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tokens: We thank Matthias Gritschneder and the whole CAST group for helpful comments and discussions. We also thank the anonymous referee for improving the structure of the paper. SH wants to thank Alexander Beck for useful comments that clarified the presentation of the paper. AB and SH are supported by the priority programme 1573 ‘ Physics of the Interstellar Medium ’ of the German Science Foundation and the Cluster of Excellence ‘ Origin and Structure of the Universe ’.