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

Shota Notsu, Kazumasa Ohno, Takahiro Ueda, Catherine Walsh, Christian Eistrup, Hideko Nomura

The disk midplane temperature is potentially affected by disk substructures such as dust traps/rings. The dust depletion beyond the water snowline will cast a shadow. 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. 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. 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 future observations with ALMA and ngVLA. 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, 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.