Schematic view of a possible evolutionary scenario. Upper left panel: most obscured phase where both the mid-infrared core and the outflow are still completely embedded in a layer of cooler dust. Upper right panel: conditions in the mid-infrared core in this phase. Lower left panel: outflow has broken through the cooler layer of dust and widened, making it visible in the far-infrared. Lower right panel: a third stage in which the outflow has cleared a path through the surrounding dust layer. In the two final stages, conditions in the core are no longer sufficient for luminous HCN-vib emission to occur.