SOFIA/GREAT [C ii] observations in nearby clouds near the lines of sight towards B0355+508 and B0212+735 ^⋆

¹ I. Physikalisches Institut Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: glueck@ph1.uni-koeln.de
² SOFIA-USRA, NASA Ames Research Center, Mail Stop 232-12, Moffett Field, CA 94035-0001, USA
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 3 May 2015
Accepted: 23 May 2016

Abstract

We present SOFIA/GREAT [C ii] 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 ≳ 30 K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots (I(CO J = 1–0)~ 5–20 K km s^-1), 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 [C ii] 158 μm emission within the rms noise of ~0.1–0.3 K. Our non-detection in [C ii] contradicts the above scenario. In diffuse clouds, the efficiency of photo-electric heating is highest. Under the assumption that [C ii] is the dominant coolant in diffuse clouds, we can calculate the predicted [C ii] emission arising from these clouds. Based on the derived hydrogen column densities for the diffuse clouds, a line width in [C ii] 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, [C ii] line intensities ≳1.5 to 5 K km s^-1 are expected, which is a factor ~3 to 15 above the upper limits of the observations. The upper limits of the [C ii] and the observed CO line intensities are, however, consistent with the intensities predicted for photon-dominated region (PDR) surfaces on regular cold (T ~ 15 K) low-density cloud fragments. Lower temperatures lead to less excited [C ii]. The assumption of lower densities, which would equally lower the [C ii] 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 ~ 15 K. 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 [C ii].