9:30 AM - 9:45 AM
[PCG21-03] Dust Chemical Evolution in a Time-Varying Protoplanetary Disk by Viscous Accretion
Keywords:protoplanetary disk, protosolar disk, dust, carbon depletion
Introduction: Dust particles in protoplanetary disks move by accretion and diffusion, and experience various physical and chemical processes. The chemical evolution of dust in the protosolar disk led to the formation of diverse Solar-System bodies, including Earth, 4.56 Ga.
Ishizaki et al. (2023) discussed progress of chemical reactions of dust particles moving in a steady accretion disk and developed formulas to predict the effective temperatures of irreversible dust chemical reactions. However, Solar-System bodies formed from dust that underwent physical and chemical evolution in a time-evolving disk. In this study, we simulated the temporal change of spatial distribution of dust particles that undergo irreversible chemical reactions in the time-evolving disk. The developed model was then applied to discuss the carbon depletion in the inner Solar System.
Methods: We performed 3D Monte-Carlo simulation that tracks trajectories of dust particles that undergo irreversible chemical reaction in a time-evolving viscous accretion disk, where the disk surface density varies according to a self-similar solution (e.g., Armitage 2020). The disk is heated by viscous heating and irradiative heating in the inner and outer regions, respectively. We did 10 simulations that track 105 dust particles for 106years. The initial particle distribution was determined using the initial disk surface density as a probability distribution. We adopted the turbulent viscosity parameter (avis) of 1.0×10–2, 3.0×10–3, and 1.0×10–3, the initial mass accretion rate near the central star (Mdot) of 10–7 and 10–8 Msun/yr, and the initial disk radius (Rdisk) of 20 and 50 au, respectively, as physical parameters of the disk.
As an irreversible dust chemical reaction, we explored thermal decomposition of refractory organics (Li et al. 2021). The kinetic parameters for thermal decomposition of kerogen (Chyba et al. 1990), expressed as an Avrami equation, was used to simulate the dust chemical reaction.
Results & Discussion: The maximum temperatures that refractory organic dust particles experience before complete decomposition ranges from 500 to 600 K with a peak at 540-550 and 570-590 K for avis of 1.0×10–2 and 1.0×10–3, respectively. This suggests that the sootline, where refractory organic carbon sublimates, is located at ~540-590 K in protoplanetary disks. This temperature estimate is in good agreement with that calculated with the formula of Ishizaki et al. (2023) for a steady-state accretion disk using the initial Mdot as a parameter. We thus conclude that the prediction formula for effective reaction temperature by Ishizaki et al. (2023) is applicable to various irreversible chemical reactions of dust occurring in time-varying viscous accretion disks.
Our simulations show that, in the disk with avis=1.0×10−3, all the dust particles at 1 au lose refractory organic carbon after 1 Myrs. On the other hand, 70 % of dust particles contain almost unreacted refractory organic carbon after 1 Myrs in the disk with avis=1.0×10−2. If the initial dust particles contain refractory organic carbon with the C/Si ratio close to Ryugu (Yokoyama et al. 2022), the avis of 1.0×10−2 case suggests that solid components at 1 au retain organic carbon with the abundance ratio comparable to carbonaceous chondrites. The C/Si ratio similar to the bulk Earth (Okamoto and Ida 2024) can be reproduced for dust at 1 au after 1 Myrs when avis=3.0×10−3.
Ishizaki et al. (2023) discussed progress of chemical reactions of dust particles moving in a steady accretion disk and developed formulas to predict the effective temperatures of irreversible dust chemical reactions. However, Solar-System bodies formed from dust that underwent physical and chemical evolution in a time-evolving disk. In this study, we simulated the temporal change of spatial distribution of dust particles that undergo irreversible chemical reactions in the time-evolving disk. The developed model was then applied to discuss the carbon depletion in the inner Solar System.
Methods: We performed 3D Monte-Carlo simulation that tracks trajectories of dust particles that undergo irreversible chemical reaction in a time-evolving viscous accretion disk, where the disk surface density varies according to a self-similar solution (e.g., Armitage 2020). The disk is heated by viscous heating and irradiative heating in the inner and outer regions, respectively. We did 10 simulations that track 105 dust particles for 106years. The initial particle distribution was determined using the initial disk surface density as a probability distribution. We adopted the turbulent viscosity parameter (avis) of 1.0×10–2, 3.0×10–3, and 1.0×10–3, the initial mass accretion rate near the central star (Mdot) of 10–7 and 10–8 Msun/yr, and the initial disk radius (Rdisk) of 20 and 50 au, respectively, as physical parameters of the disk.
As an irreversible dust chemical reaction, we explored thermal decomposition of refractory organics (Li et al. 2021). The kinetic parameters for thermal decomposition of kerogen (Chyba et al. 1990), expressed as an Avrami equation, was used to simulate the dust chemical reaction.
Results & Discussion: The maximum temperatures that refractory organic dust particles experience before complete decomposition ranges from 500 to 600 K with a peak at 540-550 and 570-590 K for avis of 1.0×10–2 and 1.0×10–3, respectively. This suggests that the sootline, where refractory organic carbon sublimates, is located at ~540-590 K in protoplanetary disks. This temperature estimate is in good agreement with that calculated with the formula of Ishizaki et al. (2023) for a steady-state accretion disk using the initial Mdot as a parameter. We thus conclude that the prediction formula for effective reaction temperature by Ishizaki et al. (2023) is applicable to various irreversible chemical reactions of dust occurring in time-varying viscous accretion disks.
Our simulations show that, in the disk with avis=1.0×10−3, all the dust particles at 1 au lose refractory organic carbon after 1 Myrs. On the other hand, 70 % of dust particles contain almost unreacted refractory organic carbon after 1 Myrs in the disk with avis=1.0×10−2. If the initial dust particles contain refractory organic carbon with the C/Si ratio close to Ryugu (Yokoyama et al. 2022), the avis of 1.0×10−2 case suggests that solid components at 1 au retain organic carbon with the abundance ratio comparable to carbonaceous chondrites. The C/Si ratio similar to the bulk Earth (Okamoto and Ida 2024) can be reproduced for dust at 1 au after 1 Myrs when avis=3.0×10−3.