cloud (depending on time). Both observed abundances are reproduced by the model at a satisfactory level. The constraints set by these observations on the physical conditions in the foreground cloud are however weak. Furthermore, the CH abundance in the envelope is strongly affected by the rate coefficient of the reaction H+CH$rightarrow$C+H$_2$ ; further investigation of its value at low temperature would be necessary to facilitate the comparison between the model and the observations.
