Transcript The ortho:para ratio of H3+ in diffuse molecular clouds

The ortho:para ratio of H3+ in
diffuse molecular clouds
Kyle N. Crabtree, Nick Indriolo,
Holger Kreckel, Brian A. Tom, and
Benjamin J. McCall
November 6, 2010
MWAM 2010
University of Illinois
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H3+, Interstellar Chemistry, and
Astrophysics
•
Starting point for
complex gas-phase
chemistry
Use as an
astrophysical probe:
•
–
–
–
–
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Gas density at the
galactic center
Imaging of Jupiter’s
polar aurorae
Interstellar cosmic ray
ionization rate
Interstellar temperature
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H3+ Spectroscopy
R(1,0)
36685 Å
R(1,1)u
36681 Å
R(1,1)l
37155 Å
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H3+ as an Interstellar Thermometer
•
Observed R(1,0) and
R(1,1)u/R(1,1)l lines 
n(1,0), n(1,1)  T(H3+)
•
Analogous to T01,
derived from UV
observations of H2 J=0
(para) and J=1 (ortho)
levels
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Temperature in Diffuse Molecular
Clouds
• Diffuse molecular clouds: diffuse clouds with
most hydrogen in molecular form.
• Survey of diffuse molecular clouds:
– <T01> ~ 70 K (N = 66) 1
– <T(H3+)> ~ 30 K (N = 18) 2
• Only 2 are in common: ζ-Per and X-Per
• Recent observations (N. Indriolo) have
extended this number to 5
1 B.
D. Savage et al., ApJ, (1977), 216, 291, B. L. Rachford et al., ApJ, (2002), 577, 221, B. L. Rachford et al., ApJS,
(2009), 180, 125.
2 N. Indriolo et al., ApJ, (2007), 671, 1736.
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Temperature Discrepancy
Why
is there
more
Which
of these
p-H3+ than the
expected
represents
“true”
for the temperature?
kinetic
T01
T(H3
HD 73882
+)
HD 154368
ζ-Per
X-Per
HD 110432
T01
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Chemistry of H3+
• Formation:
1. H2 + cosmic ray  H2+ + e- (slow)
2. H2+ + H2  H3+ + H (fast)
• Thermalization
H3+ + H2  H2 + H3+
• Destruction:
H3+ + e-  H2 + H or 3H
• Nuclear spin dependence?
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H3+ Formation
1. H2 + cosmic ray  H2+ + e- (slow)
2. H2+ + H2  H3+ + H (fast)
Reaction
Collision
Fraction
Branching Fraction
 p-H3+
p-H3+ Fraction
p-H2+ + p-H2
(p2)2
1
(p2)2
p-H2+ + o-H2
(p2) (1-p2)
2/3
(2/3) (p2) (1-p2)
o-H2+ + p-H2
(1-p2) (p2)
2/3
(2/3) (1-p2) (p2)
o-H2+ + o-H2
(1-p2)2
1/3
(1/3)(1-p2)2
Total
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(1/3) + (2/3)p2
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“Nascent” p-H3+ Fraction (p3)
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Thermalization: H3+ + H2
•
•
•
Branching fractions: Sid,
Shop, and Sexch
α ≡ Shop/Sexch (0.5?)
Selection Rules
1
“identity”
+
H5
3
“hop”
Does the steady state of
this reaction give a
thermal ortho:para H3+
ratio at low temperature?
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“exchange”
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Bimolecular Reactive Equilibrium (BRE)
•
•
•
•
n(H2)/n(e-) ~ 104; k(H2,H3+)/kDR ~ 10-9/10-7 ~ 10-2
H3+ sees ~100 collisions with H2 during its lifetime
Assume steady state [p-H3+] is determined by
nuclear-spin-changing collisions with H2:
+ + o-H
o-H3+ + p-H2 
p-H
3
2
+
Express in terms of p-H3 fraction (p3):
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Nuclear Spin Rate Coefficients
Parameter
Trot
Tcoll
Sid
Value(s)
10 K
10-160 K
0.1-0.9
Shop
0-1
0-1
Sexch
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f (T,Sid,)
koooo kooop koopo koopp
kopoo kopop koppo koppp
kpooo kpoop kpopo kpopp
kppoo kppop kpppo kpppp
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p3 vs. p2 Determination
Trot = 10 K
Choose Sid, α
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p3 vs. p2 Determination
Tcoll p2
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BRE Results
Sid = 0.1
Sid = 0.9
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BRE Results
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Steady State Model
• Include nuclear spin dependent formation and
destruction reactions:
ke,o = o-H3+ DR
ke,p = p-H3+ DR
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xe = Electron fraction (1.5 x 10-4)
f = Molecular fraction (0.9)
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Steady State: DR Rates Equal
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Steady State: DR Rates Equal
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Steady State: Theoretical DR Rates
1
p-H3+ DR 10x faster
than o-H3+
1dos
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Santos et al., J. Chem. Phys. (2007), 127, 124309.
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Steady State: DR Only
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Conclusions
• H3+ ortho:para ratio in diffuse molecular
clouds likely governed by competition
between thermalization (H3+-H2 collisions) and
destruction by DR with electrons
• H3+ + H2 reaction based on microcanonical
statistical model– quantum scattering
calculations and experimental measurements
needed
• State-selective DR measurements of H3+ also
needed to verify/invalidate model
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Acknowledgements
•
•
•
•
November 6, 2010
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McCall group
Takeshi Oka
Steve Federman
Brian Rachford
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