Thermalization of interstellar CO Takeshi Oka Department of Astronomy and Astrophysics and Department of Chemistry The Enrico Fermi Institute, University of Chicago Han Xiao and Phillip Lynch Department of Statistics, University of Chicago OSU international symposium on molecular spectroscopy, June 24, 2009 Tomoharu Oka Department of Physics, Keio University : in diffuse clouds near the Galactic center

CO toward the Galactic center Christian Andreas Doppler (3,3) metastable (1,1) ground COH3+H3+ Oka, Geballe, Goto, Usuda, McCall, ApJ 632, 882 (2005) CO GC 14 % 76 % Spiral arms 86 % 24 % H3+H3+

IR absorption versus Radio CO emission NHS 25 NHS 42 IRS1W μ 0 = Debye μ 1-0 = Debye μ 2-0 = Debye Optical depth τ ∝ μ 2 n(CO) GCS3-2 Critical density J = 1 → cm -3 J = 3 → 2 3 × 10 4 cm -3 Radiation trapping W λ → N(CO)

J Radiative processes B = MHz ≅ K μ = D 7.40 e months 6.36 e days 2.58 e days 7.15 e-7 16 days 3.58 e hours 2.23 e-5 12 hours 1.27 e-5 22 hours T = K J = 1 → J = 2 →

Collisional and radiative processes Detailed balance Steady State Green & Chapman, ApJS 37, 169 (1978) D. R. Flower, J. Phys. B: At. Mol. Opt. Phys. 34, 2731 (2001) Wernli, Valiron, Faure, Wiesenfeld, Jankowski, Swalewicz, A&A, 446, 367 (2006) Schöier, van der Tak, van Dishoeck, Black, A&A 432, 369 (2005) LAMDA Yang, Stancil, Balakrishnan, Forrey, J. Chem. Phys. 124, (2006) CO J’ CO J H2H2H2H2

Polynomial fitting is dangerous Cubic spline FMM n(1)/n(0)

Some combinations are not useful n(1)/n(0)n(2)/n(0)

Some combinations are more useful n(1)/n(0)n(3)/n(2) Spiral arms Galactic C.

Radio CO, CS, HCN, CH emission H 3 + IR abs. Understanding a monster X-rayγ-ray HII region Radio recomb. Fine structure Radio continuum H 2 FIR H 2 IR emission CO IR abs Near IR, FIR Dust emission