Oral Presentation

The Molecular Composition of Shadowed Proto-solar Disk Midplanes Beyond the Water Snowline

Author(s): Shota Notsu (RIKEN)

Presenter: Shota Notsu (Star and Planet Formation Laboratory, RIKEN)

The thermal structure in the protoplanetary disk plays an important role in controlling the disk chemical structure. The disk midplane temperature is potentially affected by the disk substructures such as dust traps/rings. The dust depletion beyond the water snowline will cast a shadow (Ohno & Ueda 2021).

In our modeling study (Notsu et al. 2022, ApJ, 936, 188), we adopted a detailed gas-grain chemical reaction network, and investigated the radial gas and ice abundance distributions of dominant carbon-, oxygen-, and nitrogen-bearing molecules in disks with shadow structures beyond the water snowline around a protosolar-like star. We also investigated the dependance of the disk chemical structures on ionisation rates and initial abundances. In shadowed disks, the dust grains at r ~ 3-8 au are predicted to have more than ~ 5-10 times amounts of ices of organic molecules such as H2CO, CH3OH, and NH2CHO, saturated hydrocarbon ices (such as CH4 and C2H6), in addition to H2O, CO, CO2, NH3, N2, and HCN ices, compared with those in non-shadowed disks which are composed mostly H2O, CO2, and NH3 ices. In the shadowed regions, we find that hydrogenation (especially of CO ice) is the dominant formation mechanism of complex organic molecules, rather than radical-radical reactions and gas-phase reactions. The gas-phase N/O ratios show much larger spatial variations than the gas-phase C/O ratios, and thus the N/O ratio is predicted to be a useful tracer of the shadowed region. N2H+ line emission is a potential tracer of the shadowed regions beyond the water snowline in observations with ALMA. We conclude that a shadowed region allows the recondensation of key volatiles onto dust grains, provides a region of chemical enrichment of ices that is much closer to the star than within a non-shadowed disk, and may explain to some degree the trapping of O2 ice in dust grains that formed comet 67P/Churyumov-Gerasimenko. We discuss that in the shadowed disks, Jupiter does not need to have migrated vast distances, and complex organic molecules can be formed in situ rather than being fully inherited from molecular clouds.

In this presentation, we will explain these results of our disk chemical modeling studies (Notsu et al. 2022), and discuss prospects for current and future disk observations with ALMA and ngVLA.