Transcript FC11_2010.pptx

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 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


Theoretical Collision rates H2O/He





Phillips et al., Ap. J. Supp. 1996
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
 PB  0.447 T T   in    j
R
 Calibration
Temperature
due to heating of gas from injector
j
j , J 'K a 'Kc '
J ' K a ' K c ' JK a K c
Convert to cross section
Theory vs. Experiment He/H2O
Theory vs. Experiment H2/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
Model with step-power function
 Rapid
drop for H2-H2O near 70-80 K
cannot be modeled with ‘usual’ power-law
T  γ0  T 
 T0 
 Modify
n'
n
 PB   0  T T  , n  n'  0.5

0

power law with step function
e (T T0 ) / T  T 
 (T )   200


(T T0 ) / T
1 e
 200 
n
Implications : Overview
•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)
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 counteracted
In 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