$T_{\ensuremath{\mathrm{ex}}}$ and $\Delta\varv$, as these parameters could not be set free in the least-squares fits. Given the similarity of the beam size ($\sim 27$[$^{\prime\!\prime}$]{}), for the $p$- $N_{Ka,\,Kc} = 2_{1,2} \leftarrow 1_{0,1}$ lines we have assumed the same excitation temperature determined for . A smaller value is expected to hold for the $o$- $N_{Ka,\,Kc} = 1_{1,1} \leftarrow 0_{0,0}$ line, as the *Herschel* beam HPBW at this frequency is as broad as 40[$^{\prime\!\prime}$]{}, thus sampling a comparatively large volume of cold gas. For this transition we have fixed $T_{\ensuremath{\mathrm{ex}}} = 4.5$K, a value determined for in the same source [@bacmann2010first] through the analysis of its $N=1-0$ fine-structure doublet centered at 534GHz. In fact, this value allows for a satisfactory fit of the absorption $o$- feature, whereas for higher excitation temperatures (e.g., already at 5.5K) the LTE model predicts the line to be observable in emission. Constraints for the FWHM line width $\Delta\varv$ have been derived from the values determined for after deconvolution from the WBS spectrometer resolution (1.1MHz). The column densities derived for $p$- using two components are consistent within $2\sigma$ and the weighted average value is $N = (0.241\pm 0.014)\times 10^{13}$cm$^{-2}$. The observed *ortho*-to-*para* ratio is $2.7\pm 0.4$. By considering 