2000A&A...364..625S

We probe the region around the protostar HH108MMS by deep mid infrared photometric and polarimetric imaging. The protostar is detected at 14µm in absorption against the diffuse background. Next to HH108MMS, we find a second absorbing core, named Q1, and the young stellar object IRAS 18331-0035 which is more advanced in its evolution and already seen in emission at 12µm and 14µm. HH108MMS, Q1 and IRAS 18331-0035 form a triplet along an extended filamentary absorption feature. From the variation of the surface brightness across the source, we derive for HH108MMS and Q1 the optical depth and density profile. Along the axes which are parallel to the filament, the density distributions follow a ρ∝r^–1.8 power law. We estimate that the intensity of the background radiation at 14µm is about two times stronger than the intensity of the interstellar radiation field in the solar neighborhood. The present photometric data of IRAS18331-0035 between 12µm and 1.3mm can be explained by a central source with a luminosity of 2.5L_☉ that is surrounded by a spherical cloud of 1.1M_☉ with a 1/r density distribution. As HH108MMS is also seen in the millimeter dust emission, we can derive the ratio of the dust extinction coefficients at 14µm and 1.3mm and obtain κ_14µm/κ_1300µm∼470. Because models for the dust in the diffuse interstellar medium predict a ratio of around 2000, our value points to fluffy composite grains which are expected to prevail in dense and cold environments. First mid infrared polarisation images of pre-stellar absorbing cores are presented. At 12µm and 14µm the polarisation of the region around HH108MMS is strong (≥15%) and tightly correlated with the source triplet. We demonstrate that the high degree of polarisation can be explained by extinction of rotationally aligned dust particles of moderate elongation.