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Glueck, C. B., Stutzki, J., Roellig, M., Chambers, E. T. and Risacher, C. (2017). SOFIA/GREAT [C II] observations in nearby clouds near the lines of sight towards B0355+508 and B0212+735. Astron. Astrophys., 600. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

We present SOFIA/GREAT [C II] P-2(3/2) -> P-2(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 greater than or similar to 30 K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots (I(CO J = 1-0) similar to 5-20 Kkm 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 mu m emission within the rms noise of similar to 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 greater than or similar to 1.5 to 5 Kkm s(-1) are expected, which is a factor similar to 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 similar to 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 similar to 15 K. The KOSMA-tau 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].

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Glueck, C. B. UNSPECIFIED UNSPECIFIED UNSPECIFIED Stutzki, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Roellig, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Chambers, E. T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Risacher, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-234640
DOI:	10.1051/0004-6361/201526454
Journal or Publication Title:	Astron. Astrophys.
Volume:	600
Date:	2017
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language CO-H-2 CONVERSION FACTOR; PHOTODISSOCIATION REGIONS; ABSORPTION-LINES; DIFFUSE CLOUDS; EMISSION; BRIGHT; CARBON; (CO)-C-12; SHOCKS; MODEL Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/23464