Water vapor in high-mass star-forming regions and PDRs: Tracing the dynamics and chemistry with Herschel/HIFI

Massive stars play a major role in the interstellar energy budget and the shaping of the galactic environment. The water molecule is thought to be a sensitive tracer of physical conditions in star-forming regions because of its large abundance variations between hot and cold regions. Herschel/HIFI allows us to observe the multiple rotational transitions of H2O including the ground-state levels, and its isotopologues toward high-mass star-forming regions in different evolutionary stages. Photodissociation regions (PDRs) are also targeted to investigate the distribution of water and its chemistry. We present line profiles and maps of H2O using data from two programs “Water In Star-forming regions with Herschel” and “Herschel observations of EXtra-Ordinary Sources”. We estimate turbulent and expansion velocities, and abundance of water in the inner and outer envelopes using the 1D radiative transfer code. We find H2O abundances of ~10-8–10-9 for the outer envelope and ~10-4–10-5 for the inner envelope, and expansion and turbulent velocities range from 1.0 km s-1 to 2.5 km s-1. The Herschel/HIFI mapping observations of H2O toward the Orion Bar show that H2O emission peaks between the shielded dense gas and the radicals position, in agreement with the theoretical and the observational PDR structure. The derived H2O abundance is ~10-7 and peaks at the depth of AV ~8 mag from the ionization front. Together with the low ortho-to-para ratio of H2O (~1) presented by Choi et al. (2014), our results show that the chemistry of water in the Orion Bar is dominated by photodesorption.