2006A&A...458..191D

Massive-star formation triggered by the expansion of HII regions. To understand if sequential star formation is taking place at the periphery of the HII region Sh2-219. We present ¹²CO J=2->1 line observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta, Spain). 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 2000M_☉ lies at the southwestern border of Sh2-219, in the Hi gap. Two molecular condensations, elongated along the ionization front, probably result from the interaction between the expanding HII 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 HII 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 7.5pc 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 HII region evolution shows that Sh2-219 is probably 10⁵yr old. The age and origin of the near-IR cluster observed on the border of Sh2-219 remain unknown.