Dust growth in the interstellar medium: How do accretion and coagulation interplay?

TL;DR

This study investigates how accretion and coagulation processes interact to grow dust grains in interstellar clouds, revealing their individual efficiencies, effects on grain size distribution, dust mass, and extinction properties.

Contribution

It provides a numerical analysis of the interplay between accretion and coagulation, highlighting their independent roles and impacts on dust evolution in molecular clouds.

Findings

Accretion rapidly depletes small grains within 10 Myr.

Coagulation occurs on a similar timescale but is less efficient in mass growth.

Accretion enhances UV slope and carbon bump in extinction curves.

Abstract

Dust grains grow in interstellar clouds by accretion and coagulation. In this paper, we focus on these two grain growth processes and numerically investigate how they interplay to increase the grain radii. We show that accretion efficiently depletes grains with radii $a\la 0.001 \micron$ on a time-scale of $\la 10$ Myr in solar-metallicity molecular clouds. Coagulation also occurs on a similar time-scale, but accretion is more efficient in producing a large bump in the grain size distribution. Coagulation further pushes the grains to larger sizes after a major part of the gas phase metals are used up. Similar grain sizes are achieved by coagulation regardless of whether accretion takes place or not; in this sense, accretion and coagulation modify the grain size distribution independently. The increase of the total dust mass in a cloud is also investigated. We show that coagulation slightly 'suppresses' dust mass growth by accretion but that this effect is slight enough to be neglected in considering the grain mass budget in galaxies. Finally we examine how accretion and coagulation affect the extinction curve: The ultraviolet slope and the carbon bump are \textit{enhanced} by accretion, while they are flattened by coagulation.