1 / 40

H 3 + : A Case Study for the Importance of Molecular Laboratory Astrophysics

H 3 + : A Case Study for the Importance of Molecular Laboratory Astrophysics. Ben McCall. Dept. of Chemistry. Dept. of Astronomy. H 3 + : Cornerstone of Interstellar Chemistry. Nature 384, 334 (1996). R (1,1) l. R (1,1) u. R (1,0). Wavelength ( Å). H 3 + in Dense Clouds.

deborah
Download Presentation

H 3 + : A Case Study for the Importance of Molecular Laboratory Astrophysics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. H3+:A Case Study for the Importance of Molecular Laboratory Astrophysics Ben McCall Dept. of Chemistry Dept. of Astronomy

  2. H3+: Cornerstone of Interstellar Chemistry

  3. Nature 384, 334 (1996) R(1,1)l R(1,1)u R(1,0) Wavelength (Å) H3+ in Dense Clouds N(H3+) ~ 31014 cm-2 Consistent with expectations McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999) UKIRT Kitt Peak

  4. Role of Laboratory Astrophysics • Four and a half years – much of it assembling the IR laser system and discharge cell • Scanned from: • 6/12-8/3 (1978) • 12/18-1/26 (1978-79) • 4/24-12/18 (1980) • Success on April 25, 1980

  5. Surprise: H3+ in Diffuse Clouds! Cygnus OB2 12 ~ 4 × 1014 cm-2 ! Similar column density to dense clouds!! observed at Kitt Peak observed at UKIRT B. J. McCall, T. R. Geballe, K. H. Hinkle & T. Oka, Science 279, 1910 (1998)

  6. Steady State [H2]  (310-17 s-1) [H3+] = =  (2400) [e-] ke (510-7 cm3 s-1) Diffuse 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 If L ~ 3 pc ~ 1019cm, expect only N(H3+) ~ 1012 cm-2!

  7. H3+ in Lots of Diffuse Clouds! Cygnus OB2 12 HD 183143 McCall, et al. ApJ 567, 391 (2002)

  8. [H2] [H3+] = [e-] ke Big Problem with the Chemistry! >1 order 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 

  9.  Persei 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)

  10. [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 

  11. Enigma of H3+ Recombination • Laboratory values of ke have varied by 4 orders of magnitude! • Problem: not measuring H3+ in ground states

  12. H3+ 20 ns 45 ns H, H2 electron beam Ion Storage Ring Measurements CRYRING • Very simple experiment • Complete vibrational relaxation • Control H3+– e- impact energy • Rotationally cold ions from supersonic expansion source 900 keV 12.1 MeV 30 kV

  13. CRYRING Results • Considerable amount of structure (resonances) in the cross-section • ke = 2.6  10-7 cm3 s-1 • Factor of two smaller McCall, et al. Phys. Rev. A 70, 052716 (2004)

  14. Agreement with Other Work • Reasonable agreement between: • CRYRING • Supersonic expansion • TSR • 22-pole trap • Theory S.F. dos Santos, V. Kokoouline and C. H. Greene, J. Chem. Phys 127 (2007) 124309

  15. [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 

  16. Implications for  Persei N(H3+) N(H2)  [H3+] = = L ke N(e-) N(e-) ke N(H3+)  L = (4.710-4) (1.610-7 cm3 s-1) (71013 cm-2) N(H2) (dense cloud value) (firm) • L =5300 cm s-1 Adopt n = 215 cm-3 → L=2.4 pc Adopt =310-17 s-1 =7.410-16 s-1 (25x higher!) L= 60 pc n = 9 cm-3 Similar results in many other sightlines N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007)

  17. Surprise → Conventional Wisdom • Higher  in diffuse (vs. dense) clouds initially greeted with “skepticism” • Incorporated into models without incident • Now generally accepted (but not understood!)

  18. Low Energy CRs? T. E. Cravens & A. Dalgarno, ApJ 219, 750 (1978) • Could there be a large flux of low energy cosmic rays? AV .13 .44 8 30 160 600 1 MeV 2 MeV 10 MeV 20 MeV 50 MeV (diffuse) (dense) M.D. Stage, G. E. Allen, J. C. Houck, J. E. Davis, Nat. Phys. 2, 614 (2006)

  19. Cosmic Ray Observations Inferred interstellar W.R.Webber, ApJ 506, 329 (1998)

  20. Theoretical Spectra

  21. Consider This Spectrum

  22. Inferred Ionization Rate dense~6×10-17 s-1

  23. Inferred Ionization Rate See poster 05.04 (Nick Indriolo) dense~6×10-17 s-1 diffuse~3.1×10-16 s-1

  24. Summary • H3+ surprisingly abundant in diffuse clouds • Enabled by laboratory spectroscopy • H3+ is now a direct probe of ionization rate • Enabled by storage ring measurements & theory • Ionization rate ~10× higher than thought • only in diffuse clouds • would still be unknown if not for laboratory astrophysics work • Two proposed explanations • MHD self-confinement (Padoan & Scalo) • high flux of low energy cosmic rays

  25. Acknowledgments Nick Indriolo (U. Illinois) Brian Fields (U. Illinois) NASA Laboratory Astrophysics Takeshi Oka (U. Chicago) Tom Geballe (Gemini) NSF Divisions of Chemistry & Astronomy http://astrochemistry.uiuc.edu

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

  27. 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

  28.  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, 44, 367 (2006) Photo: Jose Fernandez Garcia

  29. 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)

  30. 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

  31. 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)

  32. 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)

  33. Theoretical Calculations

  34. TSR CRYRING theory TSR Results • Good agreement • Different ion production • Different conditions • Experiments likely “right”! H. Kreckel, et al. Phys. Rev. Lett. 95, 263201 (2005)

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

  36. Observational Consequences? • Energy required for acceleration • about 0.2 × 1051 ergs/century • Heating of diffuse clouds • about 1/10 of photoelectric heating • Production of LiBeB (spallation) • roughly consistent with observed abundances • γ-ray line production (nuclear excitation) • below detectable limits our spectrum is not excluded by observations!

  37. The Future (The Dream?) • Improved precision in  determinations • improved density estimates • more sophisticated cloud models • Measure H3+ in wider range of sightlines • diffuse, translucent, dense clouds • Infer (AV) → cosmic ray spectrum • information on acceleration mechanism(s) • information on galactic propagation

  38. Recent Astronomical Results • Range of ζ from 1.1-7.3 10-16 s-1 • Biggest uncertainty is in adopted n N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007)

More Related