2000ApJ...531..200M

We present a map of the J=2-1 transition of ¹³CO in the starburst nucleus of IC 342 made with the Owens Valley Millimeter Array. The interferometric ¹³CO map allows us to directly compare with ¹²CO (1-0), ¹²CO (2-1), and ¹³CO (1-0) maps of nearly identical (∼4".5) resolution. While all four transitions show similar basic morphology, there are spatial differences between the ¹²CO and ¹³CO transitions that show up in ratio maps. In particular, the ¹³CO (2-1)/¹³CO (1-0) ratio has a markedly different distribution across the nuclear region than does ¹²CO (2-1)/¹²CO (1-0), indicating that ¹²CO and ¹³CO trace different components of the molecular gas. These differences are explained if ¹²CO traces the warm, photodissociation region ``skins'' of the clouds and ¹³CO traces the cooler, interior portions constituting the bulk of the molecular gas. We derive excitation temperatures for the bulk of the cloud mass of ∼10-20 K and densities of ∼10^3.7 cm^–2 along the molecular minispiral.