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Observing ro-vibrational lines of water: molecular and optical data

Presentation #218.02 in the session Laboratory Astrophysics (LAD) Division Meeting: Spectroscopy.

Published onJun 29, 2022
Observing ro-vibrational lines of water: molecular and optical data

Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses [1]. While rotational lines have been examined in great details for long, it is not at all the case for ro-vibrational lines, which mark warmer astrophysical matter. As many lines of warm water (above a few hundreds K) fall in the Mid-Infra-Red spectral regions, they are prime aims of the new JWST space telescope, and also, for the Far-Infra-Red lines, of the SOFIA airborne telescope. Also, some masing lines have been observed in the water excited vibrational state [2].

The water-molecular hydrogen being a system of paramount importance, we computed extensively the rates of vibrational quenching and the state-to-state rates for ro-vibrational (de-)excitation, by fully converged quantum methods.

We show here that the exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer. The excitation of the quantized rotor of the projectile is by far the most effective ro-vibrational quenching path of water [2].

Rates were computed for the collision of water with molecular hydrogen, both with ground state ad excited rotational states, and water in the first vibrational excited states. These rates are presented from gas kinetic temperatures between 30K to 1000K.

We shall present those rates, compare them to known data, as well as critical densities and optical depths models, to underline the relevance of this new extensive series of collisional data pertaining to water.

Work presented here has been made possible thanks to the CNRS supercomputer center IDRIS.

[1] WF Thi, I Kamp, P Woitke, G Van der Plas, R Bertelsen, L Wiesenfeld. Radiation thermo-chemical models of protoplanetary discs-IV. modelling co ro-vibrational emission from Herbig AE discs. A & A, 551:A49, 2013.

[2] K M. Menten, K Young. Discovery of Strong Vibrationally Excited Water Masers at 658 GHz Toward Evolved Stars. ApJ L 450:L67, 1995.

[3] L Wiesenfeld. Quantum nature of molecular vibrational quenching: Water–molecular hydrogen collisions. JCP, 155(7):071104, 2021.

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