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 University of Chicago

Astronomer’s Periodic Table CN O Ne Mg Fe SiS Ar H He

Molecules in Space H 2 H 3 + NH 3 CH 4 C 2 H 4 C 6 HCH 3 C 3 N COH 2 OC 2 H 2 SiH 4 CH 3 CNCH 2 CHCNHCOOCH 3 CH CO 2 H 2 COc-C 3 H 2 C 5 HCH 3 C 2 HC 7 H CH + HCOH 3 O + l-C 3 H 2 C 5 OHC 5 NH 2 C 6 OH HCO + c-C 3 HC 5 CH 3 NCHCOCH 3 CH 3 C 4 H C 2 HOC + l-C 3 HC 4 HCH 3 OHNH 2 CH 3 CH 3 CH 2 CN CN HCNC 3 NC 4 SiCH 3 SHc-C 2 H 4 O(CH 3 ) 2 O CO + C 3 C 3 OCH 2 CNHC 3 NH + CH 3 CH 2 OH NOC 2 OC 3 SHC 3 NHC 2 CHOHC 7 N AlFC 2 SHCCNHC 2 NCHCONH 2 C 8 H AlClCH 2 HCNH + HCOOHl-H 2 C 4 (CH 3 ) 2 CO CPHCS + HNCOH 2 CHNC 5 NHC 9 N CSH 2 SHNCSH 2 C 2 OHC 11 N CSiHNCHOCO + H 2 NCN HClHNOH 2 CNHNC 3 KClMgCNH 2 CSH 2 COH + NHMgNCSiC 3 NSN 2 H + NaClN 2 O PNNaCN SOOCS SO + SO 2 SiNc-SiC 2 SiOC 2 H SiSNH 2 HF 116 molecules...and counting! [updated 05/06/99]

Ion-Neutral Reactions H 3 + is abundantly produced in the interstellar medium through the cosmic-ray ionization of H 2 H 3 + initiates a network of ion-neutral reactions, which is responsible for most observed molecules

Tree of Interstellar Chemistry

T. Oka, PRL 45,531 (1980) What is H 3 + ?  2y 2x  Equilateral triangle structure  No allowed rotational spectrum  No electronic spectrum  Infrared spectrum obtained in 1980 Infrared inactive stretching mode Infrared active degenerate bending mode

Searching for H 3 + United Kingdom Infrared Telescope (UKIRT) Mauna Kea, Hawaii Cooled Grating Spectrometer 4 (CGS4) R ~ 20,000 Nicholas U. Mayall Telescope Kitt Peak, AZ Phoenix Spectrometer R ~60,000

H 3 + in Molecular Clouds McCall, Geballe, Hinkle, & Oka Astrophysical Journal, 522, 338 (1999) Geballe & Oka Nature, 384, 334 (1996) N(H 3 + ) ~ cm -2

H 3 + Chemistry Formation: Destruction: k CO n(H 3 + ) n(CO)  n(H 2 ) Steady State: (~ cm -3 ) Rate:

para-H 3 + H 3 + as a Probe ~ 1 parsec ~ 10 4 –10 5 cm -3 ~ 25–50 K Agreement with canonical dense cloud values confirms ion-neutral reaction scheme. McCall, Geballe, Hinkle, & Oka Astrophysical Journal, 522, 338 (1999) ortho-H 3 + Path Length: Mean Density: Kinetic Temperature:

Galactic Center N para = 5.1(1.7) × cm -2 N ortho = 2.4(1.1) × cm -2 N broad = 17.5(3.9) × cm -2 Geballe, McCall, Hinkle, & Oka Astrophysical Journal, 510, 251 (1999)

Galactic Rotation

About Cyg OB2 #12 Morgan, Johnson, & Roman PASP 66, 85 (1954)  Bill Morgan, 1954  distance ~ 1.7 kpc  L ~ 10 6 L  visual extinction ~ 10 mag  N(  H) ~ 2 × cm -2  no 3.08 µm ice feature  no dense clouds  strong 3.4 µm C-H band  diffuse clouds  C 2 consistent with n ~ few hundred cm -3  Less than 5% of C atoms in CO  long path of diffuse material

Diffuse Cloud H 3 + observed at UKIRT observed at Kitt Peak N para = 2.4(3) × cm -2 N ortho = 1.4(2) × cm -2 Similar column density to dense clouds!! McCall, Geballe, Hinkle, & Oka Science 279, 1910 (1998)

CO Observations N(CO) ~ few ×10 16 cm -2  CO scarce  diffuse  velocity consistent with H 3 +, C 2

Diffuse H 3 + Chemistry k e n(H 3 + ) n(e - ) Formation: Destruction:  n(H 2 ) Steady State: Rate: (~ cm -3 )

H 3 + vs. Density

Long Path, Low Density Observation: N(H 3 + ) = 4 ×10 14 cm -2  L ~ cm ~ 300 pc! Seems unreasonably long... For N(H 2 ) ~ 2 × cm -2 (inferred from visual extinction), ~ 20 cm -3 Model: n(H 3 + ) = 4 ×10 -7 cm -3 Seems unreasonably low... also inconsistent with C 2 and CO observations!

Dalgarno’s Model Several “clumps” with hierarchical structure are invoked to produce H 3 +. In this model, the vast majority of the H 3 + is not associated with CO or C 2 ! “clump” L = 6.7 pc n(H 2 ) = 100 cm -3 H 3 +, OH “cloudlet” L ~ 0.01 pc n = 10 4 C 2 “core” L ~ 1 AU n = 10 6 CO, HCO + Cecchi-Pestellini & Dalgarno, MNRAS, submitted HCO + recently observed with consistent abundance. Scappini, Cecchi-Pestellini, Codella, & Dalgarno, A&A, submitted

Problems Remain N = 2.3(3) × cm -2 N = 2.0(5) × cm -2 Cyg OB2 #12 Cyg OB2 #5 Atmosphere CH 4  Pressure balance between components difficult  Cyg OB2 #5 (2.5 pc away) shows similar H 3 + and C 2  All parameters at extremes to maximize H 3 +  A more general solution would be desirable.

Impact of H 3 + Dense Clouds: Path length H 2 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?