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Dissociative Recombination of Cold H 3 + and its Interstellar Implications

Dissociative Recombination of Cold H 3 + and its Interstellar Implications. Ben McCall Department of Chemistry Department of Astronomy University of llinois at Urbana-Champaign. T. Oka (University of Chicago) , T. R. Geballe (Gemini Observatory)

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Dissociative Recombination of Cold H 3 + and its Interstellar Implications

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  1. Dissociative Recombination of Cold H3+ and its Interstellar Implications Ben McCall Department of Chemistry Department of Astronomy University of llinois at Urbana-Champaign T. Oka (University of Chicago), T. R. Geballe (Gemini Observatory) A. J. Huneycutt, R. J. Saykally(University of California at Berkeley) N. Djuric, G. H. Dunn(University of Colorado & NIST) J. Semaniak, O. Novotny(Świetokrzyska Academy, Poland) A. Paal, F. Österdahl (Manne Siegbahn Laboratory) A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas, M. Larsson(Stockholm University)

  2. H He Ne O C N Si S Ar Mg Fe Astronomer's Periodic Table

  3. H3+: Cornerstone of Interstellar Chemistry

  4. Observing Interstellar H3+ • Equilateral triangle • “No” rotational spectrum • “No” electronic spectrum • Vibrational spectrum is only probe • Absorption spectroscopy against background or embedded star 1 2

  5.  Persei Interstellar Cloud Classification* Dense molecular clouds: • H  H2 • C  CO • n(H2) ~ 104–106 cm-3 • T ~ 20 K Diffuse clouds: • H ↔ H2 • C  C+ • n(H2) ~ 101–103 cm-3 • [~10-18 atm] • T ~ 50 K • Diffuse atomic clouds • H2 << 10% • Diffuse molecular clouds • H2 > 10% (self-shielded) Barnard 68 (courtesy João Alves, ESO) * Snow & McCall, ARAA, 2006 (in prep) Photo: Jose Fernandez Garcia

  6. H3+ in Dense Clouds R(1,1)l R(1,1)u R(1,0) Wavelength (Å) N(H3+) = 1–51014 cm-2 McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999)

  7. Dense Cloud H3+ Chemistry Formation cosmic ray [H2]  [H2]  H2 H2+ + e- H2 + H2+  H3+ + H Rate = (fast) Destruction k Rate = k [H3+] [CO] [H3+] [CO] H3+ + CO  HCO+ + H2 Steady State (310-17 s-1) = =  (6700) (210-9 cm3 s-1) Density Independent! = 10-4 cm-3 McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999)

  8. 32.9 K (forbidden) K H3+ as a Probe of Dense Clouds • Given n(H3+) from model, and N(H3+) from infrared observations: • path length L = N/n ~ 31018 cm ~ 1 pc • density n(H2) = N(H2)/L ~ 6104 cm-3 • temperature T ~ 30 K • Unique probe of clouds • Consistent with expectations • confirms dense cloud chemistry

  9. Steady State [H2]  (310-17 s-1) [H3+] = =  (2400) [e-] ke (510-7 cm3 s-1) Diffuse Molecular Cloud H3+ Chemistry Formation cosmic ray  [H2] H2 H2+ + e- H2 + H2+  H3+ + H Rate = Destruction Rate = ke [H3+] [e-] H3+ + e-  H + H2 or 3H Density Independent! = 10-7 cm-3 103 times smaller than dense clouds!

  10. Lots of H3+ in Diffuse Clouds! Cygnus OB2 12 HD 183143 N(H3+) ~ dense clouds n(H3+) ~ 1000 times less L ~ 1000 times longer ?!? McCall, et al. ApJ 567, 391 (2002)

  11. [H2] [H3+] = [e-] ke Big Problem with the Chemistry! ~2 orders of magnitude!! Steady State: • To increase the value of [H3+], we need: • Smaller electron fraction [e-]/[H2] • Smaller recombination rate constant ke • Higher ionization rate 

  12. H3+ toward  Persei [e-]/[H2] not to blame N(C+) from HST McCall, et al. Nature 422, 500 (2003) N(H2) from Copernicus Savage et al. ApJ 216, 291 (1977) Cardelli et al. ApJ 467, 334 (1996)

