Triggered massive-star formation on the borders of Galactic H II regions

Context.Massive-star formation triggered by the expansion of H ii regions. Aims.To understand if sequential star formation is taking place at the periphery of the H ii region Sh2-219. Methods.We present ^{12}$CO $J=2\rightarrow 1$ line observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta, Spain). Results.In the optical, Sh2-219 is spherically symmetric around its exciting star; furthermore it is surrounded along three quarters of its periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at infrared and millimetre wavelengths. A molecular cloud of about 2000 $M_\odot$ lies at the southwestern border of Sh2-219, in the H i gap. Two molecular condensations, elongated along the ionization front, probably result from the interaction between the expanding H ii region and the molecular cloud. In this region of interaction there lies a cluster containing many highly reddened stars, as well as a massive star exciting an ultracompact H ii region. More surprisingly, the brightest parts of the molecular cloud form a “chimney”, perpendicular to the ionization front. This chimney is closed at its south-west extremity by Hα walls, thus forming a cavity. The whole structure is 7.5 pc long. A luminous Hα emission-line star, lying at one end of the chimney near the ionization front, may be responsible for this structure. Confrontation of the observations with models of H ii region evolution shows that Sh2-219 is probably 105 yr old. The age and origin of the near-IR cluster observed on the border of Sh2-219 remain unknown.