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

Tomohiro C. Yoshida, Hideko Nomura, Takashi Tsukagoshi, Kenji Furuya, Takahiro Ueda

The gas surface density profile of protoplanetary disks is poorly constrained by observations. Since H2 does not radiate efficiently in disks, CO isotopologues and dust emissions are conventionally used as gas mass tracers. However, it is known that the CO/H2 ratio and the dust/gas ratio dramatically change in disks, which causes significant uncertainty in gas surface density. Infrared HD lines are promising, however, no current instrument can observe them. We analyzed Atacama Large Millimeter/Submillimeter Array archival data of the 12CO J=3-2 line in the protoplanetary disk around TW Hya and discovered extremely broad line wings. The broad line wings are due to pressure broadening, which enables us to determine the midplane gas density for the first time directly. The gas surface density at ~5 au from the star is ~10^3 g cm^{-2}, which suggests that the inner region of the disk has enough mass to form a Jupiter-mass planet. We also found a low CO abundance of ~10^{-6} with respect to H2. This implies the conversion of CO to less-volatile species such as CO2.