LERMA & UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: firstname.lastname@example.org
2 CESR, 9 avenue du colonel Roche, BP 44348, Toulouse Cedex 4, France e-mail: email@example.com
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: firstname.lastname@example.org
5 Department of Physics and JILA, University of Colorado, Boulder, Colorado 80309-0440, USA e-mail: email@example.com
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: firstname.lastname@example.org
8 Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, HI 96720, USA e-mail: email@example.com
Accepted: 6 October 2008
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.
Key words: ISM: abundances / ISM: clouds / ISM: structure / astrochemistry / ISM: individual objects: L183 / molecular processes
© ESO, 2009