Triggered massive-star formation at the border of the HII region Sh 104

We present millimetre-line imaging of the Galactic H ii region Sh 104. We show that it is surrounded by a ring of molecular gas and dust. Four large molecular condensations are regularly spaced around the ring. These condensations are themselves fragmented and contain several massive dense cores. A deeply embedded cluster is observed in the near IR towards the largest condensation. It contains at least one massive star ionizing an ultra-compact H ii region. The Sh 104 region is a good illustration of the “collect and collapse” model for star formation triggered by the expansion of an H ii region.