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

III. Star formation at the periphery of Sh2-219

¹ Laboratoire d'Astrophysique de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France e-mail: lise.deharveng@oamp.fr
² Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 414 rue de la Piscine, BP 53, 38041 Grenoble Cedex 9, France
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁵ Centro de Radioastronomía y Astrofísica, UNAM, Apartado Postal 3-72, 58089 Morelia, Michoacán, México

Received: 5 December 2005
Accepted: 10 July 2006

Abstract

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 CO  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   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.