DARK WATER - IMPLICATIONS OF RECENT COLLISIONAL

COOLING MEASUREMENTSBy

Brian J. Drouin, Michael J. Dick, and John C. Pearson

Jet Propulsion Laboratory,California Institute of Technology

Interest in Cold Water Observing and understanding the spectrum of water in space is

essential to expanding our knowledge of the universe.

1) Direct Importance: Water plays a central role in star and planet formation and is essential to life.

2) Indirect Importance: The spectrum of water could be used as a probe of the temperature, velocity and geometry of interstellar clouds. For example, state-to-state collision rates of H2O and H2 are essential in determination (or reconciliation) of cloud structure and composition.

To study water under interstellar conditions various experimental obstacles must be overcome.

Interest in Cold Water II

Overall results of SWAS (Submillimeter Wave Astronomy Satellite) Warm water (>100 Kelvin) is well modeled

and explained in the context of other cloud tracers

Cold water (< 100 K) is not well explained and typically ‘underabundant’

New results coming from Herschel HIFI

SWAS cold water

Effects on other water No handle on cold regions increases

uncertainty for shocked regions

Experimental Setup

Experimental Setup

Collimating optics pass radiation through the system.Using diode detector spectra are recorded in absorption in real

time using video spectroscopy.

Previous Work - Water Theoretical Collision rates H2O/H2

Phillips et al., Ap. J. Supp. 1996

Theoretical Collision rates H2O/He Green et al., Ap. J. Supp. 1993 Dubernet and Grosjean, A&A, 2002 Grosjean et al., A&A, 2003 Dubenet et al. A&A, 2006

One temperature study of water completed on the 313 ← 220 transition by Goyette et al. (1990).

Investigated the pressure broadening of this transition in He, H2, O2 and

N2 from 80K to 296K.

Pressure Broadening of Water: Data

Pressure Broadening of Water

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''',447.0 Temperature Calibration due to heating of gas from injector

Convert to cross section

Theory vs. Experiment He/H2O

• Collision theories (red / blue and grey) for water and molecular hydrogen predict small decreases or increases in the excitation cross sections

• No prior experiments constrained the theory and astronomers are forced to use it• Our collisional broadening measurements black squares (Dick et al. JQSRT

2009, Dick et al. Phys Rev. A 2010) show dramatic decreases in collisional cross sections at 50-80 K

Theory vs. Experiment H2/H2O

Model with step-power function

Rapid drop for H2-H2O near 70-80 K cannot be modeled with ‘usual’ power-law

Modify power law with step function

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• ISO, Odin and SWAS all have trouble modeling interstellar water below 80 K

• Water is a primary coolant that slows gravitational collapse when excited by collisions

• Reducing the water collisional excitation rate will affect ISM physics via:

1) Water becoming unimportant in the radiative balance of cold (< 80 K) clouds

2) Increasing the derived water abundance (i.e. the majority of the water is dark)

3) Increasing the oxygen abundance (potentially resolving the O deficit)

Implications : Overview

Implication I: There is more water, its just dark

Application to SWAS data

Implication II: Water unimportant in the radiative balance of cold clouds

Dynamics in molecular clouds are dominated by collisions with H2

Gravitational collapse is counteractedIn part from outward pressure due to

water emission slower collapse if water present

Animation (1)Previous rates for 30-100K cloud:

Animation (2)Faster collapse when water is

Not excited easily

Implication III: More oxygen

Nucleosynthetic theories predict elemental abundances Issues include observed oxygen deficit

We can breathe easier and start to count dark water as a hidden source of the missing oxygen

Future Work

Examine the state-to-state collision rates for water colliding with hydrogen using a double resonance experiment.

Explore the effect of para vs. ortho hydrogen concentrations on state-to-state collision rates.

Acknowledgements

We would like to thank Tim Crawford for technical support and

guidance. NASA’s Astronomy and Physics Research and

Analysis program (APRA) for funding Herschel Science Center

Also: Copyright 2010 California Institute of

Technology. Government sponsorship acknowledged