1996A&A...311..282S

Observations of the [C I]³P₁-³P₀492GHz fine structure line and isotopic CO(2-1, 3-2) rotational lines have been made at six positions along an east-west strip through the centre of the high-latitude dark cloud L 183 using the James Clerk Maxwell Telescope. The [C I] spectra have line strengths varying between T_MB=0.8-3.4K. The lines appear broad due to a blend of two kinematically different components around V≃1km/s and V≃2.3km/s which can be resolved at some positions. If the [C I] emission originates in the warmer, translucent envelopes (A_V≃1-5mag) of L 183 we derive total column densities in both components in the range 7x10¹⁵-3x10¹⁷cm^–2. In case it would come primarily from the cold dark core (A_V>5mag) we calculate lower limits of about 10¹⁶-10¹⁷cm^–2. The component around V≃1km/s is most likely entirely translucent. We argue that most of the observed [C I] emission from the V≃2.3km/s component also originates in the translucent regions of L 183 and has an optical depth in the range τ=0.1-6. We find a good correlation between the [C I] and 3CO(3-2, 2-1) measurements but not with C 18O(2-1). The derived C/CO column density ratio along the cut varies between 0.08-0.7 and remains constant within the uncertainty. Chemical model calculations are used to study the [C I] column density as well as the C/CO ratio as a function of density and total hydrogen column density. A close inverse correlation is found between N(C)/N(CO) and A_V for N(H₂)∼10²⁰-5x10²¹cm^–2, which is consistent with the generally observed correlation between C and CO from diffuse clouds to dense photon dominated regions.