The Chemical Structure of Young High-mass Star-forming Clumps. II. Parsec-scale CO Depletion and Deuterium Fraction of HCO+

Abstract

Abstract 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 μ m-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 ∼ 10 4 –10 5 cm −3 , we compared pairs of neighboring 70 μ m bright and dark clumps and found that (1) the H 2 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 H 13 CO + and DCO + , are anticorrelated; (3) the abundance ratio between C 18 O and DCO + shows a strong correlation with the source temperature; (4) both the C 18 O 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 × 10 4 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.