Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..11f&link_type=abstract
The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30
Other
Scientific paper
Ultracompact (UC) HII regions constitute one of the earliest phases in the formation of a massive star and are characterized by extreme physical and chemical conditions (G0 105 in units of Habing field and n> 106 cm-3). Their understanding is important for distinguishing the different processes in massive star formation (UV radiation vs shocks) and because they can be used as a template for other extreme photon-dominated regions (PDRs) such as the surface layers of circumstellar disks and the nuclei of starburst galaxies. The UC HII Mon R2 is the only one that can be spatially resolved with Herschel. It has been extensively studied at millimeter and submillimeter wavelengths and all evidences support a chemistry driven by the strong UV field while shocks play only a minor role. Its physical and kinematical structure is well known and current observations are well explained by an expanding envelope with ve=0.5×(r/Rout)-1 km s-1 triggered by the inner HII region. HIFI observations revealed that the emission of the o-H2O 11,0→10,1 line is extended over all the region. The o-H218O 11,0→10,1 line has been detected towards the ionization front. Combining the o-H2O 11,0→10,1, o-H218O 11,0→10,1 with the high-J CO and 13CO lines observed with Herschel, we have been able to derive the water abundance profile across this expanding envelope. The o-H2O abundance varies in the range a few 10-7--10-8 with the highest values for the gas closest to the ionized region. HIFI observations of the o-NH3 10→01 have also been performed toward this PDR. Contrary to o-H2O, the o-NH3 abundance is minimum for the gas with the highest expansion velocities. This is consistent with the predictions of gas phase chemical models for PDRs but also with recent HIFI results in bipolar outflows. We discuss this result in the context of the relative importance of the gas phase and grain surface chemistry in the formation of H2O in warm star forming regions. The influence of different desorption mechanisms, thermal desorption, photodesorption and grain sputtering, is under study.
Berné Olivier
Cernicharo Jose
Fuente Asuncion
Gerin Maryvonne
Goicoechea Javier R.
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