1. Interstellar Ice Formation on Dust Grains
Qiang Chang
Xinjiang Astronomical Observatory
Chinese Academy of Sciences
FRA2015，Guiyang
July 30, 2015

2. Interstellar Ice in Dense Clouds
Stellar nursery. Composed of gas and dust.
H density: >104 cm-3.
Temperature: K.
Rich chemistry. Almost 200 molecules have been found.
Dust grains are covered by ice.
Water ice is the major component of ice on dust grains.
CO and CO2 are the most abundant species other than water ice (10%-40% of water).
CH4, CH3OH, NH3(2-5%).

3. Gas-Grain Chemistry
Based on a slide of D.Semenov
Ice
Core

4. Ice Formation Chemical Reactions
1. Water: Hydration of O, O2, O3 O + H -> OH OH + H -> H2O O2 + H -> HO2 HO2 + H ->H2O2 H2O2 + H -> OH + H2O O3 + H -> OH + O2 OH + H2 -> H + H2O 2. CH4, NH3, CH3OH: Hydration of C, N and CO respectively. C + H -> CH + H -> CH2 + H -> CH3 + H ->CH4 N + H ->NH + H -> NH2 + H -> NH3 CO + H -> HCO + H -> H2CO + H ->CH2OH -> CH3OH 3. CO2: not very clear yet. CO + OH ->CO2 + H (?)

5. Simulation Methods
Unified microscopic-macroscopic Monte Carlo methods(Chang & Herbst 2012).
Surface Roughness.
Finite size problem of surface chemical kinetics.

6. Simple Microscopic Monte Carlo Simulation

7. Surface/Bulk Model Two Phase Model
Three Phase model(Hasegawa & Herbst 1993)
Inert Bulk
Reactive Bulk
Dust Grain Core
Reactive surface

8. Bulk diffusion
Substitutional
Interstitial

9. Beyond Three Phase Model
1. The ice mantle actually is not inert and can be photodissociated because photons are able to penetrate through ice mantle. (Anderson & van Dishoeck, A&A 2008)
2. Species can move around within ice bulk (bulk diffusion).
3. New surface/ice mantle bulk models have
to be developed.

10. Our Surface Model
Chang & Herbst 2014

11. Results (Ice)

12. Results (Radicals)

13. Results（Compare with Observations)
Observational results are from Oberg et al 2011

14. Conclusions
Major components of interstellar ice are not affected by bulk diffusion and photon penetration.
Interstellar ice is likely to undergo thermal processing.
Radicals frozen in ice are heavily dependent on bulk diffusion and photon penetration.

15. Thank You!