On the importance of scattering at 8 microns: Brighter than you think

TL;DR

This study demonstrates that scattering at 8 microns significantly affects observations of dense molecular clouds, challenging previous assumptions and emphasizing the need for dust models that include efficient scattering grains.

Contribution

It provides the first evidence that scattering at 8 microns is non-negligible, highlighting the importance of including such effects in dust modeling for dense clouds.

Findings

Scattering at 8 microns is significant and cannot be neglected.

Neglecting scattering leads to underestimation of column density.

Efficient scattering dust grains, like coagulated aggregates, are necessary in models.

Abstract

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.