Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 1S007

A non-equilibrium ortho-to-para ratio of H2O in the Orion PDR

Choi, Yunhee (Kapteyn Astronomical Institute & SRON, Groningen, The Netherlands)
van der Tak, Floris (SRON & Kapteyn Astronomical Institute, Groningen, The Netherlands)
Bergin, Edwin (Department of Astronomy, University of Michigan, Ann Arbor, USA)
Plume, Rene (Department of Physics and Astronomy, University of Calgary, Calgary, Canada )

The ortho-to-para ratio (OPR) of H2O is lower than 1 at low temperature (< 15 K) and increases to 3 at high temperature (> 40 K). The OPR of H2O is thus useful to study the formation mechanism of water. The measured OPRs of H2O is 2-3 in solar system comets (Mumma & Charnley, 2011) and in the interstellar medium (Lis et al. 2010; Flagey et al. 2013). In the protoplanetary disk TW Hya, the OPR is ~ 0.77, which is much lower than the interstellar value (Hogerheijde et al. 2011). We investigate the OPR of water in the Orion PDR (Photon-dominated region), at the Orion Bar and Orion S positions, using data from the HEXOS (Herschel/HIFI Observations of EXtraOrdinary Sources, PI: E. Bergin) key program for the HIFI instrument onboard the Herschel Space Observatory. We detect the ground-state lines of ortho- and para-H2O-18 in the Orion Bar and Orion S and we estimate the column densities using LTE and non-LTE methods. Based on our calculations, the OPR in the Orion Bar is 0.1-0.5, which is well below that of TW Hya. Toward Orion S, our OPR estimates range from 0.1 to 4, depending on the gas temperature and density. This low OPR in the Orion Bar is inconsistent with gas phase formation and with thermal evaporation from dust grains, but it may be explained by photodesorption (Andersson & van Dishoeck 2008; Arasa et al. 2010).

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