Dust processing in elliptical galaxies
  We reconsider the origin and processing of dust in elliptical galaxies. We
theoretically formulate the evolution of grain size distribution, taking into
account dust supply from asymptotic giant branch (AGB) stars and dust
destruction by sputtering in the hot interstellar medium (ISM), whose
temperature evolution is treated by including two cooling paths: gas emission
and dust emission (i.e. gas cooling and dust cooling). With our new full
treatment of grain size distribution, we confirm that dust destruction by
sputtering is too efficient to explain the observed dust abundance even if AGB
stars continue to supply dust grains, and that, except for the case where the
initial dust-to-gas ratio in the hot gas is as high as $\sim 0.01$, dust
cooling is negligible compared with gas cooling. However, we show that,
contrary to previous expectations, cooling does not help to protect the dust;
rather, the sputtering efficiency is raised by the gas compression as a result
of cooling. We additionally consider grain growth after the gas cools down.
Dust growth by the accretion of gas-phase metals in the cold medium increase
the dust-to-gas ratio up to $\sim 10^{-3}$ if this process lasts >10/(n_H/10^3
cm^{-3}) Myr, where $n_\mathrm{H}$ is the number density of hydrogen nuclei. We
show that the accretion of gas-phase metals is a viable mechanism of increasing
the dust abundance in elliptical galaxies to a level consistent with
observations, and that the steepness of observed extinction curves is better
explained with grain growth by accretion.
