N₂H⁺ deuterium fraction (R_D ≡ N(N₂D⁺)/N(N₂H⁺) column density ratio) obtained from the N₂D⁺ (4–3) and N₂H⁺ (3–2) lines using constant excitation temperature analysis, as a function of H₂ column density, N(H₂), from Herschel data. The different colors represent different molecular cloud complexes: Corona Australis (blue), Ophiuchus (red), Lupus (black). The stars show the cores associated with YSOs.