2020ApJ...901..145F

The physical and chemical properties of cold and dense molecular clouds are key to understanding how stars form. Using the IRAM 30 m and NRO 45 m telescopes, we carried out a Multiwavelength line-Imaging survey of the 70 µm-dArk and bright clOuds (MIAO). At a linear resolution of 0.1-0.5 pc, this work presents a detailed study of parsec-scale CO depletion and HCO⁺ deuterium (D-) fractionation toward four sources (G11.38+0.81, G15.22-0.43, G14.49-0.13, and G34.74-0.12) included in our full sample. In each source with T < 20 K and n_H ∼ 10⁴-10⁵ cm^–3, we compared pairs of neighboring 70 µm bright and dark clumps and found that (1) the H₂ column density and dust temperature of each source show strong spatial anticorrelation; (2) the spatial distribution of CO isotopologue lines and dense gas tracers, such as 1-0 lines of H¹³CO⁺ and DCO⁺, are anticorrelated; (3) the abundance ratio between C¹⁸O and DCO⁺ shows a strong correlation with the source temperature; (4) both the C¹⁸O depletion factor and D-fraction of HCO⁺ show a robust decrease from younger clumps to more evolved clumps by a factor of more than 3; and (5) preliminary chemical modeling indicates that chemical ages of our sources are ∼8 x 10⁴ yr, which is comparable to their free-fall timescales and smaller than their contraction timescales, indicating that our sources are likely dynamically and chemically young.