Pagani, L. and Vastel, C. and Hugo, E. and Kokoouline, V. and Greene, C. H. and Bacmann, A. and Bayet, E. and Ceccarelli, C. and Peng, R. and Schlemmer, S. (2009) Chemical modeling of L183 (L134N): an estimate of the ortho/para H_2 ratio. Astronomy and Astrophysics, 494 (2). pp. 623-636. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20090923-131950519
- Published Version
See Usage Policy.
Use this Persistent URL to link to this item: http://resolver.caltech.edu/CaltechAUTHORS:20090923-131950519
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) H_2D^+ abundance to the sole ortho-H_2D^+ column density measurement. Methods. To constrain this ortho/para H_2 ratio and derive its profile, we make use of the N_2D^+/N_2H^+ ratio and of the ortho-H_2D^+ 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 H_2D^+ 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 (≈0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 X 10^5 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.
|Additional Information:||© ESO 2009. Received 14 July 2008. Accepted 6 October 2008. We would like to thank D. Flower, G. Pineau des Forêts, M. Walmsley, S. Cazaux and V. Wakelam for fruitful discussions and an anonymous referee for her/his careful reading. Part of this work was supported by the National Science Foundation (NSF) grant AST 05-40882 to the CSO. The authors are grateful to the CSO and JCMT staffs for their support at the telescopes. We would like to thank Samantha Santos for the help provided in numerical calculations of thermal DR rate coefficients. This work has been supported by the NSF under Grants No. PHY-0427460 and PHY-0427376, by an allocation of NERSC and NCSA (project # PHY-040022) supercomputing resources. This work has benefited from research funding from the European Community’s Sixth Framework Programme under RadioNet contract R113CT 2003 5158187.|
|Subject Keywords:||ISM: abundances ISM: clouds ISM: structure astrochemistry ISM: individual objects: L183 molecular processes|
|Official Citation:||Chemical modeling of L183 (L134N): an estimate of the ortho/para H2 ratio L. Pagani, C. Vastel, E. Hugo, V. Kokoouline, C. H. Greene, A. Bacmann, E. Bayet, C. Ceccarelli, R. Peng and S. Schlemmer A&A 494 (2) 623-636 (2009) DOI: 10.1051/0004-6361:200810587|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||23 Sep 2009 21:18|
|Last Modified:||26 Dec 2012 11:25|
Repository Staff Only: item control page