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Observations of H 3 + The Initiator of Interstellar Chemistry

Observations of H 3 + The Initiator of Interstellar Chemistry. Benjamin McCall Oka Ion Factory University of Chicago Thomas Geballe Gemini Observatory (HI) Kenneth Hinkle National Optical Astronomy Observatories Kitt Peak National Observatory (AZ) Takeshi Oka Oka Ion Factory

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Observations of H 3 + The Initiator of Interstellar Chemistry

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  1. Observations of H3+ The Initiator of Interstellar Chemistry Benjamin McCall Oka Ion Factory University of Chicago Thomas Geballe Gemini Observatory (HI) Kenneth Hinkle National Optical Astronomy Observatories Kitt Peak National Observatory (AZ) Takeshi Oka Oka Ion Factory University of Chicago

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

  3. Molecules in Space H2H3+NH3CH4 C2H4 C6H CH3C3N COH2O C2H2 SiH4 CH3CN CH2CHCN HCOOCH3 CH CO2H2CO c-C3H2 C5H CH3C2H C7H CH+ HCO H3O+ l-C3H2 C5O HC5N H2C6 OHHCO+ c-C3H C5 CH3NC HCOCH3 CH3C4H C2HOC+ l-C3H C4H CH3OH NH2CH3 CH3CH2CN CN HCN C3N C4Si CH3SH c-C2H4O (CH3)2O CO+ C3 C3O CH2CN HC3NH+CH3CH2OH NO C2O C3S HC3N HC2CHO HC7N AlF C2S HCCN HC2NC HCONH2 C8H AlCl CH2 HCNH+HCOOH l-H2C4 (CH3)2CO CP HCS+ HNCO H2CHN C5N HC9N CS H2S HNCS H2C2O HC11N CSi HNC HOCO+ H2NCN HCl HNO H2CN HNC3 KCl MgCN H2CS H2COH+ NH MgNC SiC3 NS N2H+ NaCl N2O PN NaCN SO OCS SO+ SO2 SiN c-SiC2 SiO C2H SiS NH2 HF 116 molecules...and counting! http://www.cv.nrao.edu/~awooten/allmols.html [updated 05/06/99]

  4. Ion-Neutral Reactions H3+ is abundantly produced in the interstellar medium through the cosmic-ray ionization of H2 H3+ initiates a network of ion-neutral reactions, which is responsible for most observed molecules

  5. Tree of Interstellar Chemistry

  6. What is H3+ ?  Equilateral triangle structure  No allowed rotational spectrum  No electronic spectrum  Infrared spectrum obtained in 1980 T. Oka, PRL 45,531 (1980) n2x n1 n2y Infrared inactive stretching mode Infrared active degenerate bending mode

  7. Searching for H3+ Cooled Grating Spectrometer 4 (CGS4) R ~ 20,000 United Kingdom Infrared Telescope (UKIRT) Mauna Kea, Hawaii Nicholas U. Mayall Telescope Kitt Peak, AZ Phoenix Spectrometer R ~60,000

  8. H3+ in Molecular Clouds Geballe & Oka Nature, 384, 334 (1996) N(H3+) ~ 1014 cm-2 McCall, Geballe, Hinkle, & Oka Astrophysical Journal, 522, 338 (1999)

  9. H3+ Chemistry Formation: Rate: n(H2) Destruction: kCO n(H3+) n(CO) Steady State: (~ 10-4 cm-3)

  10. H3+ as a Probe Path Length: ~ 1 parsec Mean Density: ~ 104–105 cm-3 Kinetic Temperature: ortho-H3+ para-H3+ ~ 25–50 K Agreement with canonical dense cloud values confirms ion-neutral reaction scheme. McCall, Geballe, Hinkle, & Oka Astrophysical Journal, 522, 338 (1999)

  11. Galactic Center Npara = 5.1(1.7) × 1014 cm-2 Northo = 2.4(1.1) × 1014 cm-2 Nbroad = 17.5(3.9) × 1014 cm-2 Geballe, McCall, Hinkle, & Oka Astrophysical Journal, 510, 251 (1999)

  12. Galactic Rotation

  13. About Cyg OB2 #12  Bill Morgan, 1954  distance ~ 1.7 kpc  L ~ 106 L  visual extinction ~ 10 mag  N(H) ~ 2 × 1022 cm-2  no 3.08 µm ice feature  no dense clouds  strong 3.4 µm C-H band  diffuse clouds  C2 consistent with n ~ few hundred cm-3  Less than 5% of C atoms in CO  long path of diffuse material Morgan, Johnson, & Roman PASP 66, 85 (1954)

  14. Diffuse Cloud H3+ observed at Kitt Peak observed at UKIRT Npara = 2.4(3) × 1014 cm-2 Northo = 1.4(2) × 1014 cm-2 Similar column density to dense clouds!! McCall, Geballe, Hinkle, & Oka Science 279, 1910 (1998)

  15. CO Observations N(CO) ~ few ×1016 cm-2  CO scarce  diffuse  velocity consistent with H3+, C2

  16. Diffuse H3+ Chemistry Formation: Rate: n(H2) Destruction: ke n(H3+) n(e-) Steady State: (~ 10-6 cm-3)

  17. H3+ vs. Density

  18. Long Path, Low Density Model: n(H3+) = 4 ×10-7 cm-3 Observation: N(H3+) = 4 ×1014 cm-2  L ~ 1021 cm ~ 300 pc! Seems unreasonably long... For N(H2) ~ 2 × 1022 cm-2 (inferred from visual extinction), ~ 20 cm-3 Seems unreasonably low... also inconsistent with C2 and CO observations!

  19. Dalgarno’s Model Cecchi-Pestellini & Dalgarno, MNRAS, submitted “clump” L = 6.7 pc n(H2) = 100 cm-3 H3+, OH “core” L ~ 1 AU n = 106 CO, HCO+ “cloudlet” L ~ 0.01 pc n = 104 C2 Several “clumps” with hierarchical structure are invoked to produce H3+. In this model, the vast majority of the H3+ is not associated with CO or C2! HCO+ recently observed with consistent abundance. Scappini, Cecchi-Pestellini, Codella, & Dalgarno, A&A, submitted

  20. Problems Remain Atmosphere CH4 CH4 Cyg OB2 #12 N = 2.3(3) × 1014 cm-2 N = 2.0(5) × 1014 cm-2 Cyg OB2 #5  Pressure balance between components difficult  Cyg OB2 #5 (2.5 pc away) shows similar H3+ and C2  All parameters at extremes to maximize H3+  A more general solution would be desirable.

  21. Impact of H3+ Dense Clouds: Path length H2 number density Kinetic temperature Confirmation of ion-neutral scheme Diffuse Clouds: Apparently long path lengths? Rich chemistry even in rarefied environment? Insight into Diffuse Interstellar Bands? Probing a new ISM component?

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