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

¹ Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, Germany e-mail: emprecht@ph1.uni-koeln.de
² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 4 June 2008
Accepted: 16 September 2008

Abstract

Context. Deuterated ions, especially H₂D⁺ and N₂D⁺, are abundant in cold (~10 K), dense (~10⁵ 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 10⁴.

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, 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.

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. < 0). With decreasing deuterium fraction, the infall signature disappears and 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 ~0.03 at the Class 0/I borderline.