DARK WATER - IMPLICATIONS OF RECENT COLLISIONAL COOLING MEASUREMENTS By 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 H 2 O and H 2 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 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’ 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

 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 H 2 O/H 2 Phillips et al., Ap. J. Supp Phillips et al., Ap. J. Supp  Theoretical Collision rates H 2 O/He Green et al., Ap. J. Supp Green et al., Ap. J. Supp Dubernet and Grosjean, A&A, 2002 Dubernet and Grosjean, A&A, 2002 Grosjean et al., A&A, 2003 Grosjean et al., A&A, 2003 Dubenet et al. A&A, 2006 Dubenet et al. A&A, 2006  One temperature study of water completed on the 3 13 ← 2 20 transition by Goyette et al. (1990). Investigated the pressure broadening of this transition in He, H 2, O 2 and N 2 from 80K to 296K. Investigated the pressure broadening of this transition in He, H 2, O 2 and N 2 from 80K to 296K.

Pressure Broadening of Water: Data

Pressure Broadening of Water Temperature Calibration due to heating of gas from injector Convert to cross section

Theory vs. Experiment He/H 2 O

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 K Theory vs. Experiment H 2 /H 2 O

Model with step-power function  Rapid drop for H 2 -H 2 O near K cannot be modeled with ‘usual’ power-law  Modify power law with step function

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 H 2 Gravitational collapse is counteracted In part from outward pressure due to water emission  slower collapse if water present Animation (1) Previous rates for K 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 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