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<p>Atomic carbon ([C<sub>I</sub>]) is a key species in the carbon chemistry of the interstellar medium (ISM). Using the Submillimeter Wave Astronomy Satellite (SWAS), we conduct a [C<sub>I</sub>](<sup>3</sup>P<sub>1</sub> → <sup>3</sup>P<sub>0</sub>) 492 GHz survey covering approximately 4 deg<sup>2</sup> of the L1688 and L1689 regions in the <i>ρ</i> Oph molecular cloud, achieving a spatial resolution of 4.25′. The derived [C<sub>I</sub>] column densities, <i>N</i>([C<sub>I</sub>), range from 4.85 × 10<sup>14</sup> to 6.29 × 10<sup>17</sup> cm<sup>−2</sup>, corresponding to an abundance ratio <i>N</i>([C<sub>I</sub>])/<i>N</i>(H<sub>2</sub>) of 2.24 × 10<sup>−7</sup> to 2.39 × 10<sup>−4</sup>, with a median value of 1.8 × 10<sup>−5</sup>. Combining observations with photodissociation region (PDR) modeling, we find that [C<sub>I</sub>] abundance varies less than that of CO in regions with Ultraviolet intensity <i>G</i><sub>0</sub> &gt; 16 and <i>N</i>(H<sub>2</sub>) &lt; 4.6 × 10<sup>21</sup> cm<sup>−2</sup>, suggesting [C<sub>I</sub>] is a more reliable tracer of molecular hydrogen in low-density, high-radiation environments where the [C<sub>I</sub>]-to-CO transition occurs. Utilizing [C<sub>I</sub>] as a direct H<sub>2</sub> tracer, the CO-dark gas fraction is estimated to be 0.43, meaning that 43% of the total cloud mass will be missed by conventional calculations based on CO observations but can be calibrated by [C<sub>I</sub>] emission. The [C<sub>I</sub>] line widths are systematically broader than those of <sup>13</sup>CO, possibly due to contributions from atomic carbon. These findings provide key insights into Galactic [C<sub>I</sub>] emission and the carbon cycle evolution in the interstellar medium. Future high-sensitivity [C<sub>I</sub>](<sup>3</sup>P<sub>1</sub> → <sup>3</sup>P<sub>0</sub>) surveys with the Chinese Survey Space Telescope (CSST) will significantly advance our understanding of the carbon cycle evolution.</p>

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Large scale mapping of [CI] and the [CI]-to-CO transition in ρ Ophiuchus molecular cloud

  • 期刊论文

  • 发表日期：

    2026年4月29日

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  • Jifeng Xia,
  • Ningyu Tang,
  • Thomas G. Bisbas,
  • Chen Wang,
  • Gan Luo,
  • Sihan Jiao,
  • Xin Lv,
  • Xuejian Jiang,
  • Donghui Quan,
  • Jinzeng Li,
  • Paul F. Goldsmith,
  • Gary A. Fuller,
  • Di Li

摘要

Atomic carbon ([CI]) is a key species in the carbon chemistry of the interstellar medium (ISM). Using the Submillimeter Wave Astronomy Satellite (SWAS), we conduct a [CI](3P1 → 3P0) 492 GHz survey covering approximately 4 deg2 of the L1688 and L1689 regions in the ρ Oph molecular cloud, achieving a spatial resolution of 4.25′. The derived [CI] column densities, N([CI), range from 4.85 × 1014 to 6.29 × 1017 cm−2, corresponding to an abundance ratio N([CI])/N(H2) of 2.24 × 10−7 to 2.39 × 10−4, with a median value of 1.8 × 10−5. Combining observations with photodissociation region (PDR) modeling, we find that [CI] abundance varies less than that of CO in regions with Ultraviolet intensity G0 > 16 and N(H2) < 4.6 × 1021 cm−2, suggesting [CI] is a more reliable tracer of molecular hydrogen in low-density, high-radiation environments where the [CI]-to-CO transition occurs. Utilizing [CI] as a direct H2 tracer, the CO-dark gas fraction is estimated to be 0.43, meaning that 43% of the total cloud mass will be missed by conventional calculations based on CO observations but can be calibrated by [CI] emission. The [CI] line widths are systematically broader than those of 13CO, possibly due to contributions from atomic carbon. These findings provide key insights into Galactic [CI] emission and the carbon cycle evolution in the interstellar medium. Future high-sensitivity [CI](3P1 → 3P0) surveys with the Chinese Survey Space Telescope (CSST) will significantly advance our understanding of the carbon cycle evolution.