Chemical modeling of L183 (L134N): an estimate of the ortho/para H${_2}$ ratio

Abstract

<i>Context. <i/>The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H<sub>2<sub/> ratio through the H fractionation.<i>Aims. <i/>We want to constrain the ortho/para H<sub>2<sub/> 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) H<sub>2<sub/>D<sup>+<sup/> abundance to the sole ortho-H<sub>2<sub/>D<sup>+<sup/> column density measurement.<i>Methods. <i/>To constrain this ortho/para H<sub>2<sub/> ratio and derive its profile, we make use of the N<sub>2<sub/>D<sup>+<sup/>/N<sub>2<sub/>H<sup>+<sup/> ratio and of the ortho-H<sub>2<sub/>D<sup>+<sup/> 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.<i>Results. <i/>We estimate the H<sub>2<sub/>D<sup>+<sup/> ortho/para ratio in the L183 cloud, and constrain the H<sub>2<sub/> ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (<i>≈<i/>0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 10<sup>5<sup/> 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.<i>Conclusions. <i/>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 H<sub>2<sub/> ratio should also place useful constraints on the dynamical evolution of prestellar cores.