Top 11 panels: Selected data portions (black histograms) containing HC₉N rotational lines in the ground state, ν₁₉ = 1, and seven ν₁₉ overtones with their fit contribution highlighted in different colors over the full spectral fit (dotted magenta line shifted by –2 mK). Bottom panel: Rotational diagrams resulting from the HC₉N line areas corrected for beam coupling and dilution over a 30″ source (W = ∫T_MBdv). Here, μ is the HC₉N electric dipole moment, and S is the quantum line strength of each transition. Filled circles are used for lines with a fit intensity, open circles are for lines with a fixed intensity (due to strong overlap or low S/N), and crosses correspond to the subset of lines actually shown in the top panels. The linear regressions use only the filled circles and provide rotational temperatures and column densities listed in Table 1.