2006A&A...456..235M

The aim of the paper is to understand the emission from the photon dominated regions in Cepheus B, estimate the column densities of neutral carbon in bulk of the gas in Cepheus B and to derive constraints on the factors which determine the abundance of neutral carbon relative to CO. This paper presents 15'x15' fully sampled maps of [CI] at 492 GHz and ¹²CO 4-3 observed with KOSMA at 1' resolution. The new observations have been combined with the FCRAO ¹²CO 1-0, IRAM-30m ¹³CO 2-1 and C¹⁸O 1-0 data, and far-infrared continuum data from HIRES/IRAS. The KOSMA-τ spherical PDR model has been used to understand the [CI] and CO emission from the PDRs in Cepheus B and to explain the observed variation of the relative abundances of both C⁰ and CO. The emission from the PDR associated with Cepheus B is primarily at V_LSR between -14 and -11km/s. We estimate about 23% of the observed [CII] emission from the molecular hotspot is due to the ionized gas in the HII region. Over bulk of the material the C⁰ column density does not change significantly, (2.0±1.4)x10¹⁷cm^–2, although the CO column density changes by an order of magnitude. The observed C/CO abundance ratio varies between 0.06 and 4 in Cepheus B. We find an anti-correlation of the observed C/CO abundance ratio with the observed hydrogen column density, which holds even when all previous observations providing C/CO ratios are included. Here we show that this observed variation of C/CO abundance with total column density can be explained only by clumpy PDRs consisting of an ensemble of clumps. At high H₂ column densities high mass clumps, which exhibit low C/CO abundance, dominate, while at low column densities, low mass clumps with high C/CO abundance dominate.