Nuclear Spin of H3+ and H2 in Dense Molecular
Clouds
Kyle N. Crabtree and Benjamin J. McCall
Dense clouds and deuterium fractionation
• Cold (10-50 K), dense (~104 cm-3)
clouds of molecular gas and dust
• Cosmic D/H ratio ~10-5, but some
molecules have larger observed ratios
(e.g. DCO+/HCO+ ~0.2 in L134N)
• What causes this enrichment?
L183 (L134N)
NASA/JPL, Spitzer, L. Pagani
Barnard 68
ESO, VLT, FORS team
http://bjm.scs.illinois.edu
2
Chemistry of deuterium fractionation
• Predominant form of D in molecular clouds is HD
• “Extracting” D from HD:
H3+ + HD  H2D+ + H2 + 232 K
H2D+
+X
1

3
(DX+
2
3
+ H2) + (HX+ + HD)
• Further enrichment:
H2D+ + HD  D2H+ + H2 + 187 K
D2H+ + HD  D3+ + H2 + 234 K
http://bjm.scs.illinois.edu
3
Nuclear spin modifications of H3+ and H2
o-H2; J = 1
o-H3+
(J,K) = (1,0)
DE
170 K
DE
32 K
p-H3+
p-H2; J = 0
(J,K) = (1,1)
Chemical reactions interconvert o- and p-H2 (o- and p-H3+)
e.g. o-H2 + H+  p-H2 + H+; o-H3+ + p-H2  p-H3+ + o-H2
Non-equilibrium distributions very possible
http://bjm.scs.illinois.edu
4
o-H2 influences deuterium fractionation
• Deuterium fractionation reactions:
H3+ + HD  H2D+ + H2 + 230 K
H2D+ + HD  D2H+ + H2 + 190 K
D2H+ + HD  D3+ + H2 + 230 K
• But, with o-H2, reverse reactions become more feasible:
H2D+ + o-H2 + 60 K  H3+ + HD
D2H+ + o-H2 + 20 K  H2D+ + HD
D3+ + o-H2 + 60 K  D2H+ + HD
• Also, nuclear spin modifications of H3+, H2D+, etc. are
important
• Abundance of o-H2 is critical for deuterium fractionation
http://bjm.scs.illinois.edu
5
H3+/H2 chemistry in dense clouds
•
•
•
•
•
•
•
•
H + H  H2 (grain surface formation)
H2 + H+  H+ + H2 (H2 o/p conversion)
H2  H2+ + e- (cosmic ray ionization/H2 destruction)
H2+ + H2  H3+ + H (H2 destruction/H3+ formation)
H3+ + H2  H2 + H3+ (H3+ and H2 o/p conversion)
H3+ + CO  HCO+ + e- (H3+ destruction)
H3+ + HD  H2D+ + H2 (start of D fractionation)
CO/HD ratio impacts extent of D fractionation
http://bjm.scs.illinois.edu
6
Dense clouds vs. cores
H3+ + CO  HCO+ + H2
H3+ + HD  H2D+ + H2
T = 10-50 K
CO/HD > 10
T < 10 K
CO frozen!
CO/HD < 1
H3+ + HD  H2D+ + H2
H3+ + CO  HCO+ + H2
http://bjm.scs.illinois.edu
7
Nuclear spin dependent chemistry in dense clouds
H
75%
X
50/50
o-H2
o-H2+
o-H3+
25%
H+
H3+
p-H2
H2+
H2+
H2
p-H3+
http://bjm.scs.illinois.edu
X
8
Database of reaction rates for o/p H3+ and H2
• Inspired by OSU and UMIST databases
• 28 species, 172 reactions, 220 rates
• New platform-independent C++/Qt
solving code w/ GNU Scientific Library
differential equation solving algorithms
http://bjm.scs.illinois.edu
9
Sample model output: abundances of major species
H2
H
O
CO O2
C
eH3+
H+
http://bjm.scs.illinois.edu
10
Timescale for o/p-H2  steady state: 107 yr
p2 = 0.995 0.9999997 @ 10 K
0.92 @ 10 K
p3 = 0.68
T(H3+) = 24 K
T(H2) = 21 K
http://bjm.scs.illinois.edu
11
Trends observed in modeling
1. The o-H2 fraction is always greater than equilibrium
value
2. The timescale for o/p-H2 steady state is (nearly)
independent of density
3. Higher temperature and cosmic ray ionization rate
decrease timescale
4. The lower the temperature, the more nonthermal T(H2)
is at steady state
5. The proton abundance may play a key role (oxygen
chemistry)
http://bjm.scs.illinois.edu
12
Proton abundance and oxygen chemistry
O2
H2O
H3+
H+
H+ + O2  O2+ + H
H+ + H2O  H2O+ + H
Modeling nuclear spin dependent chemistry is critical,
and much work still needs to be done
http://bjm.scs.illinois.edu
13
Acknowledgements
NASA Earth and Space
Science Fellowship Program
http://bjm.scs.illinois.edu
14