Molecular gas properties of the starburst nucleus of IC 342: High-resolution (CO)-C-13 (2-1) imaging

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