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!