H${}_2$ 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 the non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-to-para (o-p) H${}_2$ ratio. In this letter we explore the effect of the 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 in between 330-550 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 to explain the large observed deuteration fraction $D_{\mathrm{frac}}$ 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 to better assess the overall role of this process.

Accepted for publication in ApJ Letters

References in corpus (7)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Simulation of the Formation and Morphology of Ice Mantles on Interstellar Grains
• Survey of ortho-H2D+(1_{1,0}-1_{1,1}) in dense cloud cores
• On the frequency of N2H+ and N2D+
• ATLASGAL-selected massive clumps in the inner Galaxy: I. CO depletion and isotopic ratios
• The Distribution of Ortho-H_2D+(1_1,0 - 1_1,1) in L1544: Tracing the Deuteration Factory in Prestellar Cores
• Grain Surface Reactions in Molecular Clouds: The Effect of Cosmic Rays and Quantum Tunneling

Cited by in corpus (9)

• Cosmic rays in molecular clouds probed by H${}_2$ rovibrational lines -- Perspectives for the James Webb Space Telescope
• Why does ammonia not freeze out in the center of pre-stellar cores?
• The chemical structure of young high-mass star-forming clumps: (II) parsec-scale CO depletion and deuterium fraction of $\mathrm{HC}{\mathrm{O}}^{+}$
• Adsorption of H${}_2$ on Amorphous Solid Water Studied with Molecular Dynamics Simulations
• A timeline for massive star-forming regions via combined observation of o-H${}_2$D${}^{+}$ and N${}_2$D${}^{+}$
• Rapid ortho-to-para nuclear spin conversion of H2 on a silicate dust surface
• The evolution of HCO${}^{+}$ in molecular clouds using a novel chemical post-processing algorithm
• Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogues: Diamond-like Carbon and Graphite
• Singly- and doubly-deuterated formaldehyde in massive star-forming regions