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

We have detected [C I] ³P₁-³P₀ emissions in the gaseous debris disks of 49 Ceti and β Pictoris with the 10 m telescope of the Atacama Submillimeter Telescope Experiment, which is the first detection of such emissions. The line profiles of [C I] are found to resemble those of CO(J=3-2) observed with the same telescope and the Atacama Large Millimeter/submillimeter Array. This result 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, the column density of C is evaluated to be (2.2 +/- 0.2) × 10¹⁷ and (2.5 +/- 0.7) × 10¹⁶ cm^-2 for 49 Ceti and β Pictoris, respectively. The C/CO column density ratio is thus derived to be 54 +/- 19 and 69 +/- 42 for 49 Ceti and β Pictoris, respectively. These ratios are higher than those of molecular clouds and diffuse clouds by an order of magnitude. The unusually high ratios of C to CO are likely attributed to a lack of H₂ molecules needed to reproduce CO molecules efficiently from C. This result implies a small number of H₂ molecules in the gas disk; i.e., there is an appreciable contribution of secondary gas from dust grains.