Water Absorption In The Milky Way: Herschel/hifi Insights On The History Of The Gas

Details Water Absorption In The Milky Way: Herschel/hifi Insights On The History Of The Gas Water Absorption In The Milky Way: Herschel/hifi Insights On The History Of The Gas

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21930201f&link_type=abstract

American Astronomical Society, AAS Meeting #219, #302.01

Physics

Scientific paper

The water molecule plays an essential role in the chemistry and physics of the dense regions of the interstellar medium (ISM). In particular, it is an important reservoir of oxygen and cooling agent. It is found in vapor or condensed in iced mantle on grains in the ISM. One specificity of the water molecule is due to its asymmetry. A molecule of water, at a given energy level, as defined by its quantum numbers, can either be ortho or para. The ratio between the population of ortho and para water is 3 in the high temperature limit. At lower temperature, the ortho/para ratio can be lower than 2. Collisions and interaction between the molecule and the grain are processes that may affect the ortho/para ratio. Consequently, measurements of the ortho/para ratio in water can provide insights on the history of the molecule and its environment.
The Herschel Guaranteed Time Key Progamme PRISMAS (P.I. M.Gerin) observed 25 molecules, including the five lower transitions of water, towards eight sources with strong dust emission in the Galactic plane with the HIFI instrument. We present the multiple absorption features of water due to foreground clouds that are known to intersect those lines of sight. We model the observations, taking into account background emission due to the continuum sources. We then infer optical depth along the line of sight and the ratio between ortho and para water column densities. We discuss the results and their implications on our understanding of physical and chemical processes.

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