1. November 6, 2010 MWAM 2010 University of Illinois1 The ortho:para ratio of H 3 + in diffuse molecular clouds Kyle N. Crabtree, Nick Indriolo, Holger Kreckel, Brian A. Tom, and Benjamin J. McCall

2. November 6, 2010 MWAM 2010 University of Illinois2 H 3 +, Interstellar Chemistry, and Astrophysics Starting point for complex gas-phase chemistry Use as an astrophysical probe: –Gas density at the galactic center –Imaging of Jupiter’s polar aurorae –Interstellar cosmic ray ionization rate –Interstellar temperature Starting point for complex gas-phase chemistry Use as an astrophysical probe: –Gas density at the galactic center –Imaging of Jupiter’s polar aurorae –Interstellar cosmic ray ionization rate –Interstellar temperature

3. November 6, 2010 MWAM 2010 University of Illinois3 H 3 + Spectroscopy R(1,0) 36685 Å R(1,1) u 36681 Å R(1,1) l 37155 Å

4. November 6, 2010 MWAM 2010 University of Illinois4 H 3 + 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(H 3 + ) Analogous to T 01, derived from UV observations of H 2 J=0 (para) and J=1 (ortho) levels Observed R(1,0) and R(1,1) u /R(1,1) l lines  n(1,0), n(1,1)  T(H 3 + ) Analogous to T 01, derived from UV observations of H 2 J=0 (para) and J=1 (ortho) levels

5. November 6, 2010 MWAM 2010 University of Illinois5 Temperature in Diffuse Molecular Clouds Diffuse molecular clouds: diffuse clouds with most hydrogen in molecular form. Survey of diffuse molecular clouds: – ~ 70 K (N = 66) 1 – ~ 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 Diffuse molecular clouds: diffuse clouds with most hydrogen in molecular form. Survey of diffuse molecular clouds: – ~ 70 K (N = 66) 1 – ~ 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. 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.

6. November 6, 2010 MWAM 2010 University of Illinois6 Temperature Discrepancy ζ-Per X-Per HD 73882 HD 154368 HD 110432 T(H 3 + ) T 01 Which of these represents the “true” kinetic temperature? T 01 Why is there more p-H 3 + than expected for the temperature?

7. November 6, 2010 MWAM 2010 University of Illinois7 Chemistry of H 3 + Formation: 1.H 2 + cosmic ray  H 2 + + e - (slow) 2.H 2 + + H 2  H 3 + + H (fast) Thermalization H 3 + + H 2  H 2 + H 3 + Destruction: H 3 + + e -  H 2 + H or 3H Nuclear spin dependence? Formation: 1.H 2 + cosmic ray  H 2 + + e - (slow) 2.H 2 + + H 2  H 3 + + H (fast) Thermalization H 3 + + H 2  H 2 + H 3 + Destruction: H 3 + + e -  H 2 + H or 3H Nuclear spin dependence?

8. November 6, 2010 MWAM 2010 University of Illinois8 H 3 + Formation 1.H 2 + cosmic ray  H 2 + + e - (slow) 2.H 2 + + H 2  H 3 + + H (fast) Reaction Collision Fraction Branching Fraction  p-H 3 + p-H 3 + Fraction p-H 2 + + p-H 2 (p2)2(p2)2 1(p2)2(p2)2 p-H 2 + + o-H 2 (p 2 ) (1-p 2 )2/3(2/3) (p 2 ) (1-p 2 ) o-H 2 + + p-H 2 (1-p 2 ) (p 2 )2/3(2/3) (1-p 2 ) (p 2 ) o-H 2 + + o-H 2 (1-p 2 ) 2 1/3(1/3)(1-p 2 ) 2 Total(1/3) + (2/3)p 2

9. November 6, 2010 MWAM 2010 University of Illinois9 “Nascent” p-H 3 + Fraction (p 3 )

10. November 6, 2010 MWAM 2010 University of Illinois10 Thermalization: H 3 + + H 2 “identity” “hop” “exchange” H5+H5+ H5+H5+ 1 1 3 3 6 6 Branching fractions: S id, S hop, and S exch α ≡ S hop /S exch (0.5?) Selection Rules Branching fractions: S id, S hop, and S exch α ≡ S hop /S exch (0.5?) Selection Rules Does the steady state of this reaction give a thermal ortho:para H 3 + ratio at low temperature?

