17 mu m imaging of eta Carinae: an onion-like structure for the lobes?

We have obtained a 17 mu m image of the eta Carinae nebula with a signal to noise ratio never reached before. Once deconvolved, the spatial resolution is of the order of 0.5 ". The ring and arclike structures seen at 17 mu m are very similar to those observed at 12.5 mu m; however the intensity maxi mum is located elsewhere in a dense cold structure seemingly torus-like aro und the central object. The high S/N ratio achieved in the image gives us a 2-D photometry map of the nebula at 17 mu m with a good precision. Combined with a simultaneous N band (10 mu m) image, and a model of the dus t thermal emission, we have built up a colour temperature map, which is the n used to construct a map of the dust column density in the lobes. We could test the different models found in the literature concerning the r adial density of matter in the lobes, and specifically, pay attention to th e question of the hollowness of the lobes. Our modeling of the SE lobe show s that it has a more complex structure than just an external shell, as clai med in some previous works. Our observations can be fit by a radial density model which includes a main, thick, external shell, a secondary shell betw een 0.30 and 0.55 times the lobe radius, and some material at the center of the lobe. The external shell is believed to be associated with the 1842 Gr eater Eruption, but we still lack 3D kinematic constrains to assess that th e secondary shell is due to the 1890 Lesser Eruption.