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