Analysis of OH +, H 2 O +, and H 3 + in a Diffuse Molecular Cloud Toward W51 Nick Indriolo 1, David Neufeld 1, Maryvonne Gerin 2, & Tom Geballe 3 1 – Johns Hopkins University 2 – LERMA, CNRS, Observatoire de Paris & ENS 3 – Gemini Observatory June 20, th International Symposium on Molecular Spectroscopy WH04

Motivation Hydrogen and oxygen chemistry are initiated by cosmic-ray ionization H 3 + traces ionization rate of H 2 OH + & H 2 O + trace ionization rate of H Applicable to different environments Test of consistency between 2 methods

W51 Line of Sight Massive star forming region about 7 kpc away from the sun Bright at THz and μm making it an ideal background source Diffuse foreground material is well separated in velocity from systemic, and thought to be in the local spiral arm

Velocity Components Sonnentrucker et al A&A, 521, L12 foreground W51

Hydrogen Chemistry Formation – CR + H 2  H e - + CR' – H H 2  H H Destruction – H e -  H + H + H Dense Clouds – H CO  HCO + + H 2 – H O  OH + + H 2 Atomic Clouds – H H  H 2 + H + – H e -  H + H CR H2+H2+ H2H2 H2H2 H3+H3+ e-e- CO O e-e- H

IR Observations UKIRT & Gemini South N(H 3 + ) = 2.89×10 14 cm -2 N(CO) = 2.81×10 15 cm -2 N(C + ) = 4.0×10 17 cm -2 T ex = 4.3 K n H ≈ 100 cm -3

Ionization Rate from H 3 + N(H 3 + )N(CH)N(H 2 )nHnH xexe k(H 3 + |e - )ζ2ζ2 (10 14 cm -2 )(10 13 cm -2 )(10 21 cm -2 )(cm -3 )(cm 3 s -1 )( s -1 ) × × CR + H 2 H e -

Oxygen Chemistry CR + H  H + + e - + CR' H + + O  O + + H O + + H 2  OH + + H OH + + H 2  H 2 O + + H H 2 O + + H 2  H 3 O + + H OH + + e -  products H 2 O + + e -  products H 3 O + + e -  products O + + H  H + + O H + + e -  H + hν OH + H2H2 H2O+H2O+ H2H2 H3O+H3O+ CR H H+H+ O O+O+ H2H2 H e-e- e-e- e-e- e-e-

THz Observations HIFI aboard Herschel PRISMAS program N(OH + ) = 2.97×10 13 cm -2 N(o-H 2 O + ) = 4.69×10 12 cm -2 Assume ortho/para = 3 N(H 2 O + ) = 6.25×10 12 cm -2

H 2 Fraction from OH + & H 2 O + OH + + H 2 H 2 O + + H 2 H 2 O + + e - xexe k(OH + |H 2 )k(H 2 O + |H 2 )k(H 2 O + |e - )N(OH + )N(H 2 O + )f H2 (cm 3 s -1 ) (10 13 cm -2 )(10 12 cm -2 ) 1.1× × × × Calculated from N(H) and N(H 2 )

Ionization Rate from OH + & H 2 O + Assuming ζ 2 /2.3 = ζ H /1.5, εζ 2 = 0.32× s -1 CR + H OH + + H 2 OH + + e - N(OH + )N(H)nHnH f H2 k(OH + |H 2 )k(OH + |e - )xexe εζ H (10 13 cm -2 )(10 21 cm -2 )(cm -3 )(cm 3 s -1 ) ( s -1 ) × × ×

Determination of ε From H 3 + : ζ 2 = 4.8× s -1 From OH + & H 2 O + : εζ 2 = 0.32× s -1 Cosmic-ray flux should be roughly the same throughout atomic and molecular parts of the cloud ε = 0.07 (i.e., 7% efficiency in H +  OH + ) Meudon code predicted ε = 0.5−1

Reasons for Low ε Removal of H + by PAH and PAH - can decrease ε to ~ 0.2 (Hollenbach et al. 2012) Current value k(OH + |e - ) = 6.5×10 -8 cm 3 s -1 may be too low Factor of 10 smaller than other DR rate coefficients, and never measured on an absolute scale in a storage ring At 10×k(OH + |e - ), ε = 0.23

Summary First combined analysis of H 3 +, OH +, and H 2 O + in a diffuse cloud H 3 + resides in molecular interior, while OH + & H 2 O + are in atomic outer layers In order for the ionization rate inferred from OH + & H 2 O + to be consistent with ζ 2 = 4.8× s -1 derived from H 3 +, ε must be 0.07, lower than previously thought see Indriolo et al for more details

Acknowledgments Ben McCall & Takeshi Oka John Black, Javier Goicoechea, & Karl Menten PRISMAS consortium