On the importance of scattering at 8 microns: Brighter than you think
  Context. Extinction and emission of dust models need for observational
constraints to be validated. The coreshine phenomenon has already shown the
importance of scattering in the 3 to 5 micron range and its ability to validate
dust properties for dense cores. Aims. We want to investigate whether
scattering can also play a role at longer wavelengths and to place even tighter
constraints on the dust properties. Methods. We analyze the inversion of the
Spitzer 8 micron map of the dense molecular cloud L183, to examine the
importance of scattering as a potential contributor to the line-of-sight
extinction. Results. The column density deduced from the inversion of the 8
micron map, when we neglect scattering, disagrees with all the other column
density measurements of the same region. Modeling confirms that scattering at 8
microns is not negligible with an intensity of several hundred kJy per sr. This
demonstrates the need of efficiently scattering dust grains at MIR wavelengths
up to 8 microns. Coagulated aggregates are good candidates and might also
explain the discrepancy at high extinction between E(J-K) et tau(9.7) toward
dense molecular clouds. Further investigation requires considering efficiently
scattering dust grains including ices as realistic dust models.
