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Astron. Astrophys. 330, 311-320 (1998)

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The SO-to-CS abundance ratio in molecular cirrus clouds

Andreas Heithausen 1, Uwe Corneliussen 2 and Volkmar Großmann 3

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2 I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, D-50937 Köln, Germany
3 Institut für Astronomie und Astrophysik - Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

Received 15 July 1997 / Accepted 15 September 1997


We have extended our study of sulphur monoxide towards other less opaque cirrus cores and have clearly detected 3 out of 6 sources observed, for the Draco cloud, LVC 127+20, and MBM 32. We find for at least one further cloud (MBM 32) that SO is extended. Assuming that the lines are optically thin we derive column densities of about N (SO)= 5 1012 cm-2. Based on the visual extinction estimates we derive abundances X (SO)=2 10-9, thus again higher than can be predicted by chemical models with standard assumptions on a low initial S abundance in the gas phase. Our new observations therefore confirm our previous conclusion, that in cirrus clouds sulphur must be essentially undepleted to explain the amount of SO detected.

For MCLD 126.6+24.5 we present an analysis of our extensive SO ([FORMULA]) mapping. The SO emission reveals clumpy structure of that cloud. We decompose the SO cloud into 17 individual clumps. Those clumps follow the same size-line width relation as high latitude molecular clouds (HLCs) on larger scales. Also, the size-density relation for the SO clouds has the same slope as that for other HLCs found from CO observations.

The clouds detected in SO were also observed in the CS ([FORMULA]) and ([FORMULA]) lines. Only MCLD 126.6+24.5 and MBM 32 were detected in CS. For the most intense SO clump in MCLD 126.6+24.5 we present a detailed study of SO and CS. The abundance ratio of CS and SO ranges from unity for MCLD 126.6+24.5 to less than 0.1 for the other clouds, indicating a low depletion of oxygen in that clouds. Our detailed comparison of CS and SO in MCLD 126.6+24.5 both in position and in velocity indicate that the molecules do not necessarily coexist in the cloud; we find that both molecules are not equally distributed, that the profiles of both molecules look different and that there is no correlation between the column densities of the molecules. Our findings let us suggest that these clouds are inhomogeneous clumpy objects, possibly not well described by simple equilibrium models.

Key words: ISM: abundances – ISM: clouds – ISM: individual objects: MBM 32, MCLD 126.6+24.5 – ISM: molecules – radio lines: ISM

Send offprint requests to: A. Heithausen, Internet: Heith@Astro.Uni-Bonn.De

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998