The Trifid nebula is a young HII region undergoing a burst of star formation. In this article, we report on far-infrared and millimeter continuum and line observations of several massive and bright protostellar sources in the vicinity of the exciting star fo the nebula, just behind the ionization front. These objects are probably young protostars (Class 0) and are associated with very massive cores (M ~ 8-90 Msol) powering young energetic outflows. Analysis of the far-infrared emission in the 45-200 micron range from ISO/LWS data shows that they are embedded in cold dense material. Inspection of their physical properties suggest that they are similar to the dust protostellar cores observed in Orion, although at an earlier evolutionary "Pre-Orion" stage. The cores are embedded in a compressed layer of dense gas. Based on comparison with the models, we find that the cores could have formed from the fragmentation of the layer and that the birth of the protostars was triggered by the expansion of the Trifid nebula.

22 pages, 8 figures; accepted by Astrophys. J