2019ApJ...885L..39H

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.