importance of diffusion rates in models - lorentz center vasyunin.pdf · importance of diffusion...
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Importance of diffusion rates in models
Anton Vasyunin
29.07.2014
Lorentz Center, Leiden
(MPE, Garching)
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Matter cycle in the Galaxy
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Sketch of a low mass star formation
Shu et al. 1987
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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
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Interstellar organic molecules
Herbst & van Dishoeck (2009)
6 or more atoms, contain carbon
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Formation and Destruction of Molecules
Collisions in gas phase
Desorption Accretion
Surface reactions
Silicate
Based on slide of D.Semenov
Bulk reactions?
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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:
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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.
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Possible mechanisms of grain-surface chemistry
Cuppen et al. (2013)
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Mobility of species on grain surfaces: The Langmuir-Hinshelwood mechanism
accretion desorption
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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
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Reaction-diffusion competition
e.g., Herbst&Millar (2008)
E_b E_a
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Diffusion rates on rough (real?) surfaces
Karssemeijer et al. (2014)
Chang&Herbst (2012)
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Bulk diffusion and ice morphology
Garrod (2013)
How porous is the bulk? What mechanisms provide mobility for species inside the bulk?
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Types of desorption
Thermal Desorption Photodesorption Cosmic Ray-Induced Desorption Reactive Desorption
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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)
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Example 1: Ice composition and formation of COMs in the warm-up phase
Vasyunin & Herbst, ApJ (2013a)
Öberg et al. (2011)
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Example 1: Ice composition and formation of COMs in the warm-up phase
Vasy
unin
& H
erbs
t, A
pJ (
2013
a)
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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
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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
)
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Example 3: isothermal formation of COMs
E_b/E_D = 0.3 E_b/E_D = 0.5
CH3OH
Koren et al., in prep.
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Example 3: isothermal formation of COMs
E_b/E_D = 0.3 E_b/E_D = 0.5
HCOOCH3
Koren et al., in prep.
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Example 3: isothermal formation of COMs
E_b/E_D = 0.3 E_b/E_D = 0.5
CH3OCH3
Koren et al., in prep.
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Example 4: stochastic effects in surface chemistry
Vasyunin et al. (2009)
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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.
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Thank you!