13:45 〜 15:15
[PEM11-P02] The molecular composition and elemental abundance ratios of shadowed proto-solar disk midplanes beyond the water snowline
キーワード：宇宙化学、原始惑星系円盤、惑星大気、スノーライン、分子
The chemical structures of gas and solid dust grains in the protoplanetary disks will decide planetary compositions. Especially, positions of "snowlines" always influence planet formation, since they determine the elemental content of solids and gas at different locations in protoplanetary disks. The thermal structure in the protoplanetary disk plays an important role in controlling the disk chemical structure and snowline positions of various molecules. The disk midplane temperature is potentially affected by the disk substructures such as dust traps/rings which have been found by recent observations with e,g., ALMA. 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 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 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 to explain its atmospheric composition with heavy element enrichment, and complex organic molecules can be formed in situ rather than being fully inherited from molecular clouds.
If the planets acquire their atmospheres from the gas in the shadowed region, they are expected to have super-stellar N/O ratios of >>1, super-stellar C/O ratios of around unity in most cases, and sub-stellar metallicities. In contrast, they are expected to have stellar N/O and C/O ratios and super-stellar metallicities if the planetary atmospheres are efficiently polluted by solid components (including the case of Jupiter).
In this presentation, we will explain these results based on our disk chemical modeling studies, and also discuss prospects for current and future observations of exoplanetary atmospheres (such as JWST and Ariel).
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 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 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 to explain its atmospheric composition with heavy element enrichment, and complex organic molecules can be formed in situ rather than being fully inherited from molecular clouds.
If the planets acquire their atmospheres from the gas in the shadowed region, they are expected to have super-stellar N/O ratios of >>1, super-stellar C/O ratios of around unity in most cases, and sub-stellar metallicities. In contrast, they are expected to have stellar N/O and C/O ratios and super-stellar metallicities if the planetary atmospheres are efficiently polluted by solid components (including the case of Jupiter).
In this presentation, we will explain these results based on our disk chemical modeling studies, and also discuss prospects for current and future observations of exoplanetary atmospheres (such as JWST and Ariel).