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

Interstellar Ice in Dense Clouds Stellar nursery. Composed of gas and dust. H density: >104 cm-3. Temperature: 10-50 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%).

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

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 (?)

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

Simple Microscopic Monte Carlo Simulation

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

Bulk diffusion Substitutional Interstitial

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.

Our Surface Model Chang & Herbst 2014

Results (Ice)

Results (Radicals)

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

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.

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