Discovery of Line Pressure Broadening and Direct Constraint on Gas Surface Density in a Protoplanetary Disk

The gas surface density profile of protoplanetary disks is one of the most fundamental physical properties to understand planet formation. However, it is challenging to determine the surface density profile observationally, because the H$_2$ emission cannot be observed in low-temperature regions. We analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) archival data of the \co line toward the protoplanetary disk around TW Hya and discovered extremely broad line wings due to the pressure broadening. In conjunction with a previously reported optically thin CO isotopologue line, the pressure broadened line wings enabled us to directly determine the midplane gas density for the first time. The gas surface density at $\sim5$ au from the central star reaches $\sim 10^3\ {\rm g\ cm^{-2}}$, which suggests that the inner region of the disk has enough mass to form a Jupiter-mass planet. Additionally, the gas surface density drops at the inner cavity by $\sim2$ orders of magnitude compared to outside the cavity. We also found a low CO abundance of $\sim 10^{-6}$ with respect to H$_2$, even inside the CO snowline, which suggests conversion of CO to less volatile species. Combining our results with previous studies, the gas surface density jumps at $r\sim 20$ au, suggesting that the inner region ($3<r<20$ au) might be the magnetorotational instability dead zone. This study sheds light on direct gas-surface-density constraint without assuming the CO/H$_2$ ratio using ALMA.