Post on 25-Dec-2015
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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jinTPB RT'''
''',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