2009A&A...494..623P

The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H₂ ratio through the H₃⁺ fractionation. We want to constrain the ortho/para H₂ ratio across the L183 prestellar core. This is required to correctly describe the deuteration amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H₂D⁺ abundance to the sole ortho-H₂D⁺ column density measurement. To constrain this ortho/para H₂ ratio and derive its profile, we make use of the N₂D⁺/N₂H⁺ ratio and of the ortho-H₂D⁺ observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models. We estimate the H₂D⁺ ortho/para ratio in the L183 cloud, and constrain the H₂ ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (≃0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2x10⁵-years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained. A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H₂ ratio should also place useful constraints on the dynamical evolution of prestellar cores.