thermodynamic equilibrium (LTE) approximation and Gaussian profiles, the opacity of the $i$-th HFS components is computed as $\tau_i = \tau\,R_i$, where $R_i$ is the corresponding normalised relative intensity. The $i$-th component opacity at each velocity channel is then given by: $$\label{eq:taui}
        \tau_i(\varv) = \tau_i\,\exp\left[-4\ln 2 \left(\frac{\varv - \varv_{0,i}}{\Delta\varv}\right)^2\right] \,,$$ where $\varv_{0,i}$ is the corresponding velocity position expressed as an offset with respect to the systemic velocity $\varv_0$. Finally, the continuum-subtracted antenna temperature at a given velocity channel $\varv$ is modeled as $$\label{eq:RT}
        T_\text{ant}(\varv) = \left[ J_\nu \left(T_\text{ex}\right) - J_\nu \left(T_\text{CMB}\right) - T_{\ensuremath{\mathrm{C}}} \right]
        \left(1 - {\ensuremath{\mathrm{e}}}^{-\sum_i\tau_i(\varv)}\right) \,,$$ where $J_\nu(T)$ is the Rayleigh–Jeans radiation temperature, $T_{\ensuremath{\mathrm{C}}}$ is the temperature of the continuum and $T_{\ensuremath{\mathrm{CMB}}}$ is the cosmic background temperature. These data are compared to the continuum-subtracted observed antenna temperatures, $T_{\ensuremath{\mathrm{A}}} - T_{\ensuremath{\mathrm{C}}}$, in a non-linear least-squares fashion to determine the best fit values of $N$, $T_{\ensuremath{\mathrm{ex}}}$, $\Delta\varv$, and $\varv_0$. Whenever possible, all four parameters have been adjusted during the analysis. This sometimes produced strong correlation between them, specially when fitting weak features (e.g., those of ), for which the relative intensities of the observed HFS components are poorly constrained because of the low signal-to-noise ratio (S/N). In these cases, suitable assumption 