The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars

Context. Deuterated ions, especially H2D+ and N2D+, are abundant in cold (similar to 10 K), dense (similar to 10(5) cm(-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 similar or equal to 10(4). Aims. 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. Methods. We observed N2H+ 1-0, N2D+ 1-0, 2-1 and 3-2, (CO)-O-18 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. Results. 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. delta nu < 0). With decreasing deuterium fraction, the infall signature disappears and dv 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. Conclusions. 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 similar to 0.03 at the Class 0/I borderline.