Dust in the Wolf-Rayet Nebula M1-67
  The Wolf-Rayet nebula M1-67 around WR124 is located above the Galactic plane
in a region mostly empty of interstellar medium, which makes it the perfect
target to study the mass-loss episodes associated with the late stages of
massive star evolution. Archive photometric observations from WISE, Spitzer
(MIPS) and Herschel (PACS and SPIRE) are used to construct the spectral energy
distribution (SED) of the nebula in the wavelength range of 12-500$\mu$m. The
infrared (photometric and spectroscopic) data and nebular optical data from the
literature are modeled simultaneously using the spectral synthesis code Cloudy,
where the free parameters are the gas density distribution and the dust grain
size distribution. The infrared SED can be reproduced by dust grains with two
size distributions: a MRN power-law distribution with grain sizes between 0.005
and 0.05$\mu$m and a population of large grains with representative size 0.9$
\mu$m. The latter points towards an eruptive origin for the formation of M1-67.
The model predicts a nebular ionized gas mass of $M_\mathrm{ion} =
9.2^{+1.6}_{-1.5}~\mathrm{M}_\odot$ and the estimated mass-loss rate during the
dust-formation period is $\dot{M} \approx 6 \times 10^{-4}
\mathrm{M}_\odot$yr$^{-1}$. We discuss the implications of our results in the
context of single and binary stellar evolution and propose that M1-67
represents the best candidate for a post-common envelope scenario in massive
stars.
