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Abstract:

Observational constraints on gas surface density in protoplanetary disks are crucial for understanding planet formation. However, directly measuring it is challenging due to the lack of a robust tracer, leading to uncertainties of orders of magnitude even in total disk mass estimates. To address this issue, we propose using the pressure broadening of CO emission lines. Since pressure-broadened CO lines are sensitive to hydrogen volume density, they enable a direct determination of the total gas surface density. We applied this method to three transition disks and successfully constrained their gas surface density profiles for the first time. By comparing these profiles with dust surface density distributions, we directly demonstrate that dust grains are trapped at gas pressure maxima in these disks. Additionally, our estimates of total gas mass exceed the minimum-mass solar nebula, suggesting that further planet formation remains quantitatively feasible.