Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5

Abstract

Aims. The physical conditions and chemical structure in the dark cloud of Barnard 5 and its surrounding atomic halo is studied. The impact of the halo on the line emission emerging from the molecular cloud is investigated. Methods. We present observations of the [CI] 3 P1 → 3 P0 transition of neutral carbon and the low-J transitions of 12 CO and 13 CO. The CO maps extend from the core (Av > 7) to the northern cloud edge and into the halo (Av 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 6 × 10 15 cm −2 .