Herschel and SCUBA-2 observations of dust emission in a sample of Planck
  cold clumps
  Analysis of all-sky Planck submillimetre observations and the IRAS 100um data
has led to the detection of a population of Galactic cold clumps. The clumps
can be used to study star formation and dust properties in a wide range of
Galactic environments. Our aim is to measure dust spectral energy distribution
(SED) variations as a function of the spatial scale and the wavelength. We
examine the SEDs at large scales using IRAS, Planck, and Herschel data. At
smaller scales, we compare with JCMT/SCUBA-2 850um maps with Herschel data that
are filtered using the SCUBA-2 pipeline. Clumps are extracted using the
Fellwalker method and their spectra are modelled as modified blackbody
functions. According to IRAS and Planck data, most fields have dust colour
temperatures T_C ~ 14-18K and opacity spectral index values of beta=1.5-1.9.
The clumps/cores identified in SCUBA-2 maps have T~ 13K and similar beta
values. There are some indications of the dust emission spectrum becoming
flatter at wavelengths longer than 500um. In fits involving Planck data, the
significance is limited by the uncertainty of the corrections for CO line
contamination. The fits to the SPIRE data give a median beta value slightly
above 1.8. In the joint SPIRE and SCUBA-2 850um fits the value decreases to
beta ~1.6. Most of the observed T-beta anticorrelation can be explained by
noise. The typical submillimetre opacity spectral index beta of cold clumps is
found to be ~1.7. This is above the values of diffuse clouds but lower than in
some previous studies of dense clumps. There is only tentative evidence of
T-beta anticorrelation and beta decreasing at millimetre wavelengths.
