2006A&A...448.1043B

Aims. The physical conditions and chemical structure in the dark cloud of Barnard5 and its surrounding atomic halo is studied. The impact of the halo on the line emission emerging from the molecular cloud is investigated. We present observations of the [CI] ³P₁⟶³P₀ transition of neutral carbon and the low-J transitions of ¹2^CO and ¹3^CO. The CO maps extend from the core (A_v>7) to the northern cloud edge and into the halo (A_v≲1). They are complemented by deeply integrated [CI] spectra made along a 1D cut of similar extent. Escape probability and photon-dominated region (PDR) models are employed to interpret the observations. ¹2^CO and ¹3^CO are detected in the cloud and the halo, while [CI] is detected only toward the molecular cloud. This occurs even though the neutral carbon column density is >5 times larger than the CO column density in the halo, but it can be understood in terms of excitation. The [CI] excitation is governed by collisions even at the low halo densities, while the CO excitation is dominated by the absorption of line photons emitted by the nearby molecular cloud. The upper limit on the neutral carbon column density in the halo is 6x10¹⁵/cm2. The PDR studies show that even small column densities of H₂ and CO, such as those in the B5 halo, can significantly change the [CI] and CO line emission (pre-shielding). Since this effect decreases the [CI] intensity and increases the CO intensity, the largest impact is noted for the [CI]/CO line ratios. For the B5 cloud, a PDR model with a molecular hydrogen column density of ∼6x10¹⁹/cm2 in the halo matches the observed [CI]/CO line ratios best. Models with no pre-shielding, in contrast, suggest high gas densities that are in conflict with independently derived densities. The PDR models with a χ<1 demonstrate that the [CI]/CO ratios cannot be attributed solely to a reduced FUV field.