Optical properties of amorphous carbon dust around C-stars: new
  constraints from 2MASS and Gaia observations
  In this work the optical properties of amorphous carbon (amC) dust condensed
around carbon(C)-stars are constrained by comparing the observations for the
Large Magellanic Cloud C-stars from the Two Micron All Sky Survey (2MASS) and
from the Gaia data release 2 (DR2) with the synthetic photometry obtained by
computing dust growth and radiative transfer in their circumstellar envelopes.
The set of optical constants of amC dust considered have been pre-selected
according to their ability to reproduce the infrared colour-colour diagrams in
the Small Magellanic Cloud. Only two combinations of the optical data set and
grain size are able to reproduce the infrared photometry and the Gaia
observations simultaneously. The analysis presented provides information about
the properties of amC dust grains that might be characterized by a diamond-like
structure, rather than a graphite-like one, at least around the most
dust-enshrouded C-stars, or be composed of "small" grains of size less than
0.04 $\mu$m. The selected data sets will be adopted to compute grids of spectra
as a function of the stellar parameters that will be employed to estimate the
dust return and mass-loss rates of C-stars by fitting their spectral energy
distribution, and to study the resolved stellar populations of nearby objects.
