Dissociative Recombination of Cold H3+ and its Interstellar ImplicationsT. 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 (wietokrzyska Academy, Poland)A. Paal, F. sterdahl (Manne Siegbahn Laboratory)A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas, M. Larsson (Stockholm University)Ben McCallDepartment of ChemistryDepartment of AstronomyUniversity of llinois at Urbana-Champaign

Astronomer's Periodic Table

H3+: Cornerstone of Interstellar Chemistry

Observing Interstellar H3+Equilateral triangleNo rotational spectrumNo electronic spectrumVibrational spectrum is only probe

Absorption spectroscopy against background or embedded star

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Interstellar Cloud Classification*Diffuse clouds:H H2C C+n(H2) ~ 101103 cm-3[~10-18 atm]T ~ 50 KPhoto: Jose Fernandez GarciaDiffuse atomic cloudsH2 10% (self-shielded)* Snow & McCall, ARAA, 2006 (in prep)Barnard 68 (courtesy Joo Alves, ESO)Dense molecular clouds:H H2C COn(H2) ~ 104106 cm-3T ~ 20 K

H3+ in Dense CloudsR(1,1)uR(1,0)R(1,1)lWavelength ()N(H3+) = 151014 cm-2McCall, Geballe, Hinkle, & OkaApJ 522, 338 (1999)

Dense Cloud H3+ Chemistry [H2]H2 H2+ + e-H2 + H2+ H3+ + Hcosmic rayH3+ + CO HCO+ + H2Rate =FormationDestructionRate = k [H3+] [CO] [H3+]=k[CO]Steady State=(310-17 s-1)(210-9 cm3 s-1)[H2](6700)=10-4 cm-3DensityIndependent!(fast)McCall, Geballe, Hinkle, & OkaApJ 522, 338 (1999)

H3+ as a Probe of Dense CloudsGiven n(H3+) from model, and N(H3+) from infrared observations:path length L = N/n ~ 31018 cm ~ 1 pcdensity n(H2) = N(H2)/L ~ 6104 cm-3temperature T ~ 30 K

Unique probe of cloudsConsistent with expectationsconfirms dense cloud chemistry

Diffuse Molecular Cloud H3+ ChemistryRate = ke [H3+] [e-] [H2]H2 H2+ + e-H2 + H2+ H3+ + Hcosmic rayH3+ + e- H + H2 or 3HRate =FormationDestruction=10-7 cm-3DensityIndependent!103 times smaller than dense clouds!

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

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 ~2 orders of magnitude!!

H3+ toward PerseiCardelli et al. ApJ 467, 334 (1996)Savage et al. ApJ 216, 291 (1977)N(H2) from CopernicusN(C+) from HST[e-]/[H2]not to blameMcCall, et al. Nature 422, 500 (2003)

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

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

Storage Ring MeasurementsVery simple experimentComplete vibrational relaxationControl H3+ e- impact energyRotationally hot ions producedNo rotational cooling in ringCRYRING30 kV900 keV12.1 MeV

Supersonic Expansion Ion SourceSimilar to sources for laboratory spectroscopy in many groupsPulsed nozzle designSupersonic expansion leads to rapid coolingDischarge from ring electrode downstreamSpectroscopy used to characterize ionsSkimmer employed to minimize arcing to ringH2Gas inlet2 atmSolenoid valve

R(1,0)R(1,1)uR(2,2)l33 K151 K(0,0)(2,0)(J,G)probe of temperaturenot detectedH3+ Energy Level Structure

Spectroscopy of H3+ SourceConfirmed that H3+ produced is rotationally cold, as in interstellar mediumInfrared Cavity Ringdown Laser Absorption SpectroscopyMcCall, et al.Nature 422, 500 (2003)

CRYRING ResultsConsiderable amount of structure (resonances) in the cross-sectionke = 2.6 10-7 cm3 s-1Factor of two smallerMcCall, et al.Phys. Rev. A 70, 052716 (2004)Chris Greene: new theoryAndreas Wolf: TSR results

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

Implications for Persei[H3+] L =keN(e-)N(H2)==LN(H3+)N(H2)N(e-) L = 8000 cm s-1ke N(H3+) (2.610-7 cm3 s-1)(81013 cm-2)(3.810-4)Adopt =310-17 s-1Adopt L=2.1 pcL = 85 pcn = 6 cm-3=1.210-15 s-1(40x higher!)(solid)

What Does This Mean?Enhanced ionization rate in PerseiWidespread H3+ in diffuse cloudsperhaps widespread ionization enhancement?Dense cloud H3+ is "normal"enhanced ionization rate only in diffuse cloudslow energy cosmic-ray flux?cosmic-ray self-confinement?no constraints, aside from chemistry!!New chemical models necessaryHarvey LisztFranck Le Petit

Future WorkMore experiments!Improved spectroscopy of ion sourceHigher resolution & higher sensitivityBetter characterization of ro-vib distributionTesting of new (piezo) ion sourceSingle quantum-state CRYRING measurementsproduce pure para-H3+ using para-H2

More observational data!Search for H3+ in more diffuse cloud sightlinesConfirm generality of result in classical diffuse cloudsObservations of H3+ in "translucent" sightlinesC+ C CO

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

AcknowledgementsTakeshi Oka (U. Chicago)Tom Geballe (Gemini)Staff of UKIRT (Mauna Kea)United Kingdom InfraRed TelescopeStaff of CRYRING (Stockholm)Chris Greene (Boulder)Eric Herbst (Ohio State)Mike Lindsay (UNC)Funding:Berkeley: NSF, AFOSRIllinois: NSF, NASA, ACS, Dreyfus