tokens: The authors would like to thank S. Cabrit for stimulating discussions, G. Sandell for help with scheduling the observations, and the referee for valuable comments that helped improving the clarity of the paper. Submillimeter astronomy in Copenhagen is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 646908) through ERC Consolidator Grant “S4F”. Research at the Centre for Star and Planet Formation is funded by the Danish National Research Foundation. J.C.M. acknowledges support from the European Research Council under the European Community’s Horizon 2020 framework program (20142020) via the ERC Consolidator grant “From Cloud to Star Formation (CSF)” (project number 648505). A.K. acknowledges support from the Polish National Science Center grants 2013/11/N/ST9/00400 and 2016/21/D/ST9/01098. Based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart.
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