Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only ultra-compact H ii region (UC H ii) where its associated photon-dominated region (PDR) can be resolved with the Herschel Space Observatory.
Aims: Our aim is to investigate observationally the kinematical patterns in the interface regions (i.e., the transition from atomic to molecular gas) associated with Mon R2.
Methods: We used the HIFI instrument on board Herschel to observe the line profiles of the reactive ions CH+, OH+, and H2O+ toward different positions in Mon R2. We derive the column density of these molecules and compare them with gas-phase chemistry models.
Results: The reactive ion CH+ is detected both in emission (at central and red-shifted velocities) and in absorption (at blue-shifted velocities). The OH+ ion is detected in absorption at both blue- and red-shifted velocities, with similar column densities; H2O+ is not detected at any of the positions, down to a rms of 40 mK toward the molecular peak. At this position, we find that the OH+ absorption originates in a mainly atomic medium, and therefore is associated with the most exposed layers of the PDR. These results are consistent with the predictions from photo-chemical models. The line profiles are consistent with the atomic gas being entrained in the ionized gas flow along the walls of the cavity of the H ii region. Based on this evidence, we are able to propose a new geometrical model for this region.
Conclusions: The kinematical patterns of the OH+ and CH+ absorption indicate the existence of a layer of mainly atomic gas for which we have derived, for the first time, some physical parameters and its dynamics.
Astronomy and Astrophysics
- Pub Date:
- January 2014
- ISM: abundances;
- ISM: molecules;
- ISM: individual objects: Monoceros R2;
- photon-dominated region (PDR);
- HII regions;
- Astrophysics - Astrophysics of Galaxies
- 6 pages, 5 figures. Accepted for publication in A&