Coreshine in L1506C - Evidence for a primitive big-grain component or
  indication for a turbulent core history?
  The recently discovered coreshine effect can aid in exploring the core
properties and in probing the large grain population of the ISM. We discuss the
implications of the coreshine detected from the molecular cloud core L1506C in
the Taurus filament for the history of the core and the existence of a
primitive ISM component of large grains becoming visible in cores. The
coreshine surface brightness of L1506C is determined from IRAC Spitzer images
at 3.6 micron. We perform grain growth calculations to estimate the grain size
distribution in model cores similar in gas density, radius, and turbulent
velocity to L1506C. Scattered light intensities at 3.6 micron are calculated
for a variety of MRN and grain growth distributions to compare with the
observed coreshine. For a core with the overall physical properties of L1506C,
no detectable coreshine is predicted for an MRN size distribution. Extending
the distribution to grain radii of about 0.65 $\mu$m allows to reproduce the
observed surface brightness level in scattered light. Assuming the properties
of L1506C to be preserved, models for the growth of grains in cores do not
yield sufficient scattered light to account for the coreshine within the
lifetime of the Taurus complex. Only increasing the core density and the
turbulence amplifies the scattered light intensity to a level consistent with
the observed coreshine brightness. The grains could be part of primitive
omni-present large grain population becoming visible in the densest part of the
ISM, could grow under the turbulent dense conditions of former cores, or in
L1506C itself. In the later case, L1506C must have passed through a period of
larger density and stronger turbulence. This would be consistent with the
surprisingly strong depletion usually attributed to high column densities, and
with the large-scale outward motion of the core envelope observed today.
