Galactic cold cores V. Dust opacity
  The project Galactic Cold Cores has made Herschel observations of
interstellar clouds where the Planck satellite survey has located cold and
compact clumps. The sources range from starless clumps to protostellar cores.
We examine 116 Herschel fields to estimate the submillimetre dust opacity and
its variations. The submillimetre dust opacity was derived from Herschel data,
and near-infrared observations of the reddening of background stars are
converted into near-infrared optical depth. We studied the systematic errors
affecting these parameters and used modelling to correct for the expected
biases. The ratio of 250um and J band opacities is correlated with the cloud
location and star formation activity. We find a median ratio of
tau(250um)/tau(J)= (1.6+-0.2)*10^-3, which is more than three times the mean
value in diffuse medium. Assuming a spectral index beta=1.8 instead of
beta=2.0, the value would be lower by ~30%. No significant systematic variation
is detected with Galactocentric distance or with Galactic height. The
tau(250um)/tau(J) maps reveal six fields with clear increase of submillimetre
opacity of up to tau(250um)/tau(J) ~ 4*10^-3. These are all nearby fields with
spatially resolved clumps of high column density. We interpret the increase in
the far-infrared opacity as a sign of grain growth in the densest and coldest
regions of interstellar clouds.
