Effect of grain size distribution and size-dependent grain heating on molecular abundances in starless and pre-stellar cores

TL;DR

This study introduces a detailed gas-grain chemical model that accounts for grain size distribution and size-dependent heating, revealing complex effects on molecular and ice abundances in starless and pre-stellar cores, with implications for early star formation stages.

Contribution

The paper presents a novel model incorporating grain size dependence for CR desorption and grain temperature, tracking ice on multiple grain populations, to better understand molecular abundances in star-forming regions.

Findings

Ice abundance is highest on small grains for molecules originating in the gas phase.

Certain molecules like HCN are concentrated on large grains, while others like N2 shift from large to small grains over time.

Water ice is predominantly on small grains at high densities, with significant variations in ice composition based on grain size and environmental conditions.

Abstract

We present a new gas-grain chemical model to constrain the effect of grain size distribution on molecular abundances in starless and pre-stellar cores. We introduce grain-size dependence simultaneously for cosmic-ray (CR)-induced desorption efficiency and for grain equilibrium temperatures. We keep explicit track of ice abundances on a set of grain populations. We find that the size-dependent CR desorption efficiency affects ice abundances in a highly non-trivial way that depends on the molecule. Species that originate in the gas phase follow a simple pattern where the ice abundance is highest on the smallest grains (the most abundant in the distribution). Some molecules, such as HCN, are instead concentrated on large grains throughout the time evolution, while others (like $\rm N_2$) are initially concentrated on large grains, but at late times on small grains, due to grain-size-dependent competition between desorption and hydrogenation. Most of the water ice is on small grains at high medium density ($n({\rm H_2}) \gtrsim 10^6 \, \rm cm^{-3}$), where the water ice fraction, with respect to total water ice reservoir, can be as low as $\sim 10^{-3}$ on large (> 0.1 $\mu$m) grains. Allowing the grain equilibrium temperature to vary with grain size induces strong variations in relative ice abundances in low-density conditions where the interstellar radiation field and in particular its ultraviolet component are not attenuated. Our study implies consequences not only for the initial formation of ices preceding the starless core stage, but also for the relative ice abundances on the grain populations going into the protostellar stage. In particular, if the smallest grains can lose their mantles due to grain-grain collisions as the core is collapsing, the ice composition in the beginning of the protostellar stage could be very different to that in the pre-collapse phase.