2:00 PM - 2:15 PM
[PPS07-12] Oxygen isotopic evolution of dust in the protosolar disk: 3D Monte Carlo simulation
Keywords:protoplanetary disk, protosolar disk, oxygen isotope composition, dust
Oxygen isotope composition of solar system objects like Earth, Moon, Mars, and comets are depleted in 16O relative to that of the Sun (e.g., McKeegan et al., 2004). This difference has been explained by the oxygen isotopic evolution of original silicate dust having the Solar oxygen isotope composition with disk gas including CO gas enriched in 16O and water vaper depleted in 16O, relative to the Sun, owing to the CO self-shielding effect (Yurimoto & Kuramoto, 2004).
There are experimental studies that focus on the oxygen isotope exchange reaction and kinetics between amorphous silicate dust and disk gas (e.g., H2O, CO) in a protoplanetary disk (e.g., Yamamoto et al., 2018; 2020; under review), but none has connected these kinetics with disk models. However, dust dynamics must be considered for the oxygen isotope evolution of dust in the Solar system because the actual reactions occur on dust particles moving around in a protoplanetary disk, and dust particles that experience the reactions must be transported to outside of the disk.
In this work, we aim to constrain the conditions of the protosolar disk required to explain the oxygen isotopic compositions of the Solar system materials by simulation of progresses of oxygen isotope exchange reactions between disk H2O/CO gas and silicate dust moving in a protoplanetary disk.
We performed 3D Monte Carlo simulation tracking dust particles moving by advection and diffusion in a protoplanetary disk (Ciesla 2010,2011; Okamoto & Ida 2022) and calculated the progress of oxygen isotope exchange between amorphous silicate dust and disk H2O gas and CO gas, respectively. We assumed a steady accretion disk with vertical/radial temperature structure heated by viscous heating proportional to the local spatial gas density (α-viscosity model). We adopted α = 10–2, 10–3, and the accretion rate Mdot = 10–6, 10–7, 10–8 Msun/yr, and 6 disks in total were considered. Dust particles are ~0.1 μm in diameter with forsterite composition (Mg2SiO4) and move completely coupled with disk gas. The oxygen isotope exchange reaction was calculated by the JMA equation using experimentally determined kinetics (Yamamoto et al. 2018, 2020). The similar simulations were done with the oxygen isotope exchange between silicate dust and disk CO gas (Yamamoto et al., under review).
The highest temperature that silicate dust particles experienced before they complete each reaction showed that both oxygen isotope exchange reactions progress and complete in the limited temperature ranges. The temperatures for the oxygen isotope exchange reactions of forsterite with H2O and CO gases are ~650–750 K and ~450–650 K, respectively. It means that dust should have exchanged oxygen isotopes not only with H2O gas but also CO gas in the protosolar disk.
These results imply that the enhancement in the concentration of water vapor relative to CO is required to achieve the oxygen isotope composition of our Solar system. Because the oxygen isotope exchange reactions occur inside the snowline, the evaporation of icy dust or pebbles is likely to be a key factor for the enrichment of water vapor (Yurimoto & Kuramoto, 2004).
Following Ishizaki et al. (2023), we are currently developing a new model including two points: (1) oxygen isotope exchange of amorphous silicate dust with forsterite stoichiometry with both H2O and CO gas simultaneously, and (2) the enhancement of H2O inside the snowline due to accretion of H2O pebbles. The 16O-depleted region can arise inside the snowline line and silicate dust that exchange oxygen isotopes with gas in this region can achieve the composition of “Earth-type dust.”
In this presentation, we will discuss the results of our new model for oxygen isotope exchange between amorphous silicate dust and disk H2O and CO gases.
There are experimental studies that focus on the oxygen isotope exchange reaction and kinetics between amorphous silicate dust and disk gas (e.g., H2O, CO) in a protoplanetary disk (e.g., Yamamoto et al., 2018; 2020; under review), but none has connected these kinetics with disk models. However, dust dynamics must be considered for the oxygen isotope evolution of dust in the Solar system because the actual reactions occur on dust particles moving around in a protoplanetary disk, and dust particles that experience the reactions must be transported to outside of the disk.
In this work, we aim to constrain the conditions of the protosolar disk required to explain the oxygen isotopic compositions of the Solar system materials by simulation of progresses of oxygen isotope exchange reactions between disk H2O/CO gas and silicate dust moving in a protoplanetary disk.
We performed 3D Monte Carlo simulation tracking dust particles moving by advection and diffusion in a protoplanetary disk (Ciesla 2010,2011; Okamoto & Ida 2022) and calculated the progress of oxygen isotope exchange between amorphous silicate dust and disk H2O gas and CO gas, respectively. We assumed a steady accretion disk with vertical/radial temperature structure heated by viscous heating proportional to the local spatial gas density (α-viscosity model). We adopted α = 10–2, 10–3, and the accretion rate Mdot = 10–6, 10–7, 10–8 Msun/yr, and 6 disks in total were considered. Dust particles are ~0.1 μm in diameter with forsterite composition (Mg2SiO4) and move completely coupled with disk gas. The oxygen isotope exchange reaction was calculated by the JMA equation using experimentally determined kinetics (Yamamoto et al. 2018, 2020). The similar simulations were done with the oxygen isotope exchange between silicate dust and disk CO gas (Yamamoto et al., under review).
The highest temperature that silicate dust particles experienced before they complete each reaction showed that both oxygen isotope exchange reactions progress and complete in the limited temperature ranges. The temperatures for the oxygen isotope exchange reactions of forsterite with H2O and CO gases are ~650–750 K and ~450–650 K, respectively. It means that dust should have exchanged oxygen isotopes not only with H2O gas but also CO gas in the protosolar disk.
These results imply that the enhancement in the concentration of water vapor relative to CO is required to achieve the oxygen isotope composition of our Solar system. Because the oxygen isotope exchange reactions occur inside the snowline, the evaporation of icy dust or pebbles is likely to be a key factor for the enrichment of water vapor (Yurimoto & Kuramoto, 2004).
Following Ishizaki et al. (2023), we are currently developing a new model including two points: (1) oxygen isotope exchange of amorphous silicate dust with forsterite stoichiometry with both H2O and CO gas simultaneously, and (2) the enhancement of H2O inside the snowline due to accretion of H2O pebbles. The 16O-depleted region can arise inside the snowline line and silicate dust that exchange oxygen isotopes with gas in this region can achieve the composition of “Earth-type dust.”
In this presentation, we will discuss the results of our new model for oxygen isotope exchange between amorphous silicate dust and disk H2O and CO gases.