1. Harvey LisztLondon, January 2006 Harvey Liszt NRAO, CHARLOTTESVILLE

2. Harvey LisztLondon, January 2006 Hydrogen-bearing molecules in diffuse ISM

3. Harvey LisztLondon, January 2006

4. Harvey LisztLondon, January 2006 E BV =0.07 mag, N(H)~4x10 20 cm -2

5. Harvey LisztLondon, January 2006 E BV =0.07 mag, N(H)~4x10 20 cm -2

6. Harvey LisztLondon, January 2006 E BV =0.07 mag, N(H)~4x10 20 cm -2

7. Harvey LisztLondon, January 2006 E BV =0.07 mag, N(H)~4x10 20 cm -2

8. Harvey LisztLondon, January 2006

9. Harvey LisztLondon, January 2006 Copernicus

10. Harvey LisztLondon, January 2006

11. Harvey LisztLondon, January 2006 Copernicus N(CH) ~ 2-4x10 13 cm -2 => N(H 2 ) < 10 21 cm -2 But N(H)~7-10x10 21 cm -2

12. Harvey LisztLondon, January 2006 Copernicus N(CO)/N(H) < 1-5x10 -6 N(CH) ~ 2-4x10 13 cm -2 => N(H 2 ) < 10 21 cm -2 But N(H)~7-10x10 21 cm -2

13. Harvey LisztLondon, January 2006

14. Harvey LisztLondon, January 2006 But what about CO emission?

15. Harvey LisztLondon, January 2006 But what about CO emission?

16. Harvey LisztLondon, January 2006 “Getting it” Previously (1974, actually) we learned that H I clouds are full of H 2 In the mid-1990’s we found out that they are full of trace polyatomics (more later) Ca. 1999 - now we learn that diffuse gas has a relatively high abundance of H 3 + DEAL WITH IT! Let’s move on and talk in these terms

17. Harvey LisztLondon, January 2006 Nature makes H 2 … can we?

18. Harvey LisztLondon, January 2006 Nature makes H 2 … can we?

19. Harvey LisztLondon, January 2006 Nature makes H 2 … can we?

20. Harvey LisztLondon, January 2006 Nature makes H 2 … can we? = 70-80K = 1-3,000=> n ~ 12-40/cc

21. Harvey LisztLondon, January 2006 Nature makes H 2 … can we? = 70-80K = 2-3,000=> n ~ 25-40/cc

22. Harvey LisztLondon, January 2006 Nature makes H 2 … can we? = 70-80K = 2-3,000=> n ~ 25-40/cc

23. Harvey LisztLondon, January 2006 Nature makes H 2 … can we?

24. Harvey LisztLondon, January 2006 Nature makes H 2 … can we?

25. Harvey LisztLondon, January 2006 What did that take? Wolfire et al. (1995) heating/cooling –Small grains/PAH heat gas, neutralize atomic ions Spitzer’s (1978) H 2 - formation rate –H 2 forms as H atoms land on large grains –3x10 -18 n(H) n(H I) T K 1/2 cm -3 s -1 –Slights details of grain conditions! Lee et al. (1996) H 2 - shielding factors EQUILIBRIUM with ambient radiation In other words, an exercise in model-building

26. Harvey LisztLondon, January 2006 Chemistry of H 3 + in partly-atomic gas

27. Harvey LisztLondon, January 2006 Chemistry of H 3 + in partly-atomic gas

28. Harvey LisztLondon, January 2006 Chemistry of H 3 + in partly-atomic gas

29. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

30. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

31. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

32. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

33. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

34. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

35. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H

36. Harvey LisztLondon, January 2006 Taking a breather … H 2, H 3 + & HD can be understood in the very simplest terms –Late-time equilibrium of optically transparent, low density, quiescent gas clots –Atomic-ion neutralization by PAH very important –One twist, enhanced low-level ionization of H, H 2 HD needs the kick every bit as much as H 3 + Dense dark molecular gas ain’t needed – or wanted for that matter So what’s the rub?

37. Harvey LisztLondon, January 2006 Taking a breather … H 2, H 3 + & HD can be understood in the very simplest terms –Late-time equilibrium of optically transparent, low density, quiescent gas clots –Atomic-ion eutralization by PAH very important –One twist, enhanced low-level ionization of H, H 2 HD needs the kick every bit as much as H 3 + Dense dark molecular gas ain’t needed – or wanted for that matter So what’s the rub?

38. Harvey LisztLondon, January 2006 H 3 + is no silver bullet Abundances of trace molecules which were unexplained before H 3 + remain unexplained

39. Harvey LisztLondon, January 2006 H 3 + is no silver bullet Abundances of trace molecules which were unexplained before H 3 + remain unexplained N(HCO + )=2.4x10 12 N(H 2 ) ~1x10 21

40. Harvey LisztLondon, January 2006 H 3 + is no silver bullet Abundances of trace molecules which were unexplained before H 3 + remain unexplained CO + H 3 + will not explain HCO + –Instead, HCO + + e  CO O + H 3 + competes with O + + H 2 only for n(H) > few hundred/cc N(HCO + )=2.4x10 12 N(H 2 ) ~1x10 21

41. Harvey LisztLondon, January 2006 How quickly does H 2 form?

42. Harvey LisztLondon, January 2006 How quickly does H 2 form?

43. Harvey LisztLondon, January 2006 How quickly does H 2 form?

44. Harvey LisztLondon, January 2006 How quickly does H 2 form?

45. Harvey LisztLondon, January 2006 How quickly does H 2 form?

46. Harvey LisztLondon, January 2006 How quickly does H 2 form?

47. Harvey LisztLondon, January 2006 How quickly does H 2 form?

48. Harvey LisztLondon, January 2006 How quickly does H 2 form?

49. Harvey LisztLondon, January 2006 How quickly does H 2 form?

50. Harvey LisztLondon, January 2006 How quickly does H 2 form?

51. Harvey LisztLondon, January 2006 Time evolution of H 3 +

52. Harvey LisztLondon, January 2006 Time evolution of H 3 +

53. Harvey LisztLondon, January 2006 Time evolution of H 3 + High abundances of H 3 + and effects of varying  H do not appear until the last minute

54. Harvey LisztLondon, January 2006 Time evolution of H 3 +

55. Harvey LisztLondon, January 2006 Summing up: H 2, H 3 + & HD

56. Harvey LisztLondon, January 2006

57. Harvey LisztLondon, January 2006 Both HD and H 3 + want the same change –Gratifying to see that ionization balance works In diffuse gas ionization balance is EVERYTHING –Confirming contribution of PAH Isn’t this paradoxical, wanting more ionizing radiation? –Is the enhanced low-level H-ionization really  H ? If not, what then? Whatever it is, does it extend beyond diffuse ISM? –35 year history of dense cloud chemistry with low  H »Is it even possible to invert the chemistry to check  H ?

58. Harvey LisztLondon, January 2006 Summing up: epistemology We are left with TWO puzzles –How to understand the trace molecules? H 3 + per se doesn’t help –Why doesn’t the study of hydrogen (which is after all the dominant gas constituent) in its so many forms – H I, D I, H +, H 2, HD, H 3 + – provide more insight? Observation of neutral diffuse gas alone is a closed system, lacking needed information –Answers must come from outside

59. Harvey LisztLondon, January 2006 The end, thanks for having me

60. Harvey LisztLondon, January 2006 H 2, H 3 +, HD &  H