1. Unified Microscopic -Macroscopic Monte Carlo Simulation of Ice Formation on Interstellar Dust Grains
Qiang Chang, Eric Herbst
Chemistry department, University of Virginia
June 19, 2013

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

3. Observational studies about Ice on dust grains
1. CO and CO2 are the most abundant species other than water. 10%-40% of water abundance depending on sources.
2. CH4 and NH3 are also abundant. 2%-5%of water abundance.
Öberg et al, Apj 740, 109, 2011

4. Numerical approaches 1. Rate equation.
Efficient and easy to implement. May cause mistake when number of reactive species is small on grain surface and fluctuation is important.
2. Modified rate equation.
Efficient but it can only empirically solve the stochastic problem.
3. Master equation.
Very hard to be generalized to large system. Solves the problem of stochastic effect.
4. Macro Monte Carlo simulation.
Slow but able to solve problem of stochastic effect without spatial information.
5. Moment equation.
Fast. But not always right because of the cutoff of higher moments.
6. Micro Monte Carlo simulation.
Very slow. But because it follows the movement of each species on grain surface, we can simulate any surface processes .

5. Multilayer Approach
Based on a slide by A . Vasyunin

6. Beyond Multilayer Approach
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 bulk.
3. New surface/ice mantle bulk models have
to be developed.

7. Our Surface Model

8. New Algorithm
Surface species are divided into three group based on their diffusion barriers.
Group A: H atoms.
Group B: O, N, C atoms.
Group C: all other species.

9. Gas-Grain Chemical model
1. Gas phase chemical reaction network is from the KIDA database (Administrator: V. Wakelam, published version: Semenov et al. 2010). We use macroscopic Monte Carlo to simulate gas phase chemistry.
2. Surface reactions are from Vasyunin & Herbst, APJ About 300 reactions. We use microscopic Monte Carlo to simulate surface reactions.
3. We simulate three models.
Model 0: Photons can only penetrate top two layers.
Model 1: photons can penetrate into ice mantle until it is absorbed by a species. The diffusion barrier of species in the bulk is 70% of their desorption energy.
Model 2: Similar to model 1. The diffusion barrier of species in the bulk is equal to their desorption energy.

10. Results

11. Results

12. Results

13. Results

14. Conclusions
Bulk diffusion and photon penetration are important for radicals formation on dust grain surfaces.
2. However, they are not important for the formation of major stable species on grain surfaces.

15. Thank you for your attention!