H$_2$ ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?

TL;DR

This paper investigates how the conversion of ortho- to para-H$_2$ on dust grains influences deuterium fractionation timescales in dense cloud cores, suggesting it can significantly accelerate deuteration and impact star formation timescale estimates.

Contribution

It introduces the role of grain-surface ortho-para H$_2$ conversion in shortening deuteration timescales, a factor previously underexplored in star-forming region models.

Findings

O-P H$_2$ conversion on grains can reduce deuteration timescales by orders of magnitude.

The process is mainly controlled by temperature and H$_2$ binding energy on grains.

More precise measurements of binding energies are needed for accurate modeling.

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.