# Presentation: 352170

## Chemistry in metal-free and metal-poor gas

- Simon Glover
- (AIP)

## Collaborators

- Paul Clark (ITA, Heidelberg)
- Katharina Jappsen (CITA)
- Ralf Klessen (ITA, Heidelberg)
- Mordecai-Mark Mac Low (AMNH)
- Daniel Savin (Columbia)

## The problem of chemical complexity

- Number of possible reactions is large, even in primordial gas
-  To reduce complexity to a managable level, focus on dominant  processes regulating major coolants

## Primordial coolants: H2

-  At low densities, forms primarily via H-
- H + e-  H- + 
- H- + H  H2 + e-
-  At high densities, forms by three-body reactions
- H + H + H  H2 + H
- H + H + H2  H2 + H2

- In hot gas, destroyed by collisions with H, H2,
-    H+ and e- :
- H2 + H  H + H + H
- H2 + H2  H + H + H2
- H2 + H+  H2+ + H
- H2 + e-  H + H + e
- Also photodissociated by Lyman-Werner band photons:
- H2 +   H + H

## Primordial coolants: HD

- Chemistry dominated by:
- H2 + D+  HD + H+
- HD + H+  H2 + D+
- Inverse reaction endothermic by ≈ 500K, so at low temperatures, get significant chemical fractionation:
- [HD] / [H2] >> [D] / [H]

- HD dominates cooling at T < 150 K
- Best way to reach low T is to form large amounts of H2
- Regions cooling from ionized ICs form large amounts of H2
-  Hence: in general, HD cooling dominates only in regions that were once ionized

## Slide 8

## Slide 9

## Other primordial coolants

- LiH:
- provides much more cooling per molecule   than H2 or HD
- rapidly destroyed:  LiH + H  Li + H2
- never a significant coolant
-    (Misuzawa, Omuaki & Nishi, 2005)
- H3+:
- marginally effective if the cosmic ray ionization rate is large
-    (Glover & Savin, 2006)

## Uncertain rate coefficients

- Associative detachment of H- by H
- H- + H  H2 + e-
- Mutual neutralization of H- with H+
- H- + H+  H + H
- Both rates have large uncertainties

## Glover, Savin & Jappsen 2006

## GSJ06

## GSJ06

## GSJ06

- Three-body H2 formation:
- H + H + H  H2 + H
- Rates differ by 2-3 orders of magnitude at     T < 1000 K
- Consequences of this are not yet clear...

## Slide 17

## Chemistry at low metallicity

- Major coolants:
-  C, C+, O, Si, Si+, Fe, Fe+   (fine structure)
-  CO, H2O  (ro-vibrational lines)
-  Dust
- More coolants  more complexity; now need 100+ reactions to properly represent chemistry

## Four basic processes

- Ion-neutral exchange reactions:
- O+ + H2  OH+ + H
- Neutral-neutral exchange reactions:
- O + H2  OH + H

- Radiative associations:
- O + H  OH + 
- Three-body reactions:
- O + H + H  OH + H

## When do the metals matter?

- Santoro & Shull 2006

## Jappsen et al, 2007

## Omukai et al, 2005

- Importance of metal-line cooling may depend on environment as well as Z
-    [See also talks by Bromm, Smith
-     & Shull, and poster by Jappsen]
- Dust cooling becomes effective at lower Z than metal-line coolants
-    [See also talks by Omukai, Schneider, poster by Clark]

## t=0

- Clark, Glover & Klessen, 2007

## t=53 yr

## t=233 yr

## t=420 yr

## CGK07

## Summary

- Primordial chemistry:
- basic processes understood
- some important rates remain uncertain
- still much work to do on consequences
- Low metallicity chemistry:
- not yet clear which processes are the most important
- huge uncertainties in some areas (esp. regarding grain surface chemistry)