High atomic carbon abundance in molecular clouds in the Galactic center region

This Letter presents a Nyquist-sampled, high-resolution [C I] ³ P ₁-³ P ₀ map of the -02 < l < 12 × -01 < b < 0° region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [C I]/¹³CO intensity ratios: a bulk component consisting of clouds with a low, uniform [C I]/¹³CO ratio (0.45) and another component consisting of clouds with high [C I]/¹³CO ratios (>0.8). The [C I]-enhanced regions appear in M-0.02-0.07, the circumnuclear disk, the 180 pc ring, and the high-velocity compact cloud CO+0.02-0.02. We have carried out a large velocity gradient analysis and have derived the C⁰/CO column density ratio for M-0.02-0.07 as 0.47, which is approximately twice that of the bulk component of the CMZ (0.26). We propose several hypotheses on the origin of high C⁰ abundance in M-0.02-0.07, including cosmic-ray/X-ray dissociation and mechanical dissociation of CO in the pre-existing molecular clouds. We also suggest the possibility that M-0.02-0.07 is a cloud at an early stage of chemical evolution from diffuse gas, which was possibly formed by the bar-induced mass inflow in the Galactic center region.