The chemical structure of young high-mass star-forming clumps: (II)
  parsec-scale CO depletion and deuterium fraction of $\rm HCO^+$
  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 parsec-scale CO depletion and $\rm
HCO^+$ 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 $\rm T<20$ 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 found that
(1) the $\rm H_2$ 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 $\rm H^{13}CO^+$ and $\rm
DCO^+$, are anticorrelated; (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 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 ${\sim}8\times10^4$ 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.
