Mixing and Transport of Short-Lived and Stable Isotopes and Refractory Grains in Protoplanetary Disks

TL;DR

This study uses models of marginally gravitationally unstable disks to explain how the early solar system achieved rapid mixing and transport of isotopes and grains, matching meteorite and comet data.

Contribution

It demonstrates that MGU disks can rapidly homogenize heterogeneity and sustain isotope fractionation, providing a plausible mechanism for early solar system isotopic signatures.

Findings

MGU disks achieve large-scale transport within ~10^4 years.

Single-shot heterogeneity reduces to ~1%, matching solar system data.

Continuous injection maintains ~10% heterogeneity, consistent with observations.

Abstract

Analyses of primitive meteorites and cometary samples have shown that the solar nebula must have experienced a phase of large-scale outward transport of small refractory grains as well as homogenization of initially spatially heterogeneous short-lived isotopes. The stable oxygen isotopes, however, were able to remain spatially heterogenous at the $\sim$ 6% level. One promising mechanism for achieving these disparate goals is the mixing and transport associated with a marginally gravitationally unstable (MGU) disk, a likely cause of FU Orionis events in young low-mass stars. Several new sets of MGU models are presented that explore mixing and transport in disks with varied masses (0.016 to 0.13 $M_\odot$) around stars with varied masses (0.1 to 1 $M_\odot$) and varied initial $Q$ stability minima (1.8 to 3.1). The results show that MGU disks are able to rapidly (within $\sim 10^4$ yr) achieve large-scale transport and homogenization of initially spatially heterogeneous distributions of disk grains or gas. In addition, the models show that while single-shot injection heterogeneity is reduced to a relatively low level ($\sim$ 1%), as required for early solar system chronometry, continuous injection of the sort associated with the generation of stable oxygen isotope fractionations by UV photolysis leads to a sustained, relatively high level ($\sim$ 10%) of heterogeneity, in agreement with the oxygen isotope data. These models support the suggestion that the protosun may have experienced at least one FU Orionis-like outburst, which produced several of the signatures left behind in primitive chondrites and comets.