Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio

By L. Pagani, C. Vastel, E. Hugo, V. Kokoouline, C. Greene, A. Bacmann, E. Bayet, Cecilia Ceccarelli, R. Peng, S. Schlemmer. 2008

146 citations recorded by the source corpus.

Context: The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H2 ratio through the H3+ 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. Appendices A and B are only available in electronic form at http://www.aanda.org

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/609ba628cf50765a34462c0a90ce73e78a11a623

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In Astronomy and Astrophysics

1. P. Caselli
2. M. Tafalla
3. E. Bergin
4. Cecilia Ceccarelli
5. Y. Aikawa
6. C. Walmsley
7. E. Herbst
8. P. Myers

In Atmospheric Science

1. P. Caselli
2. M. Tafalla
3. E. Bergin
4. Cecilia Ceccarelli
5. C. Walmsley
6. Y. Aikawa
7. E. V. van Dishoeck
8. E. Keto

In Spectroscopy

1. P. Caselli
2. Cecilia Ceccarelli
3. E. Bergin
4. M. Tafalla
5. Y. Aikawa
6. E. V. van Dishoeck
7. E. Keto
8. E. Herbst

Citations in the corpus

• Our astrochemical heritage (2012, background): “This reaction cannot proceed from right to left when the kinetic temperature is below 20 K and if a large fraction of H2 molecules is in para form, as expected in cold and dense cores (Flower et al. 2006; Pagani et al. 2009; Troscompt et al. 2009).”
• FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE (2012, result): “Lower ortho:para ratios (expected in regions with a large amount of freeze out; Pagani et al. 2009; Troscompt et al. 2009; Parise et al. 2011) are not consistent with the present observations, unless a higher abundance of water vapor is present in the gas phase at high densities.”