2009A&A...493...89E

Deuterated ions, especially H₂D⁺ and N₂D⁺, are abundant in cold (∼10 K), dense (∼10⁵cm^–3) regions, in which CO is frozen out onto dust grains. In such environments, the N₂D⁺/N₂H⁺ ratio can exceed the elemental abundance ratio of D/H by a factor of ≃10⁴. We use deuterium fractionation to investigate the evolutionary state of Class 0 protostars. In particular, we expect the N₂D⁺/N₂H⁺ ratio to decrease as temperature (a sign of the evolution of the protostar) increases. We observed N₂H⁺ 1-0, N₂D⁺ 1-0, 2-1 and 3-2, C¹⁸O 1-0 and HCO⁺ 3-2 in a sample of 20 Class 0 and borderline Class 0/I protostars. We determined the deuteration fraction and searched for correlations between the N₂D⁺/N₂H⁺ ratio and well-established evolutionary tracers, such as T_Dust and the CO depletion factor. In addition, we compared the observational result with a chemical model. In our protostellar sample, the N₂H⁺ 1-0 optical depths are significantly lower than those found in prestellar cores, but the N₂H⁺ column densities are comparable, which can be explained by the higher temperature and larger line width in protostellar cores. The deuterium fractionation of N₂H⁺ in protostellar cores is also similar to that in prestellar cores. We found a clear correlation between the N₂D⁺/N₂H⁺ ratio and evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show the largest deuterium fractionation. Furthermore, we find that sources with a high N₂D⁺/N₂H⁺ ratio show clear indications of infall (e.g. δv<0). With decreasing deuterium fraction, the infall signature disappears and δv tends to be positive for the most evolved objects. The deuterium fractionation of other molecules deviates clearly from that of N₂H⁺. The DCO⁺/HCO⁺ ratio stays low at all evolutionary stages, whereas the NH₂D/NH₃ ratio is >0.15 even in the most evolved objects. The N₂D⁺/N₂H⁺ ratio is known to trace the evolution of prestellar cores. We show that this ratio can be used to trace core evolution even after star formation. Protostars with an N₂D⁺/N₂H⁺ ratio above 0.15 are in a stage shortly after the beginning of collapse. Later on, deuterium fractionation decreases until it reaches a value of ∼0.03 at the Class 0/I borderline.