H₂ Ortho-to-para Conversion on Grains: A Route to Fast Deuterium Fractionation in Dense Cloud Cores?

Abstract

Deuterium fractionation, i.e., the enhancement of deuterated species with respect to non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-topara H 2 ratio. In this Letter we explore the effect of the ortho-para (o-p) H 2 conversion on grains on the deuteration timescale in fully-depleted dense cores, including the most relevant uncertainties that affect this complex process. We show that (i) the o-p H 2 conversion on grains is not strongly influenced by the uncertainties on the conversion time and the sticking coefficient, and (ii) that the process is controlled by the temperature and the residence time of ortho-H 2 on the surface, i.e., by the binding energy. We find that for binding energies between 330 and 550 K, depending on the temperature, the o-p H 2 conversion on grains can shorten the deuterium fractionation timescale by orders of magnitude, opening a new route for explaining the large observed deuteration fraction D frac in dense molecular cloud cores. Our results suggest that the star formation timescale, when estimated through the timescale to reach the observed deuteration fractions, might be shorter than previously proposed. However, more accurate measurements of the binding energy are needed in order to better assess the overall role of this process.

C-type shock modelling – the effect of new H2–H collisional rate coefficients

“…Other important parameter is the H 2 OPR in the preshock gas. The ortho-H 2 decay time is about 0.5 − 1 Myr at ζ = 10 −17 s −1 and gas density of 10 4 cm −3 , and is in inverse ratio with the cosmic ray ionization rate and the gas density (Pagani et al 2013;Bovino et al 2017;Furuya et al 2019). In molecular gas around young protostars, where the ionization rate is relatively low, the H 2 OPR in the preshock gas may not reach its equilibrium value at the time the protostellar outflow arrives the cloud.…”

The Chemical Structure of Young High-mass Star-forming Clumps. I. Deuteration

“…Moreover, The o/p ratio of H 2 can also be altered by gas phase processes. The initial o/p ratio of H 2 which was adopted by chemical models is subject to large uncertainties (e.g., Pagani et al 2011;Bovino et al 2017). One issue is that we do not know how long it takes for atomic hydrogen to become molecular, which affects the evolution of the o/p ratio, especially under the circumstance when a molecular cloud may have gone through many dispersal-reassembly cycles (with H 2 molecules may mostly be kept intact while other species may be destroyed and reformed, e.g., Chevance et al 2020).…”

“…In the future, such comparative analysis will be applied to the entire sample. More comprehensive chemical modeling taking into account the spin states of H 2 and other relevant species (Hugo et al 2009;Sipilä et al 2010;Kong et al 2015;Bovino et al 2017) and coupling with dynamical evolution (e.g., Goodson et al 2016;Bovino et al 2019) are needed, to understand the feedback of protostellar heating on the depletion efficiency, and therefore to profile the entire evolutionary processes of these sources.…”

On the low ortho-to-para H2 ratio in star-forming filaments

“…Recent experimental works both on amorphous solid water (Ueta et al 2016) and on bare silicates (Tsuge et al 2021), show the efficiency of the ortho-to-para H 2 conversion on the surface of dust grains. To consistently follow this process within our simulations and evaluate its overall impact on the OPR, we employ state-of-the-art frameworks (Bovino et al 2017;Furuya et al 2019) in which the conversion rates in units of s −1 are defined as…”

“…This allows us to include both the ortho-to-para conversion and its inverse process (para-to-ortho) in the H 2 rate equations. However, since the inverse process is, in general, negligible at low temperatures (Bovino et al 2017), we opt for completely neglecting it in our simulations, which in practice corresponds to assuming γ = 0 (Bovino et al 2017). We note that our approach is based on a single average binding energy approximation, in particular E b = 600 K as reported in the literature (Perets & Biham 2006;Vidali & Li 2010;He & Vidali 2014), and refer to other recent works (Furuya et al 2019) for a more comprehensive treatment, which also includes the thermal hopping between different adsorption sites.…”