Chemical modeling of L183 (L134N): an estimate of the ortho/para H-2 ratio
Context. The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H-2 ratio through the H-3(+) fractionation. Aims. We want to constrain the ortho/para H-2 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. Methods. To constrain this ortho/para H-2 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. Results. We estimate the H2D+ ortho/para ratio in the L183 cloud, and constrain the H-2 ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (approximate to 0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 x 105 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. Conclusions. 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-2 ratio should also place useful constraints on the dynamical evolution of prestellar cores.
Astronomy & Astrophysics
"Chemical modeling of L183 (L134N): an estimate of the ortho/para H-2 ratio" (2009). Faculty Bibliography 2000s. 1977.