Importance of diffusion rates in models

Anton Vasyunin

29.07.2014

Lorentz Center, Leiden

(MPE, Garching)

Matter cycle in the Galaxy

Sketch of a low mass star formation

Shu et al. 1987

Molecules and ions detected in the ISM (> 160):

Two atoms: AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl NH NO NS NaCl OH PN SO SO+ SiN SiO SiS HF SH Three atoms: C3 C2H C2O C2S CH2 HCN HCO HCO+ HCS+ HOC+ H2O H2S HNC HNO MgCN MgNC N2H+ N2O NaCN OCS SO2 c-SiC2 CO2 NH2 H3

+ SiCN Four atoms: c-C3H l-C3H C3N C3O C3S C2H2 CH2D+? HCCN HCNH+ HNCO HNCS HOCO+ H2CO H2CN H2CS H3O+ NH3 SiC3 Five atoms: C5 C4H C4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N HC2NC HCOOH H2CHN H2C2O H2NCN HNC3 SiH4 H2COH+ Six atoms: C5H C5O C2H4 CH3CN CH3NC CH3OH CH3SH HC3NH+ HC2CHO HCONH2 l-H2C4 C5N Seven atoms: C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 c-C2H4O CH2CHOH C7

–(?)

Eight atoms: CH3C3N HCOOCH3 CH3COOH C7H H2C6 CH2OHCHO Nine atoms: CH3C4H CH3CH2CN (CH3)2O CH3CH2OH HC7N C8H Ten atoms: CH3C5N? (CH3)2CO NH2CH2COOH? Eleven atoms: HC9N Thirteen atoms: HC11N

Interstellar organic molecules

Herbst & van Dishoeck (2009)

6 or more atoms, contain carbon

Formation and Destruction of Molecules

Collisions in gas phase

Desorption Accretion

Surface reactions

Silicate

Based on slide of D.Semenov

Bulk reactions?

Key roles of grain-surface chemistry

Formaton of molecular hydrogen: H + H H2 Hydrogenation: CO HCO H2CO H3CO CH3OH CH CH2 CH3 CH4 NH NH2 NH3 Formation of complex organic molecules: HCO + H3CO HCOOCH3

At low temperature:

At moderate temperature:

New models vs. older models: advanced rate-equations and Monte Carlo treatment of gas-grain chemistry

Collisions in gas phase

Desorption

Accretion

Reactive bulk

Core Core

Collisions in gas phase

Desorption Accretion

Inert or reactive bulk

Previous models MAGICKAL, GRAINOBLE, MONACO, microscopic models etc.

Possible mechanisms of grain-surface chemistry

Cuppen et al. (2013)

Mobility of species on grain surfaces: The Langmuir-Hinshelwood mechanism

accretion desorption

Diffusion rates and rates of two-body reactions

- Thermal hopping

- Quantum tunneling for H, H2, O through a rectangular potential barrier

t_diff =Ns/k(diff,tf)

R_ij = K(1/t_diff_i+1/t_diff_j)*N_i*N_j

Ediff = X*Edes

Reaction-diffusion competition

e.g., Herbst&Millar (2008)

E_b E_a

Diffusion rates on rough (real?) surfaces

Karssemeijer et al. (2014)

Chang&Herbst (2012)

Bulk diffusion and ice morphology

Garrod (2013)

How porous is the bulk? What mechanisms provide mobility for species inside the bulk?

Types of desorption

Thermal Desorption Photodesorption Cosmic Ray-Induced Desorption Reactive Desorption

Reactive desorption

Heat of reaction = Σ Heats of formation of reactants - Σ Heats of formation of products

Heat of reaction ejects certain fraction of products to the gas: grX+grY -> grZ + Z(gas)

Dul

ieu

et a

l. (2

013)

Example 1: Ice composition and formation of COMs in the warm-up phase

Vasyunin & Herbst, ApJ (2013a)

Öberg et al. (2011)

Example 1: Ice composition and formation of COMs in the warm-up phase

Vasy

unin

& H

erbs

t, A

pJ (

2013

a)

B1-b, Oeberg et al. 2010

L1689b, Bacmann et al. 2012 B1-b, Cernicharo et al. 2012

Similar abundances of all COMs: 10-10 – 10-11 wrt. H

Example 2: COMs in the cold gas

Example 2: COMs in the cold gas

Vasyunin & Herbst, ApJ (2013b)

Efficient reactive desorption + gas-phase reactions of radiative association

grH+grH2CO grCH3OH (90%) grH+grH2CO CH3OH (10%) – ejected to gas

CH3OH+H3O+ CH3OH2+ + H2O

CH3OH2+ + CH3OH CH3OHCH3

+

CH3OHCH3+ + e- CH3OCH3 + H

n(X)

/n(H

)

Example 3: isothermal formation of COMs

E_b/E_D = 0.3 E_b/E_D = 0.5

CH3OH

Koren et al., in prep.

Example 3: isothermal formation of COMs

E_b/E_D = 0.3 E_b/E_D = 0.5

HCOOCH3

Koren et al., in prep.

Example 3: isothermal formation of COMs

E_b/E_D = 0.3 E_b/E_D = 0.5

CH3OCH3

Koren et al., in prep.

Example 4: stochastic effects in surface chemistry

Vasyunin et al. (2009)

Conclusions

Rates of surface diffusion are one of the key parameters that determine outcome of surface chemistry in models Diffusion rates impact directly or indirectly: - Rates of surface reactions - Rate of reactive (chemical) desorption - Need for stochastic approaches for correct modeling of surface

chemistry

Accurate estimation of diffusion rates for the conditions of astrophysical interest will significantly improve reliability of models.

Thank you!