Smoke in the Pipe Nebula: dust emission and grain growth in the starless
  core FeSt 1-457
  (abridged) Methods: We derive maps of submillimeter dust optical depth and
effective dust temperature from Herschel data that were calibrated against
Planck. After calibration, we then fit a modified blackbody to the
long-wavelength Herschel data, using the Planck-derived dust opacity spectral
index beta, derived on scales of 30' (or ~1 pc). We use this model to make
predictions of the submillimeter flux density at 850 micron, and we compare
these in turn with APEX-Laboca observations. Results: A comparison of the
submillimeter dust optical depth and near-infrared extinction data reveals
evidence for an increased submillimeter dust opacity at high column densities,
interpreted as an indication of grain growth in the inner parts of the core.
Additionally, a comparison of the Herschel dust model and the Laboca data
reveals that the frequency dependence of the submillimeter opacity, described
by the spectral index beta, does not change. A single beta that is only
slightly different from the Planck-derived value is sufficient to describe the
data, beta=1.53+/-0.07. We apply a similar analysis to Barnard 68, a core with
significantly lower column densities than FeSt 1-457, and we do not find
evidence for grain growth but also a single beta. Conclusions: While we find
evidence for grain growth from the dust opacity in FeSt 1-457, we find no
evidence for significant variations in the dust opacity spectral index beta on
scales 0.02<x<1 pc (or 36"<x<30'). The correction to the Planck-derived dust
beta that we find in both cases is on the order of the measurement error, not
including any systematic errors, and it would thus be reasonable to directly
apply the dust beta from the Planck all-sky dust model. As a corollary,
reliable effective temperature maps can be derived which would be otherwise
affected by beta variations.
