1998A&A...338..262S

We present an extended map in the [CII] 158µm fine structure line of the Rosette Molecular Cloud Complex, obtained with the Far Infrared Fabry-Perot Imaging Spectrometer FIFI onboard the NASA Kuiper Airborne Observatory. Very weak but significant C⁺ emission was found deep in the cloud at a distance of around 25 pc from the central OB cluster. The observed intensity distribution is consistent with the picture of a clumpy molecular cloud, exposed to an UV flux of around 200 G_o from the central OB cluster NGC 2244 at the cloud edge. The UV radiation creates Photon Dominated Regions (PDRs) on the clump surfaces throughout the cloud which give rise to the observed weak [CII] 158µm line emission (peak intensity 5x10^–4erg/s/sr/cm²). In contrast to more luminous PDR regions, the emergent C⁺ intensity scales logarithmically with the incident UV field but is in agreement with PDR models for a weak UV illumination. The density of the [CII] emitting gas is around 10⁴cm^–3. The rather high value of 1% for the [CII] to FIR flux ratio in Rosette indicates a large grain photoelectrical heating efficiency which might be due to the low incident UV field. With an average density of 300cm^–3, obtained from CO observations, we derive a high (30-300) clump to interclump density contrast, where the UV radiation can penetrate deep into the cloud and induces emission from many PDRs along the line of sight.