First Detection of Submillimeter-wave [ 13 C i] 3 P 1 – 3 P 0 Emission in a Gaseous Debris Disk of 49 Ceti with ALMA

We have detected the submillimeter-wave fine-structure transition (3P1–3P0) of 13C, [13C i], in the gaseous debris disk of 49 Ceti with the Atacama Large Millimeter/submillimeter Array (ALMA). Recently, the [C i] 3P1–3P0 emission has been spatially resolved in this source with ALMA. In this data set, the F = 3/2–1/2 hyperfine component of [13C i], which is blueshifted by 2.2 km s−1 from the normal species line, [C i], has been identified in the outer part of the 49 Ceti disk, thanks to the narrow velocity widths of the gas components. The [C i]/[13C i] line intensity ratio is found to be 12 ± 3, which is significantly lower than the 12C/13C abundance ratio of 77 in the interstellar medium. This result clearly reveals that the [C i] emission is optically thick in 49 Ceti at least partly, as speculated by the previous work. As far as we know, this is the first detection of [13C i] 3P1–3P0 emission at 492 GHz not only in debris disks but also in the interstellar medium.