1. Nuclear spin of H3+ in diffuse molecular clouds
Kyle N. Crabtree, Nick Indriolo, Holger Kreckel, Brian A. Tom, and Benjamin J. McCall
University of Illinois

2. 66th OSU International Symposium on Molecular Spectroscopy
Interstellar clouds
T. P. Snow and B. J. McCall, Annu. Rev. Astron. Astrophys. (2006), 44,
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

3. Temperature in diffuse molecular clouds: H2
p-H2; J = 0
o-H2; J = 1 DE
170 K
UV measurements
of H2 absorption
99% of H2 in J=0 (para) and J=1 (ortho) levels
Profile fitting gives accurate column densities for N(0) and N(1)  T01
B. L. Rachford et al., ApJ (2002), 577,
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

4. Temperature in diffuse molecular clouds: H3+
36685 Å
R(1,1)u
36681 Å
R(1,1)l
37155 Å
High-resolution IR spectroscopy of H3+
Column densities N(1,0), N(1,1)  T(H3+)
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

5. H3+/H2 temperature discrepancy
Average T01 in diffuse molecular clouds: 70 K (N = 66)
Average T(H3+) in diffuse molecular clouds: 30 K (N = 18)
Only 2 sight lines in common
Recent observations expand this number to 6
Target
Obs. p3 p2
T(H3+)
T01 DT
z Per
UKIRT
0.65(4)
0.68(6)
25(3)
58(6) 33
X Per
0.66(5)
0.69(4)
24(4)
57(4) HD
Gem. S.
0.69(6)
0.76(7)
22(4)
51(8) 29
HD 73882
VLT
0.67(4)
0.76(5)
23(3)
51(6) 28 HD
0.60(2)
0.57(3)
30(2)
68(5) 38
l Cep
UKIRT/Keck
0.57(7)
0.54(3)
34(10)
73(4) 39
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

6. H3+/H2 temperature discrepancy
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

7. Chemistry of H3+ Formation: H2 + cosmic ray  H2+ + e- (slow)
H2+ + H2  H3+ + H (fast)
Thermalization:
H3+ + H2  H2 + H3+
Destruction via dissociative recombination (DR):
H3+ + e-  H2 + H or 3H
Nuclear spin dependence?
1st order approximation: #H3+-H2 collisions prior to H3+ DR:
kt n(H2) n(H3+)
(10-9)(102)
# collisions = =
= 100
kdr n(e-) n(H3+)
(10-7)(10-2)
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

8. 66th OSU International Symposium on Molecular Spectroscopy
H3+ + H2  H2 + H3+
Sid
Nuclear spin selection rules
Conservation of angular momentum 1
“identity”
H5+ 3
“hop”
Recent laboratory measurements show that kH/kE = 0.5 at low temperatures (<130 K) 6
K. N. Crabtree, B. A. Tom, B. J. McCall, J. Chem. Phys. (2011) 134,
K. N. Crabtree et al., J. Chem. Phys. (2011) 134,
“exchange”
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

9. H3+ + H2 rate coefficients
f (T,Sid)
Parameter
Value(s)
Trot K
Tcoll
Sid
Shop
0-1
Sexch
koooo
kooop
koopo
koopp
kopoo
kopop
koppo
koppp
kpooo
kpoop
kpopo
kpopp
kppoo
kppop
kpppo
kpppp
Shop/Sexch = 0.5
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

10. Model: H3+ + H2 reaction only
10 K
p2 linked
to Tkin
No agreement; H3+ + H2 reaction not solely responsible for T discrepancy
20 K
Sid=0.1
30 K
40 K
Sid=0.9
50 K
etc
80 K
70 K
60 K
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

11. Chemistry of H3+ Formation: H2 + cosmic ray  H2+ + e- (slow)
H2+ + H2  H3+ + H (fast)
Thermalization:
H3+ + H2  H2 + H3+
Destruction via dissociative recombination (DR):
H3+ + e-  H2 + H or 3H
Nuclear spin dependence?
ke,o = o-H3+ DR
xe = Electron fraction (1.5 x 10-4)
f = Molecular fraction (0.9)
ke,p = p-H3+ DR
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

12. 66th OSU International Symposium on Molecular Spectroscopy
DR rates equal
Good agreement when Sid = 0.9
B. J. McCall et al., Nature (2004), 422,
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

13. 66th OSU International Symposium on Molecular Spectroscopy
p-H3+ DR = 10× o-H3+ DR
No agreement
F. dos Santos et al., J. Chem. Phys (2007), 127,
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

14. 66th OSU International Symposium on Molecular Spectroscopy
Summary
H3+ in diffuse molecular clouds is nonthermal
The ortho:para ratio of H3+ lies between its nascent value and its thermalized value
Competition between thermalization (H3+ + H2) and DR
Low-temperature state-specific DR rate coefficients for H3+ needed
Future: modeling environments in which H2 column densities are unavailable, but H3+ column densities are
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy

15. 66th OSU International Symposium on Molecular Spectroscopy
Acknowledgements
Kisam Park
More information:
K. N. Crabtree et al., ApJ (2011) 729, 15.
21 June 2011
66th OSU International Symposium on Molecular Spectroscopy