The chemical structure of young high-mass star-forming clumps: (II) parsec-scale CO depletion and deuterium fraction of \(\rm HCO^+\)

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\,\(\mu\)m-dArk and bright clOuds (MIAO). At a linear resolution of 0.1--0.5\,pc, this work presents a detailed study of pc-scale CO depletion and \(\rm HCO^+\) deuterium (D-) fractionation toward four sources (G\,11.38+0.81, G\,15.21-0.43, G\,14.49-0.13, and G\,34.74-0.12) included in our full sample. In each source with \(\rm T<20\)\,K and \(n_{\rm H}\rm\sim10^4\)--\(\rm 10^5\,cm^{-3}\), we compared pairs of neighboring 70\,\(\mu\)m bright and dark clumps and find that: (1) The \(\rm H_2\) column density and dust temperature of each source show strong spatial anti-correlation; (2) The spatial distribution of CO isotopologue lines and dense gas tracers such as 1--0 lines of \(\rm H^{13}CO^+\) and \(\rm DCO^+\) are anti-correlated; (3) The abundance ratio between \(\rm C^{18}O\) and \(\rm DCO^+\) shows a strong correlation with the source temperature; (4) Both the \(\rm C^{18}O\) depletion factor and D-fraction of \(\rm HCO^+\) show robust decrease from younger clumps to more evolved clumps by a factor of more than 3; (5) Preliminary chemical modeling indicates chemical ages of our sources are \({\sim}8\times10^4\) 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.