Clumpy photon-dominated regions in Carina - I. [C I] and mid-J CO lines in two 4'x4' fields

Context. The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. Aims. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. Methods. The NANTEN2-4 m submillimeter telescope was used to map the [C I] P-3(1) - P-3(0), P-3(2) - P-3(1) and CO 4 - 3, 7 - 6 lines in two 4' x 4' regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr 14, and the second region is in the molecular cloud south of eta Car and Tr 16. These data were combined with (CO)-C-13 SEST spectra, HIRES/IRAS 60 mu m and 100 mu m maps of the FIR continuum, and maps of 8 mu m IRAC/Spitzer and MSX emission. Results. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 10(3) in Draine units while the field of the southern region is about a factor 10 weaker. While the IRAC 8 mu m emission lights up at the edges of the molecular clouds, CO and also [C I] appear to trace the H-2 gas column density. The northern region shows a complex velocity and spatial structure, while the southern region shows an edge-on PDR with a single Gaussian velocity component. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38 '' beam (0.43 pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2 x 10(5) cm(-3) is needed to reproduce the observed line emission at two selected interface positions. Conclusions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.