Chemical modeling of L183 (L134N): an estimate of the ortho/para H ratio^*

¹ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: laurent.pagani@obspm.fr
² CESR, 9 avenue du colonel Roche, BP 44348, Toulouse Cedex 4, France e-mail: vastel@cesr.fr
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany e-mail: [hugo;schlemmer]@ph1.uni-koeln.de
⁴ Department of Physics, University of Central Florida, Orlando, 32816, USA e-mail: slavako@physics.ucf.edu
⁵ Department of Physics and JILA, University of Colorado, Boulder, Colorado 80309-0440, USA e-mail: chris.greene@colorado.edu
⁶ Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier, UMR 5571 du CNRS, 414 rue de la Piscine, 38041 Grenoble Cedex 09, France e-mail: [cecilia.ceccarelli;aurore.bacmann]@obs.ujf-grenoble.fr
⁷ Department of physics and astronomy, University College London, Gower street, London WC1E 6BT, UK e-mail: eb@star.ucl.ac.uk
⁸ Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, HI 96720, USA e-mail: peng@submm.caltech.edu

Received: 14 July 2008
Accepted: 6 October 2008

Abstract

Context. The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H₂ ratio through the H fractionation.

Aims. 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.

Methods. 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.

Results. 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 1.5-2 10⁵ 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 H₂ ratio should also place useful constraints on the dynamical evolution of prestellar cores.