Plots of integrated intensity vs. J_up (from top to bottom) for the C¹⁸O (red stars) and C¹⁷O (red squares), HCO⁺ (stars) and H¹³CO⁺ (squares), and N₂H⁺ (stars) spectral line emission, obtained from the LVG model. The blue and pink lines represent, respectively, the theoretical model for the outer and inner envelope components, and the green lines represent the sum of the two components. The solid lines represent the model results for C¹⁸O, HCO⁺, and N₂H⁺, while the dashed lines represent the model results for C¹⁷O and H¹³CO⁺.