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