The chemical structure of young high-mass star-forming clumps: (II) parsec-scale CO depletion and deuterium fraction of $\mathrm{HC}{\mathrm{O}}^{+}$

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 $\hat{I}¼$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 $\mathrm{HC}{\mathrm{O}}^{+}$ 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 $\mathrm{T}<20$ K and $n_{\mathrm{H}}\sim 10^4$--$10^5\mathrm{c}{\mathrm{m}}^{-3}$, we compared pairs of neighboring 70 $\hat{I}¼$m bright and dark clumps and found that (1) the ${\mathrm{H}}_2$ 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 ${\mathrm{H}}^{13}\mathrm{C}{\mathrm{O}}^{+}$ and $\mathrm{DC}{\mathrm{O}}^{+}$, are anticorrelated; (3) the abundance ratio between ${\mathrm{C}}^{18}\mathrm{O}$ and $\mathrm{DC}{\mathrm{O}}^{+}$ shows a strong correlation with the source temperature; (4) both the ${\mathrm{C}}^{18}\mathrm{O}$ depletion factor and D-fraction of $\mathrm{HC}{\mathrm{O}}^{+}$ show a robust decrease from younger clumps to more evolved clumps by a factor of more than 3; and (5) preliminary chemical modeling indicates chemical ages of our sources are $\sim 8\times 10^4$ 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.

accepted by ApJ