  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 clearly 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.
