Sequential star formation at the periphery of the H II regions Sh 217 and Sh 219 ⋆ ⋆ ⋆

The H  regions Sh 217 and Sh 219 are textbook examples of a Strömgren sphere surrounded by an annular photodissociation region (PDR). The annular PDR is observed in both the 21 cm atomic hydrogen emission and the dust (PAH) emission near 8 μm (MSX Survey). An ultracompact radio continuum source is observed in the direction of the annular PDR, in both Sh 217 and Sh 219. JHK observations show the presence of highly reddened stellar clusters (AV ∼ 20 mag) in the directions of these radio sources. These clusters are also IRAS sources, of luminosities 22 700 L⊙ for Sh 217 and 5900 L⊙ for Sh 219. Each cluster contains at least one luminous star with an IR colour excess; the one in the Sh 219 cluster shows Hα emission. The cluster associated with Sh 217 is almost spherical and contains luminous objects at its centre. The cluster associated with Sh 219 is elongated along the ionization front of this H  region. We argue that these are ‘second-generation clusters’, which means that the physical conditions present in the PDRs, close to the ionization fronts, have favoured the formation of clusters containing massive objects. We discuss the physical mechanisms which may be at the origin of the observed triggered star formation.