# Mass inventory of the giant-planet formation zone in a solar nebula analog

## TL;DR

This study provides the first spatially resolved measurements of bulk gas mass distribution in a protoplanetary disk, offering new insights into planet formation zones and dust growth processes.

## Contribution

It introduces a novel method combining $^{13}$C$^{18}$O and C$^{18}$O observations to directly measure gas mass distribution in a protoplanetary disk.

## Key findings

- Gas mass distribution in the 5-21 AU zone of TW Hya was quantified.
- The gas-to-dust mass ratio was found to be approximately 140.
- The radial gas distribution aligns with a viscous disk model but is flatter than in our Solar System.

## Abstract

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today's Solar System. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks, but this approach suffers from large uncertainties in grain growth and gas-to-dust ratio. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir. Here we present the first partially spatially resolved observations of the $^{13}$C$^{18}$O J=3-2 line emission in the closest protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass distribution. Combining it with the C$^{18}$O J=3-2 emission and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution of 13$^{+8}_{-5}\times$(R/20.5AU)$^{-0.9^{+0.4}_{-0.3}}$ g cm$^{-2}$ in the expected formation zone of gas and ice giants (5-21AU). We find the total gas/millimeter-sized dust mass ratio is 140 in this region, suggesting that at least 2.4M_earth of dust aggregates have grown to >centimeter sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.