Radiative transfer predictions from RADEX for an HCO⁺/CO abundance ratio typical of the dense (top) and diffuse gas (bottom). The blue and green areas show the range of observed CO(1–0) and HCO⁺(1–0) integrated intensities, while the black and magenta lines indicate the upper limits of HCN(1–0) and SiO(2–1). The columns correspond to predictions for relative electron abundances of n_e/n_H = 0 (left), n_e/n_H = 10⁻⁵ (middle), and n_e/n_H = 10⁻⁴ (right). The red areas correspond to the models that reproduce the CO and HCO⁺ intensities and the HCN upper limit. The dotted lines indicate the models that reproduce the mid-J CO lines emission from Salomé et al. (2019).