Pre‐Orion Cores in the Trifid Nebula

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 lineobservations of several massive and bright protostellar sources in the vicinityof the exciting star fo the nebula, just behind the ionization front. Theseobjects are probably young protostars (Class 0) and are associated with verymassive cores (M ~ 8-90 Msol) powering young energetic outflows. Analysis ofthe far-infrared emission in the 45-200 micron range from ISO/LWS data showsthat they are embedded in cold dense material. Inspection of their physicalproperties suggest that they are similar to the dust protostellar coresobserved in Orion, although at an earlier evolutionary "Pre-Orion" stage. Thecores are embedded in a compressed layer of dense gas. Based on comparison withthe models, we find that the cores could have formed from the fragmentation ofthe layer and that the birth of the protostars was triggered by the expansionof the Trifid nebula.Comment: 22 pages, 8 figures; accepted by Astrophys.