Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying
  deuteration at high angular resolution toward protoplanetary disks
  Deuterium fractionation is dependent on various physical and chemical
parameters. Thus, the formation location and thermal history of material in the
solar system is often studied by measuring its D/H ratio. This requires
knowledge about the deuteration processes operating during the planet formation
era. We aim to study these processes by radially resolving the DCN/HCN (at 0.3"
resolution) and N$_2$D$^+$/N$_2$H$^+$ (0.3 to 0.9") column density ratios
toward the five protoplanetary disks observed by the Molecules with ALMA at
Planet-forming scales (MAPS) Large Program. DCN is detected in all five
sources, with one newly reported detection. N$_2$D$^+$ is detected in four
sources, two of which are newly reported detections. We derive column density
profiles that allow us to study the spatial variation of the DCN/HCN and
N$_2$D$^+$/N$_2$H$^+$ ratios at high resolution. DCN/HCN varies considerably
for different parts of the disks, ranging from $10^{-3}$ to $10^{-1}$. In
particular, the inner disk regions generally show significantly lower HCN
deuteration compared with the outer disk. In addition, our analysis confirms
that two deuterium fractionation channels are active, which can alter the D/H
ratio within the pool of organic molecules. N$_2$D$^+$ is found in the cold
outer regions beyond $\sim$50 au, with N$_2$D$^+$/N$_2$H$^+$ ranging between
$10^{-2}$ and 1 across the disk sample. This is consistent with the theoretical
expectation that N$_2$H$^+$ deuteration proceeds via the low-temperature
channel only. This paper is part of the MAPS special issue of the Astrophysical
Journal Supplement.
