1. On the Formation of Molecules on Interstellar Grains
Qiang Chang
Xinjiang Astronomical Observatory
Chinese Academy of Sciences
July 16, 2014

2. Astrochemistry vs Cosmochemistry
Lis et al, The molecular Universe, 2010

3. Life Cycle of Interstellar Matter

4. Interstellar Clouds Stellar nursery. Composed of gas and dust.
Diffuse clouds.
1. H density: cm-3
2. Temperature: K
3. Simple molecules such as CH, CO and OH have been found.
Dense clouds.
1. H density: >104 cm-3
2. Temperature: K
3. Almost 200 molecules have been found.

5. Dust Grains
1% of the mass in a typical molecular cloud is made up of dusts.
Composed of silicates or carbon.
Formation sites of some important molecules such as H2, complex organic molecules.
Typically covered by thick layer of ice in cold dark clouds.

6. Dust Grains Desorption Accretion (UV, X-ray, cosmic rays)
Bulk
Reactive surface
Gas-phase reactions, accretion & desorption (UV, cosmic rays, collision?), surface chemistry (thermal hopping but unlikely quanum tunneling)
Core
Surface reaction
(thermal hopping, tunneling)
Based on a slide of D.Semenov

7. Dust Surface

8. Molecular Hydrogen Formation in Diffuse Clouds
The molecular hydrogen formation efficiency in gas phase is too low to explain observations.
Formation efficiency on dust grain surface is high enough, however, the temperature range is too narrow.
Stronger binding sites have to be introduced.

9. Physical Processes

10. A Math Model
Instead of keeping the desorption energy of H fixed, we introduced a distribution of the atomic H desorption energy(Chang, Cuppen and Herbst, A&A 2005).
A new simulation method, microscopic Monte Carlo simulation is used.

11. Results Single Desorption Energy .
Exponential and Gaussian Distribution.

12. Interstellar Ice in Dense Clouds
Observation
Simulation
Dust grains in dense clouds are covered by ice mantle.
The major component of interstellar ice is water.
CO and CO2: 10-40%
CH3OH, NH3, CH4: 2-5%

13. Ice Formation
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 (?)

14. CO2 Formation
CO2 is one of the major species in the ice mantle that covers dust grains in dense clouds. However, it was also difficult to explain how it is formed.
CO + OH -> CO2 + H
Most astrochemical models (two phase) use very low diffusion barriers to produce CO2 on grain surface.
Multilayer astrochemical models keep track of the position of OH and CO to produce CO2 .
(Chang, Cuppen & Herbst, A&A 2007,
Garrod, ApJ, 2011,
Chang & Herbst ApJ, 2012)

15. Two Phase Model vs Multilayer Model :
Collisions in gas phase
Collisions in gas phase
Desorption
Accretion
Desorption
Accretion
Core
Core
Reactive surface
Inert bulk
Reactive bulk
Previous models
Our model

16. CO2 Formation Mechanism in Multilayer ModelsOH CO
CO2

17. Complex Organic Molecules
Organic molecules that contain 6 atoms or more(COMs).
Have been found from different astronomical sources, in both gas phase and ice mantle on dust grains. Unfortunately, so far they are not found in protoplanetary disks, although simple molecules such as HCN and C3H2 have been found.
Good probes of the physical condition and history of astronomical sources. Some of them may help to explain the origin of life.

18. COMs in Gas Phase
Herbst & van Dishoeck, ARAA, 2009

19. Formation of Organic Molecules
Some molecules, such as HC9N can be synthesized in gas phase directly. Astrochemical models agree well with observations(Quan & Herbst, A&A, 2007, Smith, Herbst & Chang, MNRAS,2004).
More saturated species such as methanol have to be produced on dust grain surfaces.
Dissociative recombination?
CH3+ + H2O -> CH3OH2+
CH3OH2+ + e- -> CH3OH + H
However, no support from laboratory experiment. (Luca, Voulot & Gerlich, 2002, WDS’02 Proc. Contributed paper)

20. Formation Routes of Zeroth Generation Organic Molecules on Cold Grains
Charnley, 2001, the Bridge between the big bang and biology

21. Formation Routes of First Generation COMs on Warmer Grains
Rely on the zeroth generation organic molecules such as methanol that were formed on colder dust grains.
Zeroth generation organic molecules are photodissociated to produce radicals. These radicals will recombine to form first generation complex organic molecules.
H2CO ->HCO + H
CH3OH ->CH3O + H
HCO + CH3O ->HCOOCH3

22. Physical Conditions for First Generation COMs Formation
Temperature must be high enough to desorb fast diffusing and reactive species such as H and O.
Temperature must be high enough so that radicals produced by photodissociation of zeroth generation organic molecules can diffuse on grain surfaces . On the other hand, temperature must be low enough to keep radicals on grain surface .
Most astrochemical models can only produce first generation molecules at temperature between 30 K and 40 K.

23. First Generation COMs in Cold Core L1689 and B1-b
Temperature is around 10 K.
2. CH3OCH3 and HCOOCH3 have been detected. Their fractional abundances are estimated to be in L1689 and in B-1b.
Backmann et al, 2012, A&A
Cernicharo et al, 2012, APJL

24. First Astrochemical Model to explain CMOs formation in Cold Core
One gas phase reaction was added to chemical reaction network in Vasyunin & Herbst, ApJ, 2013.
CH3 + CH3O -> CH3OCH3
No reaction that form HCOOCH3 was added to chemical reaction because laboratory results show that gas phase formation pathways are not efficient( Horn et al 2004, ApJ).
The abundance of CH3OCH3 was reproduced by the model, however, HCOOCH3 abundance was one order of magnitude lower than observation.

25. CMOs formation in Cold Cores
1. Microscopic Monte Carlo simulation, which automatically keeps track the position of each species on grain surface, is used to perform the calculation.
2. Standard dark cloud physical conditions, T=10K, nH=2*104 cm-3 , is used in the simulation.
3. Three different models with different physical parameters are simulated.
(Chang & Herbst, ApJ, 2014 ).
4. Moderate amount of HCOOCH3 and CH3OCH3 are formed on grain surfaces.

26. COMs formation mechanisms
CH3OCH3
H + (CH2 , CH3O) -> (CH3, CH3O) ->CH3OCH3
HCOOCH3
O + (CH , CH3O) ->(HCO, CH3O) ->HCOOCH3

27. Continue

28. Desorption of Molecules from Grain Surface
Desorption mechanisms.
Thermal desption, photodesorption, cosmos ray desorption, reactive desorption and encounter desorption.
Photodesorption and thermal desorption are included in the simulation, however, we are not able to produce gas phase COMs in our models.
Reactive desorption may be answer. Initial simulation shows that if 10% of products can leave grain surface, gas phase COMs abundances can be reproduced.

29. ALMA

30. Protoplanetary Disks Bergin et al, 2007, Protostars and Planets V.
HH30 in Taurus
By ESO/L. Calcada

31. COMs Formation in Protoplanetary Disks
2D steady state physical model is used. Walsh, et al, A&A 2014.
CMOs can be efficiently formed on grain surafces in disk midplane.
CH3OH should be observed by ALMA.
However, both the physical model and the chemical model are questionable.

32. Conclusions
Dust grains are very important for the formation of molecules in astronomical sources.
Advanced astrochemical models have to be used to simulate the formation of some COMs.

33. Astrochemistry and Star Formation Workshop Urumuqi, August 26-27, 2014
THANK YOU!
Astrochemistry and Star Formation Workshop Urumuqi, August 26-27, 2014