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

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 ${\mathrm{N}}_2$) 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 ($n({\mathrm{H}}_2)\gtrsim 10^6\mathrm{c}{\mathrm{m}}^{-3}$), 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.

Accepted by A&A; 16 pages incl. appendices; abstract abridged to meet ArXiV requirements

References in corpus (8)

• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Formation of Complex Molecules in Prestellar Cores: a Multilayer Approach
• Benchmarking spin-state chemistry in starless core models
• The observed chemical structure of L1544
• Why does ammonia not freeze out in the center of pre-stellar cores?
• Rapid Elimination of Small Dust Grains in Molecular Clouds

Cited by in corpus (8)

• The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
• Methanol Mapping in Cold Cores: Testing Model Predictions
• Ice mantles on dust grains: dramatic variation of thickness with grain size
• Icy molecule desorption in interstellar grain collisions
• The Effects of Cosmic Rays on the Chemistry of Dense Cores
• Towards a better understanding of ice mantle desorption by cosmic rays
• The Cosmic-Ray Induced Sputtering Process On Icy Grains
• Effects of grain temperature distribution on organic protostellar envelope chemistry