Complex Organic Molecules formation on Interstellar Grains Qiang Chang Xinjiang Astronomical Observatory Chinese Academy of Sciences April 22, 2014

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

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 C 3 H 2 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.

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

Formation of Organic Molecules Some molecules, such as HC 9 N 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? CH H 2 O -> CH 3 OH 2 + CH 3 OH e - -> CH 3 OH + H However, no support from laboratory experiment. (Luca, Voulot & Gerlich, 2002, WDS’02 Proc. Contributed paper)

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 H 2, complex organic molecules. Typically covered by thick layer of ice in cold dark clouds.

7 Dust Grains Bulk Desorption (UV, X-ray, cosmic rays) Accretion Surface reaction (thermal hopping, tunneling) Core Based on a slide of D.Semenov Reactive surface

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

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. H 2 CO ->HCO + H CH 3 OH ->CH 3 O + H HCO + CH 3 O ->HCOOCH 3

Physical Processes

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.

First Generation COMs in Cold Core L Temperature is around 10 K. 2. CH 3 OCH 3 and HCOOCH 3 have been detected. Their fractional bundances are estimated to be Backmann et al, 2012, A&A

First Astrochemical Model to explain CMOs formation in Cold Core One gas phase reaction was added to chemical reaction network in Vasyunin & Herbst, ApJ, CH 3 + CH 3 O -> CH 3 OCH 3 No reaction that form HCOOCH 3 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 CH 3 OCH 3 was reproduced by the model, however, HCOOCH 3 abundance was one order of magnitude lower than observation.

CO 2 Formation CO 2 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 -> CO 2 + H Most astrochemical models (two phase) use very low diffusion barriers to produce CO 2 on grain surface. Multilayer astrochemical models keep track of the position of OH and CO to produce CO 2. (Chang, Cuppen & Herbst, A&A 2007, Garrod, ApJ, 2011, Chang & Herbst ApJ, 2012)

Two Phase Model vs Multilayer Model : Collisions in gas phase Desorption Accretion Reactive bulk Core Collisions in gas phase Desorption Accretion Inert bulk Previous modelsOur model Reactive surface

CO 2 Formation Mechanism in Multilayer Models O CO H OH CO 2

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, n H =2*10 4 cm -3, is used in the simulation. 3. Three different models with different physical parameters are simulated. (Chang & Herbst, accepted for publication in ApJ ). 4. Moderate amount of HCOOCH 3 and CH 3 OCH 3 are formed on grain surfaces.

COMs formation mechanisms CH 3 OCH 3 H + (CH 2, CH 3 O) -> (CH 3, CH 3 O) ->CH 3 OCH 3 HCOOCH 3 O + (CH, CH 3 O) ->(HCO, CH 3 O) ->HCOOCH 3

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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.

Conclusions Some first generation COMs are actually zeroth generation COMs, i.e., COMs that are produced on cold dust grains and do not reply the formation of zeroth generation COMs. Multilayer astrochemical models have to be used to simulate the formation of some COMs.

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