1. 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

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

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

4. Radiative processes B = 57635.9687 MHz ≅ 2.766 K μ = 0.112 DJ 7
3.58 e-5
7.8 hours
B = MHz ≅ K μ = D 6
2.23 e-5
12 hours 5
1.27 e-5
22 hours 4
6.36 e days
T = K 3
2.58 e days
J = 1 →
J = 2 → 2
7.15 e days 1
7.40 e months

5. 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, (2006)

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

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

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

9. 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