Abstract
Context. Cometary ices are formed during star and planet formation, and their molecular and elemental makeup can be related to the early solar system via the study of inter- and protostellar material. Aims. We set out to place the first observational constraints on the interstellar gas-phase abundance of bromine (Br). We further aim to compare the protostellar Br abundance with that measured by Rosetta in the ices of comet 67P/Churyumov–Gerasimenko. Methods. Archival Herschel data of Orion KL, Sgr B2(N), and NGC 6334I are examined for the presence of HBr and HBr+ emission or absorption lines. A chemical network for modelling HBr in protostellar molecular gas is compiled to aid in the interpretation. Results. HBr and HBr+ were not detected towards any of our targets. However, in the Orion KL Hot Core, our upper limit on HBr/H2O is a factor of ten below the ratio measured in comet 67P. This result is consistent with the chemical network prediction that HBr is not a dominant gas-phase Br carrier. Cometary HBr is likely predominantly formed in icy grain mantles which lock up nearly all elemental Br.
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