  We present a comprehensive infrared (IR) study of the iconic Wolf-Rayet (WR)
wind-blown bubble NGC6888 around WR136. We use Wide-field Infrared Survey
Explorer (WISE), Spitzer IRAC and MIPS and Herschel PACS IR images to produce a
sharp view of the distribution of dust around WR136. We complement these IR
photometric observations with Spitzer IRS spectra in the 5-38 $\mu$m wavelength
range. The unprecedented high-resolution IR images allowed us to produce a
clean spectral energy distribution, free of contamination from material along
the line of sight, to model the properties of the dust in NGC6888. We use the
spectral synthesis code Cloudy to produce a model for NGC6888 that consistently
reproduces its optical and IR properties. Our best model requires a double
distribution with the inner shell composed only of gas, whilst the outer shell
requires a mix of gas and dust. The dust consists of two populations of grain
sizes, one with small sized grains $a_\mathrm{small}$=[0.002-0.008] $\mu$m and
another one with large sized grains $a_\mathrm{big}$=[0.05-0.5] $\mu$m. The
population of big grains is similar to that reported for other red supergiants
stars and dominates the total dust mass, which leads us to suggest that the
current mass of NGC6888 is purely due to material ejected from WR136, with a
negligible contribution of swept up interstellar medium. The total mass of this
model is 25.5$^{+4.7}_{-2.8}$ M$_{\odot}$, a dust mass of
$M_\mathrm{dust}=$0.14$^{+0.03}_{-0.01}$ M$_{\odot}$, for a dust-to-gas ratio
of $5.6\times10^{-3}$. Accordingly, we suggest that the initial stellar mass of
WR136 was $\lesssim$50 M$_{\odot}$, consistent with current single stellar
evolution models.

Solar and Stellar Astrophysics;Astrophysics of Galaxies