Submillimeter imaging spectroscopy of the Horsehead nebula

We present similar to 15 arcsecond resolution single-dish imaging of the Horsehead nebula in the CI(1-0) and CO(4-3) lines, carried out using the CHAMP array at the Caltech Submillimeter Observatory (CSO). The data are used together with supporting observations of the (2-1) transitions of the CO isotopologues to determine the physical conditions in the atomic and molecular gas via Photon Dominated Region (PDR) modeling. The CO (4-3)/(2-1) line ratio, which is an excellent tracer of the direction of the incoming UV photons, increases at the western and northern edges of the nebula, confirming that the illumination is provided mostly by the stars sigma and zeta Orionis. The observed line intensities are consistent with PDR models with an H nuclei volume density of similar to 3-7 x 10(4) cm(-3). The models predict a kinetic temperature of similar to 12 K and a (CO)-O-18 fractional abundance with respect to H atoms of 2.4 x 10(-7) in the shielded region, which in turn imply a total molecular mass of similar to 24 M-circle dot in the (CO)-O-18 filament. The outer halo, devoid of (CO)-O-18, but traced by the CI emission has a comparable density and contributes additional similar to 13 M-circle dot of material, resulting in an upper limit of similar to 37 M circle dot for the total molecular mass of the nebula.