SIMBAD references

We present SOFIA/GREAT [CII] ²P_3/2-²P_1/2 (1.9005369 THz) observations of nearby clouds near the lines of sight towards the quasars B0355+508 and B0212+735. These clouds have previously been identified as warm non-LTE diffuse clouds with a temperature of T≥30K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots (I(CO J=1-0)∼5-20K.km/s), both spectrally and spatially. This small-scale structure has been interpreted as small-scale variations in the chemistry, not as density and column-density structure. We did not detect [CII] 158µm emission within the rms noise of ∼0.1-0.3K. Our non-detection in [CII] contradicts the above scenario. In diffuse clouds, the efficiency of photo-electric heating is highest. Under the assumption that [CII] is the dominant coolant in diffuse clouds, we can calculate the predicted [CII] emission arising from these clouds. Based on the derived hydrogen column densities for the diffuse clouds, a line width in [CII] of similar order to that of CO, and when at least the minimum amount of heating in the clouds is due to cosmic-ray heating, [CII] line intensities ≥1.5 to 5K.km/s are expected, which is a factor ∼3 to 15 above the upper limits of the observations. The upper limits of the [CII] and the observed CO line intensities are, however, consistent with the intensities predicted for photon-dominated region (PDR) surfaces on regular cold (T∼15K) low-density cloud fragments. Lower temperatures lead to less excited [CII]. The assumption of lower densities, which would equally lower the [CII] excitation, contradicts the observed cloud sizes and column densities. The CO(2-1)/CO(1-0) line ratios observed in these clouds are consistent with cloud temperatures of T∼15K. The KOSMA-τ PDR-model of a cold moderate-density clump or an ensemble of such clumps with a canonical mass-size relation and mass spectrum, consistent with the total column densities derived for low density, shows that cold PDRs can reproduce the observed CO intensities, the observed CO(1-0) and CO(2-1) ratio, and the observed upper limits for [CII].