tokens: ACKNOWLEDGEMENTS We thank the IRAM 30-m staff for precious help during the observations. We thank the anonymous referee for his/her constructive comments. VMR and MP acknowledge the financial support received from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 664931. LC acknowledges support from the Italian Ministero dell’Istruzione, Università e Ricerca through the grant Progetti Premiali 2012 - iALMA (CUP C52I13000140001). PC acknowledges support from the European Research Council (ERC project PALs 320620).
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