Deuterium fractionation as a multi-phase component tracer in the
  Galactic Centre
  The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but
its star formation rate is one order of magnitude lower than in the Galactic
disc. This is likely related to the fact that the bulk of the gas in the CMZ is
in a warm ($>$100 K) and turbulent phase with little material in the
pre-stellar phase. We present in this Letter observations of deuterium
fractionation (D/H ratios) of HCN, HNC, HCO$^{+}$, and N$_{2}$H$^{+}$ towards
the CMZ molecular cloud G+0.693-0.027. These observations clearly show, for the
first time, the presence of a colder, denser, and less turbulent narrow
component, with a line width of $\sim$9 km s$^{-1}$, in addition to the warm,
less dense and turbulent broad component with a line width of $\sim$20 km
s$^{-1}$. The very low D/H ratio $\le$6$\times$10$^{-5}$ for HCO$^{+}$ and
N$_{2}$H$^{+}$, close to the cosmic value ($\sim$2.5$\times$10$^{-5}$), and the
high D/H ratios $>$4$\times$10$^{-4}$ for HCN and HNC derived for the broad
component, confirm the presence of high-temperatures deuteration routes for
nitriles. For the narrow component we have derived D/H ratios $>$10$^{-4}$ and
excitation temperatures of $7$ K for all molecules, suggesting kinetic
temperatures $\le$30 K and H$_2$ densities $\ge$5$\times$10$^{4}$ cm$^{-3}$, at
least one order of magnitude larger than for the broad component. The method
presented in this Letter allows to identify clouds on the verge of star
formation, i.e. under pre-stellar conditions, towards the CMZ. This method can
also be used for the identification of such clouds in external galaxies.
