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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.