Presentation on theme: "Dissociative Recombination of Cold H 3 + and its Interstellar Implications T. Oka (University of Chicago), T. R. Geballe (Gemini Observatory) A. J."— Presentation transcript:
Dissociative Recombination of Cold H 3 + and its Interstellar Implications 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) Ben McCall Department of Chemistry Department of Astronomy University of llinois at Urbana-Champaign
Astronomer's Periodic Table H He CN O Ne Mg Fe SiS Ar
H 3 + : Cornerstone of Interstellar Chemistry
Observing Interstellar H 3 + Equilateral triangle “No” rotational spectrum “No” electronic spectrum Vibrational spectrum is only probe Absorption spectroscopy against background or embedded star 1 2
Interstellar Cloud Classification* Diffuse clouds: H ↔ H 2 C C + n(H 2 ) ~ 10 1 –10 3 cm -3 –[~ atm] T ~ 50 K Persei Photo: Jose Fernandez Garcia Diffuse atomic clouds –H 2 << 10% Diffuse molecular clouds –H 2 > 10% (self-shielded) * Snow & McCall, ARAA, 2006 (in prep) Barnard 68 (courtesy João Alves, ESO) Dense molecular clouds: H H 2 C CO n(H 2 ) ~ 10 4 –10 6 cm -3 T ~ 20 K
H 3 + in Dense Clouds R(1,1) u R(1,0) R(1,1) l Wavelength (Å) N(H 3 + ) = 1–5 cm -2 McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999)
[H 2 ] Dense Cloud H 3 + Chemistry H 2 H e - H 2 + H 2 + H H cosmic ray H CO HCO + + H 2 Rate = Formation Destruction Rate = k [H 3 + ] [CO] [H 3 + ] = k [CO] Steady State = (3 s -1 ) (2 cm 3 s -1 ) [H 2 ] (6700) = cm -3 Density Independent! (fast) McCall, Geballe, Hinkle, & Oka ApJ 522, 338 (1999)
H 3 + as a Probe of Dense Clouds Given n(H 3 + ) from model, and N(H 3 + ) from infrared observations: –path length L = N/n ~ 3 cm ~ 1 pc –density n(H 2 ) = N(H 2 )/L ~ 6 10 4 cm -3 –temperature T ~ 30 K Unique probe of clouds Consistent with expectations –confirms dense cloud chemistry (forbidden) 32.9 K K
Rate = k e [H 3 + ] [e - ] [H 2 ] Diffuse Molecular Cloud H 3 + Chemistry H 2 H e - H 2 + H 2 + H H cosmic ray H e - H + H 2 or 3H Rate = Formation Destruction [H 3 + ] = keke [e - ] Steady State [H 2 ] = (3 s -1 ) (5 cm 3 s -1 ) (2400) = cm -3 Density Independent! 10 3 times smaller than dense clouds!
Lots of H 3 + in Diffuse Clouds! HD McCall, et al. ApJ 567, 391 (2002) Cygnus OB2 12 N(H 3 + ) ~ dense clouds n(H 3 + ) ~ 1000 times less L ~ 1000 times longer ?!?
Big Problem with the Chemistry! Steady State: [H 3 + ] = keke [e - ] [H 2 ] To increase the value of [H 3 + ], we need: Smaller electron fraction [e - ]/[H 2 ] Smaller recombination rate constant k e Higher ionization rate ~2 orders of magnitude!!
H 3 + toward Persei Cardelli et al. ApJ 467, 334 (1996)Savage et al. ApJ 216, 291 (1977) N(H 2 ) from Copernicus N(C + ) from HST [e - ]/[H 2 ] not to blame McCall, et al. Nature 422, 500 (2003)
Big Problem with the Chemistry! Steady State: [H 3 + ] = keke [e - ] [H 2 ] To increase the value of [H 3 + ], we need: Smaller electron fraction [e - ]/[H 2 ] Smaller recombination rate constant k e Higher ionization rate
H 3 + Dissociative Recombination Laboratory values of k e have varied by 4 orders of magnitude! Theory unreliable (until recently)... Problem (?): not measuring H 3 + in ground states
Storage Ring Measurements 20 ns 45 ns electron beam H3+H3+ H, H 2 +Very simple experiment +Complete vibrational relaxation +Control H 3 + – e - impact energy -Rotationally hot ions produced -“No” rotational cooling in ring CRYRING 30 kV 900 keV 12.1 MeV
Supersonic Expansion Ion Source 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 H2H2 Gas inlet 2 atm Solenoid valve Insulating spacer Skimmer -900 V ring electrode Pinhole flange/ground electrode H3+H3+
R(1,0) R(1,1) u R(2,2) l 33 K 151 K (0,0) (2,0) (J,G) probe of temperature not detected H 3 + Energy Level Structure
Spectroscopy of H 3 + Source Confirmed that H 3 + produced is rotationally cold, as in interstellar medium Infrared Cavity Ringdown Laser Absorption Spectroscopy McCall, et al. Nature 422, 500 (2003)
CRYRING Results Considerable amount of structure (resonances) in the cross-section k e = 2.6 cm 3 s -1 Factor of two smaller McCall, et al. Phys. Rev. A 70, (2004) Chris Greene: new theory Andreas Wolf: TSR results
Back to the Interstellar Clouds! Steady State: [H 3 + ] = keke [e - ] [H 2 ] To increase the value of [H 3 + ], we need: Smaller electron fraction [e - ]/[H 2 ] Smaller recombination rate constant k e Higher ionization rate
Implications for Persei [H 3 + ] L = keke N(e - ) N(H 2 ) = = L N(H 3 + ) N(H 2 ) N(e - ) L = 8000 cm s -1 keke N(H 3 + ) (2.6 cm 3 s -1 ) (8 cm -2 ) (3.8 ) Adopt =3 s -1 Adopt L=2.1 pc L = 85 pc n = 6 cm -3 =1.2 s -1 (40x higher!) (solid)
What Does This Mean? Enhanced ionization rate in Persei Widespread H 3 + in diffuse clouds –perhaps widespread ionization enhancement? Dense cloud H 3 + 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
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-H 3 + using para-H 2 More observational data! –Search for H 3 + in more diffuse cloud sightlines Confirm generality of result in classical diffuse clouds –Observations of H 3 + in "translucent" sightlines C + → C → CO
Rich Diffuse Cloud Chemistry From 1930s through the mid-1990s, only diatomic molecules thought to be abundant in diffuse clouds Recently, many polyatomics observed: –H 3 + in infrared –HCO +, C 2 H, C 3 H 2, etc. in radio (Lucas & Liszt) –C 3 in near-UV (Maier, et al.) Diffuse Interstellar Bands!
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