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We have detected [C I] 3P1-3P0 emissions in the gaseous debris disks of 49Ceti and Beta Pictoris with the 10 m telescope of the Atacama SubmillimeterTelescope Experiment, which is the first detection of such emissions. The lineprofiles of [C I] are found to resemble those of CO(J=3-2) observed with thesame telescope and the Atacama Large Millimeter/submillimeter Array. Thisresult suggests that atomic carbon (C) coexists with CO in the debris disks,and is likely formed by the photodissociation of CO. Assuming an optically thin[C I] emission with the excitation temperature ranging from 30 to 100 K, thecolumn density of C is evaluated to be (2.2+-0.2)x10^17 and (2.5+-0.7)x10^16cm^-2 for 49 Ceti and Beta Pictoris, respectively. The C/CO column densityratio is thus derived to be 54+-19 and 69+-42 for 49 Ceti and Beta Pictoris,respectively. These ratios are higher than those of molecular clouds anddiffuse clouds by an order of magnitude. The unusually high ratios of C to COare likely attributed to a lack of H2 molecules needed to reproduce COmolecules efficiently from C. This result implies a small number of H2molecules in the gas disk; i.e., there is an appreciable contribution ofsecondary gas from dust grains.

Detection of submillimeter-wave [C I] emission in gaseous debris disks of 49 Ceti and Beta Pictoris