Interstellar bromine abundance is consistent with cometary ices from Rosetta

Abstract: 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. The first cometary abundance of the halogen element bromine (Br) was recently made available by the Rosetta mission. Its abundance in protostellar gas is thus far unconstrained, however. 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. 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. HBr and HBr$^{+}$ were not detected towards any of our targets. However, in the Orion KL Hot Core, our upper limit on HBr/H$_{2}$O 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.