Effect of Grain Size on Differential Desorption of Volatile Species and on Non-ideal MHD Diffusivity

TL;DR

This study models how grain size distribution affects chemical desorption processes and non-ideal MHD diffusivities in dense molecular clouds, revealing the importance of grain size in chemical abundance and magnetic diffusion.

Contribution

It introduces a reformulated cosmic-ray desorption rate considering grain size variations and analyzes their impact on chemistry and magnetic diffusivities in star-forming regions.

Findings

Larger grains (>0.1 μm) amplify differential desorption of volatile species.

Atomic nitrogen becomes significantly more abundant than CO due to differential desorption.

Small grains (< a few 100 Å) weaken ambipolar diffusion and Hall effect efficiency.

Abstract

We developed a chemical network for modeling the chemistry and non-ideal MHD effects from the collapsing dense molecular clouds to protostellar disks. First, we re-formulated the cosmic-ray desorption rate by considering the variations of desorption rate over the grain size distribution. We find that the differential desorption of volatile species is amplified by the grains larger than 0.1 $\mu$m, because larger grains are heated to a lower temperature by cosmic-rays and hence more sensitive to the variations in binding energies. As a result, atomic nitrogen N is $\sim$2 orders of magnitude more abundant than CO; N$_2$H$^+$ also becomes a few times more abundant than HCO$^+$ due to the increased gas-phase N$_2$. However, the changes in ionization fraction due to freeze-out and desorption only have minor effects on the non-ideal MHD diffusivities. Our chemical network confirms that the very small grains (VSGs: below a few 100 $\AA$) weakens the efficiency of both ambipolar diffusion and Hall effect. In collapsing dense cores, a maximum ambipolar diffusion is achieved when truncating the MRN size distribution at 0.1 $\mu$m, and for a maximum Hall effect, the truncation occurs at 0.04 $\mu$m. We conclude that the grain size distribution is crucial to the differential depletion between CO and N$_2$ related molecules, as well as to the non-ideal MHD diffusivities in dense cores.