11:00 AM - 1:00 PM
[PCG20-P01] Oxygen Isotope Exchange and Crystallization of Amorphous Silicate Dust in the Protosolar Disk: Monte Carlo Simulation
Keywords:protoplanetary disk, numerical, amorphous silicate, crystallization, isotope exchange
Silicate dust that formed meteoritic parent bodies and rocky planets in the Solar System should have been depleted in 16O than that of the Solar System (e.g., McKeegan et al., 2011; Yurimoto et al., 2008). The difference in oxygen isotopic composition requires the oxygen isotopic exchange between silicate dust and 16O-depleted H2O vapor (Yurimoto & Kuramoto, 2004). Yamamoto et al. (2018, 2019) experimentally determined diffusive oxygen isotope exchange kinetics between amorphous Mg silicates and water vapor. They showed that the diffusive isotope exchange rate is faster than the rate of oxygen self-diffusion in crystalline silicates and proposed that the oxygen isotope exchange could have been prohibited for crystalline silicate dust in the protosolar disk. This implies that oxygen isotope exchange should have occurred for amorphous silicate dust prior to its crystallization.
Previous studies for crystallization of silicate dust in protoplanetary disks have assumed instantaneous crystallization above a threshold temperature “annealing line” (e.g., Gail, 2001; Dullemond et al., 2006). Ciesla (2011) performed the Monte Carlo simulation for kinetic crystallization of amorphous silicate dust, radially moving in a steady-state accretion disk. Dichotomy was found in the degree of crystallization; complete crystallization or no crystallization depending on the dust experienced the annealing-line temperature (Ciesla, 2011). However, such kinetic discussion has not yet been made for oxygen isotope exchange of amorphous silicate dust. In this study we aim to examine both reactions of oxygen isotope exchange and crystallization of amorphous silicate dust particles that vertically move in a turbulent disk with the vertical temperature structure.
Following Ciesla(2010), we made Monte Carlo simulations to evaluate vertical trajectories of dust particles in the disk with a gas density of ρg(z)=(Σ/(√(2π)H)exp(-z2/2H2) at 1 au, where Σ is the gas surface density, and H is the local scale-height. The disk is heated by viscous heating that is proportional to the local spatial gas density. The dust particles are 80 nm in diameter and are well coupled to the gas (ρd/ρg=const.). They are released at random locations following Gaussian distribution with average location of z=0, and standard deviation of σ=H. We adopt alpha viscosity with the values of α=10-2 and 10-4. The crystallization and oxygen isotope exchange rates are given as Avrami equations with time constants (τ) and Avrami exponents (n) that are obtained from Yamamoto and Tachibana (2018) and Yamamoto et al. (2018).
Simulation results show that the dust particles experience similar thermal histories at a larger turbulent viscosity (α=10-2) because of vigorous dust movements, which results in uniform progress of crystallization and isotope exchange of dust grains. In the case of the lower turbulent viscosity (α=10-4), dust particles tend to show different degrees of crystallization and isotope exchange because the thermal history of each dust particle is more diverse than for α=10-2. It is also found that the oxygen isotope exchange occurs always faster than crystallization when the disk midplane temperature (Tc) is lower than ~800 K, while some dust may crystallize before completion of isotope exchange when Tc > ~800 K. In the presentation, we will also report preliminary results of crystallization and isotope exchange of amorphous silicate dust moving radially in the steady-state accretion disk.
Previous studies for crystallization of silicate dust in protoplanetary disks have assumed instantaneous crystallization above a threshold temperature “annealing line” (e.g., Gail, 2001; Dullemond et al., 2006). Ciesla (2011) performed the Monte Carlo simulation for kinetic crystallization of amorphous silicate dust, radially moving in a steady-state accretion disk. Dichotomy was found in the degree of crystallization; complete crystallization or no crystallization depending on the dust experienced the annealing-line temperature (Ciesla, 2011). However, such kinetic discussion has not yet been made for oxygen isotope exchange of amorphous silicate dust. In this study we aim to examine both reactions of oxygen isotope exchange and crystallization of amorphous silicate dust particles that vertically move in a turbulent disk with the vertical temperature structure.
Following Ciesla(2010), we made Monte Carlo simulations to evaluate vertical trajectories of dust particles in the disk with a gas density of ρg(z)=(Σ/(√(2π)H)exp(-z2/2H2) at 1 au, where Σ is the gas surface density, and H is the local scale-height. The disk is heated by viscous heating that is proportional to the local spatial gas density. The dust particles are 80 nm in diameter and are well coupled to the gas (ρd/ρg=const.). They are released at random locations following Gaussian distribution with average location of z=0, and standard deviation of σ=H. We adopt alpha viscosity with the values of α=10-2 and 10-4. The crystallization and oxygen isotope exchange rates are given as Avrami equations with time constants (τ) and Avrami exponents (n) that are obtained from Yamamoto and Tachibana (2018) and Yamamoto et al. (2018).
Simulation results show that the dust particles experience similar thermal histories at a larger turbulent viscosity (α=10-2) because of vigorous dust movements, which results in uniform progress of crystallization and isotope exchange of dust grains. In the case of the lower turbulent viscosity (α=10-4), dust particles tend to show different degrees of crystallization and isotope exchange because the thermal history of each dust particle is more diverse than for α=10-2. It is also found that the oxygen isotope exchange occurs always faster than crystallization when the disk midplane temperature (Tc) is lower than ~800 K, while some dust may crystallize before completion of isotope exchange when Tc > ~800 K. In the presentation, we will also report preliminary results of crystallization and isotope exchange of amorphous silicate dust moving radially in the steady-state accretion disk.