The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse

By D.R.Flower, G. P. D. Forêts, C.M.Walmsley. 2006

151 citations recorded by the source corpus.

Context. We have studied the evolution of molecular gas during the early stages of protostellar collapse, when the freeze-out of “heavy” species on to grains occurs. Aims. In addition to studying the freeze-out of “heavy” species on to grains, we wished to compute the variation of the population densities of the different nuclear spin states of `tracer' molecular ions, such as H 2 D + and D 2 H + , which are currently observed only in their ortho and para forms, respectively. Methods. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly. Nuclear spin-changing reactions have received much less attention in the literature than those leading to deuteration; but, in fact, the former processes are as significant as the latter and often involve the same reactants. A “free-fall” model of gravitational collapse was adopted. Results. We found that the ortho:para ratios of some species, e.g. H 2 D + , vary considerably as the density increases. Because the dynamical timescale is much shorter than some of the chemical timescales, there can be large departures of the predictions of the free-fall model from the steady-state solution at the same density and temperature. In the case of H 2 , it seems unlikely that the steady state value of the ortho:para ratio is attained before protostellar collapse from the progenitor molecular cloud commences. Values of the ortho:para H 2 ratio much higher than in steady state, which would prevail in “young” molecular clouds, are found to be inconsistent with high levels of deuteration of the gas. The internal energy of ortho-H 2 acts as a reservoir of chemical energy which inhibits the deuteration of H$_3^+$ and hence of other species, such as N 2 H + and NH 3 . Conclusions. The principal conclusion is that the degree of deuteration of molecular ions and molecules is sensitive to the ortho:para H 2 ratio and hence to the chemical and thermal history of the precursor molecular cloud.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Organic Chemistry

View Paper: https://www.semanticscholar.org/paper/426aec05e60916aeae9978293c33e6304b98a2a6

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. P. Caselli
2. E. Bergin
3. Cecilia Ceccarelli
4. M. Tafalla
5. K. Oberg
6. A. Crapsi
7. Cfa
8. Oan

In Atomic and Molecular Physics, and Optics

1. E. Bergin
2. M. Tafalla
3. C. Greene
4. K. Oberg
5. Cecilia Ceccarelli
6. P. Caselli
7. M. Walmsley
8. Simon Bruderer

In Organic Chemistry

1. K. Oberg
2. E. Bergin
3. Cecilia Ceccarelli
4. P. Caselli

Citations in the corpus

• Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio (2008, background): “…of various H/D carriers in the chemistry of the trihydrogen cation and isotopologues was first discussed by Pagani et al. (1992) and subsequently expandedin a series of papers by Flower and coworkers (Walmsley et al. 2004, Flower et al. 2004, 2006a,b, hereafter collectively rferred to as FPdFW).”
• Astrochemistry and compositions of planetary systems (2020, background): “This is slightly more complicated as the endothermicity of the back reaction depends on the spin states of the products in particular that of H2 (Pineau des Forets et al. 1991; Pagani et al. 1992; Flower et al. 2006; Sipilä et al. 2015).”
• Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “It is worth emphasizing that the observed form of H2D + in interstellar clouds is ortho-H2D + and models show that the H2D + ortho/para ratio (o/p) will change with density and is a sensitive function of the molecular hydrogen ortho/para ratio (Flower et al. 2006).”
• Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, methodology): “Large deuterium enrichment is a characteristic of the high density nuclei of pre-stellar cores and it is a useful test of our understanding of the chemistry (e.g. Aikawa et al. 2005; Flower et al. 2006) to measure the [NH2D]/[NH3] ratio at high angular resolution.”
• 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).”