Spectra obtained by integrating the line emission over the whole core. Top: ground-state K-components of the J = 12 → 11 transition of CH₃CN (numbers above the spectrum) and of the CH₃¹³CN isotopomer (numbers below the spectrum). Note how most K-lines are blended with each other and/or with lines of other molecules. Middle: vibrationally excited (v₈ = 1) lines of CH₃CN. The K, l quantum numbers of the transitions are indicated above the spectrum. Bottom: ground state K = 3 component. The dotted lines mark the velocity ranges used to produce the maps in Figs. 2 and 3.