Observations of HDO are an important complement for studies of water, because they give
strong constraints on the formation processes – grain surfaces versus
energetic process in the gas phase, e.g. in shocks. The HIFI
observations of multiple transitions of HDO in Sgr B2(M) presented here allow the
determination of the HDO abundance throughout the envelope, which has not been possible
before with ground-based observations only. For more details, see Comito et al.