High Atomic Carbon Abundance in Molecular Clouds in the Galactic Center Region

TL;DR

This study maps atomic carbon in the Galactic Center's molecular clouds, revealing high [CI]/13CO ratios in specific regions and proposing mechanisms like cosmic-ray dissociation and early-stage cloud evolution.

Contribution

It provides high-resolution [CI] mapping of the CMZ, identifying distinct molecular cloud groups and analyzing their chemical properties with LVG modeling, highlighting high atomic carbon abundance.

Findings

High [CI]/13CO ratios in specific CMZ regions

Derived C^0/CO ratio of 0.47 in M-0.02-0.07

Proposed mechanisms include cosmic-ray dissociation and early cloud evolution

Abstract

This letter presents a Nyquist-sampled, high-resolution [CI] 3P1-3P0 map of the -0.2 deg < l < 1.2 deg x -0.1 deg < b < 0 deg region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [CI]/13CO intensity ratios: a bulk component consisting with clouds with a low, uniform [CI]/13CO ratio (0.45) and another component consisting of clouds with high [CI]/13CO ratios (> 0.8). The [CI]-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 (LVG) analysis and have derived the C^0/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 C^0 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.