Thermalization of interstellar CO : in diffuse clouds near the Galactic center 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 Tomoharu Oka Department of Physics, Keio University OSU international symposium on molecular spectroscopy, June 24, 2009

CO toward the Galactic center (1,1) ground Christian Andreas Doppler 1803-1853 (3,3) metastable Oka, Geballe, Goto, Usuda, McCall, ApJ 632, 882 (2005) CO H3+ GC 14 % 76 % Spiral arms 86 % 24 %

IR absorption versus Radio CO emission NHS 25 NHS 42 μ0 = 0.112 Debye 150 μ1-0 = 0.106 Debye μ2-0 = 0.0093 Debye 1 IRS1W Optical depth τ ∝ μ2n(CO) Critical density J = 1 → 0 700 cm-3 J = 3 → 2 3 × 104 cm-3 Radiation trapping GCS3-2 Wλ → N(CO)

Radiative processes B = 57635.9687 MHz ≅ 2.766 K μ = 0.112 D J 7 3.58 e-5 7.8 hours B = 57635.9687 MHz ≅ 2.766 K μ = 0.112 D 6 2.23 e-5 12 hours 5 1.27 e-5 22 hours 4 6.36 e-6 1.8 days T = 2.725 K 3 2.58 e-6 4.5 days 0.1512 J = 1 → 0 0.4536 0.0058 J = 2 → 1 0.0092 2 7.15 e-7 16 days 1 7.40 e-8 5.2 months

Collisional and radiative processes Steady State CO J’ CO J H2 Detailed balance 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, 104304 (2006)

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) Galactic C. Spiral arms

Understanding a monster X-ray γ-ray Radio continuum HII region Radio recomb. Fine structure H3+ IR abs. CO IR abs H2 FIR H2 IR emission Radio CO, CS, HCN, CH emission