  13. [H2] [H3+] = [e-] ke Big Problem with the Chemistry! Steady State: • To increase the value of [H3+], we need: • Smaller electron fraction [e-]/[H2] • Smaller recombination rate constant ke • Higher ionization rate 

  14. H3+ Dissociative Recombination • Laboratory values of ke have varied by 4 orders of magnitude! • Theory unreliable (until recently)... • Problem (?): not measuring H3+ in ground states

  15. H3+ 20 ns 45 ns H, H2 electron beam Storage Ring Measurements CRYRING • Very simple experiment • Complete vibrational relaxation • Control H3+– e- impact energy • Rotationally hot ions produced • “No” rotational cooling in ring 900 keV 12.1 MeV 30 kV

  16. Pinhole flange/ground electrode -900 V ring electrode Insulating spacer Skimmer H3+ Supersonic Expansion Ion Source Gas inlet 2 atm H2 • Similar to sources for laboratory spectroscopy in many groups • Pulsed nozzle design • Supersonic expansion leads to rapid cooling • Discharge from ring electrode downstream • Spectroscopy used to characterize ions • Skimmer employed to minimize arcing to ring Solenoid valve

  17. H3+ Energy Level Structure R(2,2)l R(1,1)u R(1,0) (2,0) probe of temperature not detected 151 K (0,0) 33 K (J,G)

  18. Spectroscopy of H3+ Source Infrared Cavity Ringdown Laser Absorption Spectroscopy • Confirmed that H3+ produced is rotationally cold, as in interstellar medium McCall, et al. Nature 422, 500 (2003)

  19. CRYRING Results • Considerable amount of structure (resonances) in the cross-section • ke = 2.6  10-7 cm3 s-1 • Factor of two smaller • Chris Greene: new theory • Andreas Wolf: TSR results McCall, et al. Phys. Rev. A 70, 052716 (2004)

  20. [H2] [H3+] = [e-] ke Back to the Interstellar Clouds! Steady State: • To increase the value of [H3+], we need: • Smaller electron fraction [e-]/[H2] • Smaller recombination rate constant ke • Higher ionization rate 

  21. Implications for  Persei N(H3+) N(H2)  [H3+] = = L ke N(e-) N(e-) ke N(H3+)  L = (3.810-4) (2.610-7 cm3 s-1) (81013 cm-2) N(H2) (solid) • L =8000 cm s-1 Adopt =310-17 s-1 Adopt L=2.1 pc =1.210-15 s-1 (40x higher!) L = 85 pc n = 6 cm-3

  22. What Does This Mean? • Enhanced ionization rate in  Persei • Widespread H3+ in diffuse clouds • perhaps widespread ionization enhancement? • Dense cloud H3+ is "normal" • enhanced ionization rate only in diffuse clouds • low energy cosmic-ray flux? • cosmic-ray self-confinement? • no constraints, aside from chemistry!! • New chemical models necessary • Harvey Liszt • Franck Le Petit

  23. Future Work • More experiments! • Improved spectroscopy of ion source • Higher resolution & higher sensitivity • Better characterization of ro-vib distribution • Testing of new (piezo) ion source • Single quantum-state CRYRING measurements • produce pure para-H3+ using para-H2 • More observational data! • Search for H3+ in more diffuse cloud sightlines • Confirm generality of result in classical diffuse clouds • Observations of H3+ in "translucent" sightlines • C+→ C → CO

  24. Rich Diffuse Cloud Chemistry • From 1930s through the mid-1990s, only diatomic molecules thought to be abundant in diffuse clouds • Recently, many polyatomics observed: • H3+ in infrared • HCO+, C2H, C3H2, etc. in radio (Lucas & Liszt) • C3 in near-UV (Maier, et al.) • Diffuse Interstellar Bands!

  25. Acknowledgements • Takeshi Oka (U. Chicago) • Tom Geballe (Gemini) • Staff of UKIRT (Mauna Kea) • United Kingdom InfraRed Telescope • Staff of CRYRING (Stockholm) • Chris Greene (Boulder) • Eric Herbst (Ohio State) • Mike Lindsay (UNC) • Funding: • Berkeley: NSF, AFOSR • Illinois: NSF, NASA, ACS, Dreyfus

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