Interstellar extinction in Orion: variation of the strength of the ultraviolet bump across the complex

There is growing observational evidence of dust coagulation in the dense filaments within molecular clouds. Infrared observations show that the dust grain size distribution becomes shallower and the relative fraction of small to large dust grains decreases as the local density increases. Ultraviolet (UV) observations show that the strength of the 2175-angstrom feature, the so-called UV bump, also decreases with cloud density. In this paper, we apply the technique developed for a study of Taurus to the Orion molecular cloud and confirm that the UV bump decreases over the densest cores of the cloud as well as in the heavily UV-irradiated. Orionis shell. The study has been extended to the Rosette cloud with uncertain results, given the distance (1.3 kpc).