3:45 PM - 4:00 PM
[PPS07-20] Oxygen Isotope Evolution of Silicate Dust, H2O, and CO Disk Gas in the Protosolar Disk
Keywords:protoplanetary disk, protosolar disk, oxygen isotope exchange, dust
Introduction: Solar system bodies such as the Earth, asteroids, and a comet are depleted in 16O relative to Sun (McKeegan et al. 2011). This difference is likely due to oxygen isotope exchange of original silicate dust with 16O-enriched CO gas and 16O-depleted water vaper, compared with Sun, owing to the CO self-shielding (Yurimoto & Kuramoto 2004). To achieve the oxygen isotopic compositions of the current Solar system materials, it has been suggested that the oxygen isotope exchange proceeded in a region in which 16O-depleted water vapor is concentrated in the early Solar System. The most plausible region is inside snowline, where water vapor is thought to be enhanced due to evaporation of icy pebbles come from outer disk.
We here model the oxygen isotope exchange between silicate dust, H2O and CO gas moving in protoplanetary disks, focusing on water vapor concentration inside the H2O snowline (Yurimoto & Kuramoto 2004).
Methods: We performed Monte Carlo simulation to track trajectories of silicate dust, H2O, and CO super-particles moving in a steady accretion disk (e.g., Okamoto & Ida 2022) for ~200,000 years. Water vapor freezes outside the snowline and moves inward effectively, while evaporates inside the snowline. Silicate dust is generally well coupled with disk gas, but it sticks onto icy pebbles with a certain probability (C×collision probability between a dust particle and icy pebbles; C=0.01-1) outside the snowline. The disk is heated by viscous heating and irradiative heating. We adopted avis of 10–2 and the steady mass accretion rate Mdot of 10–7 Msun yr–1.
Progresses of two reactions, oxygen isotope exchange of amorphous silicate dust (δ17O, δ18O~–60 ‰) with water vapor (δ17O, δ18O~180 ‰) and CO gas (δ17O, δ18O~–220 ‰), are calculated based on the JMA equation using experimentally-determined isotope exchange kinetics(Yamamoto et al. 2018; Yamamoto, Ishizaki et al. 2024). Original silicate dust (solar-like composition) exchanges oxygen isotope with water vapor brought effectively by icy pebbles and with accreting CO gas. In the simulation, we assume that water vapor and CO do not exchange oxygen isotope with each other due to slow reaction kinetics and that dust is crystallized at “crystallization line (Ishizaki et al. 2023)” and keeps its oxygen isotope composition at crystallization.
Results & Discussion: Inside the “H2O exchange line (Ishizaki et al. 2023)” (T=677 K), O-isotope exchange reactions are completed, and the three species reach almost the same isotope composition (e.g., δ17O, δ18O~–45‰ at t=180,000 yr). Outside the H2O exchange line, on the other hand, dust is devided into three groups: (I) unreacted dust preserving its original isotope composition, (II) dust reacted mostly with CO gas having 16O-poorer compositions than Sun, and (III) dust having the same O-isotope composition as those inside the H2O exchange line.
Water vapor was enhanced by ~25‰ inside snowline in the early phase, but it tended to be reduced because it reaches value determined by boundary condition in a steady state. Besides, dust particles in group I did not achieve “Earth-type composition,” δ17&18O~0‰, even in the early phase. We found that enhancement of ice ratio of icy pebbles enriches water vapor inside snowline effectively. In the case where ice ratio of pebbles is enhanced by a factor of 1.5 in mass, dust particles in group (III) can achieve δ17&18O~0‰.
However, in the steady state, it will reach a lower value than Earth-type. Considering this, we must consider evolution of a protoplanetary disk to constrain conditions and regions in which dust can achieve Earth-type composition as the next step. We especially focused on an inside-out evolving disk model, where dust particles are transported from inner region to outer region effectively, due to infall from a molecular cloud. In this presentation, we will also discuss results of the new model
We here model the oxygen isotope exchange between silicate dust, H2O and CO gas moving in protoplanetary disks, focusing on water vapor concentration inside the H2O snowline (Yurimoto & Kuramoto 2004).
Methods: We performed Monte Carlo simulation to track trajectories of silicate dust, H2O, and CO super-particles moving in a steady accretion disk (e.g., Okamoto & Ida 2022) for ~200,000 years. Water vapor freezes outside the snowline and moves inward effectively, while evaporates inside the snowline. Silicate dust is generally well coupled with disk gas, but it sticks onto icy pebbles with a certain probability (C×collision probability between a dust particle and icy pebbles; C=0.01-1) outside the snowline. The disk is heated by viscous heating and irradiative heating. We adopted avis of 10–2 and the steady mass accretion rate Mdot of 10–7 Msun yr–1.
Progresses of two reactions, oxygen isotope exchange of amorphous silicate dust (δ17O, δ18O~–60 ‰) with water vapor (δ17O, δ18O~180 ‰) and CO gas (δ17O, δ18O~–220 ‰), are calculated based on the JMA equation using experimentally-determined isotope exchange kinetics(Yamamoto et al. 2018; Yamamoto, Ishizaki et al. 2024). Original silicate dust (solar-like composition) exchanges oxygen isotope with water vapor brought effectively by icy pebbles and with accreting CO gas. In the simulation, we assume that water vapor and CO do not exchange oxygen isotope with each other due to slow reaction kinetics and that dust is crystallized at “crystallization line (Ishizaki et al. 2023)” and keeps its oxygen isotope composition at crystallization.
Results & Discussion: Inside the “H2O exchange line (Ishizaki et al. 2023)” (T=677 K), O-isotope exchange reactions are completed, and the three species reach almost the same isotope composition (e.g., δ17O, δ18O~–45‰ at t=180,000 yr). Outside the H2O exchange line, on the other hand, dust is devided into three groups: (I) unreacted dust preserving its original isotope composition, (II) dust reacted mostly with CO gas having 16O-poorer compositions than Sun, and (III) dust having the same O-isotope composition as those inside the H2O exchange line.
Water vapor was enhanced by ~25‰ inside snowline in the early phase, but it tended to be reduced because it reaches value determined by boundary condition in a steady state. Besides, dust particles in group I did not achieve “Earth-type composition,” δ17&18O~0‰, even in the early phase. We found that enhancement of ice ratio of icy pebbles enriches water vapor inside snowline effectively. In the case where ice ratio of pebbles is enhanced by a factor of 1.5 in mass, dust particles in group (III) can achieve δ17&18O~0‰.
However, in the steady state, it will reach a lower value than Earth-type. Considering this, we must consider evolution of a protoplanetary disk to constrain conditions and regions in which dust can achieve Earth-type composition as the next step. We especially focused on an inside-out evolving disk model, where dust particles are transported from inner region to outer region effectively, due to infall from a molecular cloud. In this presentation, we will also discuss results of the new model