11. November 6, 2010 MWAM 2010 University of Illinois11 Bimolecular Reactive Equilibrium (BRE) n(H 2 )/n(e - ) ~ 10 4 ; k(H 2,H 3 + )/k DR ~ 10 -9 /10 -7 ~ 10 -2 H 3 + sees ~100 collisions with H 2 during its lifetime Assume steady state [p-H 3 + ] is determined by nuclear-spin-changing collisions with H 2 : Express in terms of p-H 3 + fraction (p 3 ): n(H 2 )/n(e - ) ~ 10 4 ; k(H 2,H 3 + )/k DR ~ 10 -9 /10 -7 ~ 10 -2 H 3 + sees ~100 collisions with H 2 during its lifetime Assume steady state [p-H 3 + ] is determined by nuclear-spin-changing collisions with H 2 : Express in terms of p-H 3 + fraction (p 3 ): o-H 3 + + p-H 2  p-H 3 + + o-H 2

12. November 6, 2010 MWAM 2010 University of Illinois12 Nuclear Spin Rate Coefficients ParameterValue(s) T rot 10 K T coll 10-160 K S id 0.1-0.9 S hop 0-1 S exch 0-1 k oooo k ooop k oopo k oopp k opoo k opop k oppo k oppp k pooo k poop k popo k popp k ppoo k ppop k pppo k pppp f (T,S id,  )

13. November 6, 2010 MWAM 2010 University of Illinois13 p 3 vs. p 2 Determination T rot = 10 K Choose S id, α

14. November 6, 2010 MWAM 2010 University of Illinois14 p 3 vs. p 2 Determination T coll p2p2

15. November 6, 2010 MWAM 2010 University of Illinois15 BRE Results S id = 0.9 S id = 0.1

16. November 6, 2010 MWAM 2010 University of Illinois16 BRE Results

17. November 6, 2010 MWAM 2010 University of Illinois17 Steady State Model Include nuclear spin dependent formation and destruction reactions: k e,o = o-H 3 + DR k e,p = p-H 3 + DR x e = Electron fraction (1.5 x 10 -4 ) f = Molecular fraction (0.9) x e = Electron fraction (1.5 x 10 -4 ) f = Molecular fraction (0.9)

18. November 6, 2010 MWAM 2010 University of Illinois18 Steady State: DR Rates Equal

19. November 6, 2010 MWAM 2010 University of Illinois19 Steady State: DR Rates Equal

20. November 6, 2010 MWAM 2010 University of Illinois20 Steady State: Theoretical 1 DR Rates p-H 3 + DR 10x faster than o-H 3 + 1 dos Santos et al., J. Chem. Phys. (2007), 127, 124309.

21. November 6, 2010 MWAM 2010 University of Illinois21 Steady State: DR Only

22. November 6, 2010 MWAM 2010 University of Illinois22 Conclusions H 3 + ortho:para ratio in diffuse molecular clouds likely governed by competition between thermalization (H 3 + -H 2 collisions) and destruction by DR with electrons H 3 + + H 2 reaction based on microcanonical statistical model– quantum scattering calculations and experimental measurements needed State-selective DR measurements of H 3 + also needed to verify/invalidate model H 3 + ortho:para ratio in diffuse molecular clouds likely governed by competition between thermalization (H 3 + -H 2 collisions) and destruction by DR with electrons H 3 + + H 2 reaction based on microcanonical statistical model– quantum scattering calculations and experimental measurements needed State-selective DR measurements of H 3 + also needed to verify/invalidate model

23. November 6, 2010 MWAM 2010 University of Illinois23 Acknowledgements McCall group Takeshi Oka Steve Federman Brian Rachford McCall group Takeshi Oka Steve Federman Brian Rachford