The protostellar system HH108MMS

We probe the region around the protostar HH108MMS by deep mid infrared photometric and polarimetric imaging. The protostar is detected at 14mu in absorption against the diffuse background. Next to HH108MMS, we find a second absorbing core, named Q1, and the young stellar object IRAS18331-0035 which is more advanced in its evolution and already seen in emission at 12mu and 14mu. HH108MMS, Q1 and IRAS18331-0035 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. As HH108MMS is also seen in the millimeter dust emission, we can derive the ratio of the dust extinction coefficients at 14mu and 1.3mm and obtain kappa(14mu) / kappa(1300mu) ~ 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 12mu and 14mu 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.