First Detection of [C I] ³P₁-³P₀ Emission from a Protoplanetary Disk

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We performed single point [C I] ³P₁-³P₀ and CO J=4-3 observations toward three T Tauri stars, DM Tau, LkCa 15, and TW Hya, using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 qualification model receiver installed on the Atacama Submillimeter Telescope Experiment (ASTE). Two protostars in the Taurus L1551 region, L1551 IRS 5 and HL Tau, were also observed. We successfully detected [C I] emission from the protoplanetary disk around DM Tau as well as the protostellar targets. The spectral profile of the [C I] emission from the protoplanetary disk is marginally single-peaked, suggesting that atomic carbon (C) extends toward the outermost disk. The detected [C I] emission is optically thin and the column densities of C are estimated to be < 10¹⁶ cm⁻² and 10¹⁷ cm⁻² for the T Tauri star targets and the protostars, respectively. We found a clear difference in the total mass ratio of C to dust, M(C)/M(dust), between the T Tauri stars and protostellar targets; the M(C)/M(dust) ratio of the T Tauri stars is one order of magnitude smaller than that of the protostars. The decrease of the estimated M(C)/M(dust) ratios for the disk sources is consistent with a theoretical prediction that the atomic C can survive only in the near surface layer of the disk and C+/C/CO transition occurs deeper into the disk midplane.