Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?

International audienceContext. With the initial steps of the star formation process in the densest part of the interstellar medium (ISM) still under debate, much attention is paid to the formation and evolution of pre-stellar cores. The recently discovered coreshine effect can aid in exploring the core properties and in probing the large grain population of the ISM. Aims: 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. Methods: The coreshine surface brightness of L1506C is determined from Spitzer IRAC images at 3.6 μm. 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 μm are calculated for a variety of MRN and grain growth distributions using the DIRBE 3.5 μm all-sky map as external interstellar radiation field, and are compared to the observed coreshine surface brightness. Results: For a core with the overall physical properties of L1506C, no detectable coreshine is predicted with a size distribution following the shape and size limits of an MRN distribution. Extending the distribution to grain radii of about 0.65 μm 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. Conclusions: The coreshine observed from L1506C requires the presence of grains with sizes exceeding the common MRN distribution. 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