1. Benjamin McCall and Takeshi Oka University of Chicago Kenneth H. Hinkle National Optical Astronomy Observatories Thomas R. Geballe Joint Astronomy Centre Observation of Interstellar H 3 +

2. H 3 toward GL2136 36680.8 Å 2 (J=2, K=2) 2 (J=2, K=1) 36685.2 Å ortho para (cm -1 ) +

3. Column Density Peak Area  Column Density (N) # molecules in 1 cm 2 cross section along line of sight [n  number density] dx N para = 2.07 ± 0.18 x  10 14 cm -2 N ortho = 1.60 ± 0.11 x 10 14 cm -2 GL2136

4. Doppler Shift July April GL2136  GL2136

5. Other Detections N para = 3.32 ± 0.54 x  10 14 cm -2 N ortho = 2.20 ± 0.33 x 10 14 cm -2 N para = 1.16 ± 0.21 x  10 14 cm -2 N ortho = 0.58 ± 0.13 x 10 14 cm -2 N para = 1.38 ± 0.21 x  10 14 cm -2 N ortho = 0.71 ± 0.11 x 10 14 cm -2 GL961E MonR2 IRS3 W33A

6. Monoceros R2 N para = 1.38 ± 0.21 x  10 14 cm -2 N ortho = 0.71 ± 0.11 x 10 14 cm -2 MonR2 IRS3 N para = 1.5 ± 0.3 x  10 14 cm -2 N ortho = 0.5 ± 0.2 x 10 14 cm -2 MonR2 IRS2 ? ? ? ?

7. NGC 2024 IRS 2 N para < 0.5 x  10 14 cm -2 N ortho < 0.3 x 10 14 cm -2 (3  limits) ?? Palomar plate Klinglesmith & Hollis (1987)

8. Formation Rate=Destruction Rate (Cosmic Ray Ionization)(Reaction with CO)  n(H 2 )­k n(H 3 ) n(CO) +  -- k n(H 2 ) ------- n(CO) ­ n(H 3 ) + Constant! In all molecular clouds, n(H 3 ) ~ 3 x 10 -5 cm -3 observed to be constant in many varied conditions ~ 6.7 x 10 3 measured ~ 2 x 10 -9 cm 3 s -1 assumed constant ~ 10 -17 s -1 + H 3 Chemistry +

9. Calculations ÷ n(H 3 ) || 3 x 10 -5 + + + +++ This work Other observations (1) Tielens et al. (1991) (2) Lacy et al. (1994)

10. Temperatures N ortho --------= e 2 N para ²E - ----- kT H 3 + H 2  H 2 + H 3 (“proton transfer”)  H 3 is thermalized with H 2 ++ + GL2136 35 ± 4 K W33A 30 ± 6 K MonR2 IRS324 ± 4 K GL 961 E24 ± 5 K (MonR2 IRS2)  (18 ± 4 K) ?

11. Future Work H Survey of other molecular clouds H Variation of H 3 column density? H Comparison with CO temperatures H Observe H 2, CO, HCO + in absorption H Comparison with models of interstellar chemistry +