Interstellar bromine abundance is consistent with cometary ices from
  Rosetta
  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.
