  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 dataset, the
F=3/2-1/2 hyperfine component of [13C I], which is blue-shifted by 2.2 km/s
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 about
one-fifth of 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.
