Integrated ¹²CO and ¹³CO line intensities in the nucleus of NGC 253 with our adopted model of the excitation. The data are compiled from the literature and smoothed or adjusted to correspond to a 15^{﹩\arcsec﹩} beam (see Table 1). The solid lines are the total; the thin dotted lines in the bottom left show a diffuse component that is energetically negligible except for a contribution to the ﹩J=1\rightarrow 0﹩ lines.