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

Abstract

Aims. We wish to establish whether sequential star formation is taking place at the periphery of the Galactic H II region Sh2-212.Methods. We present CO millimetre observations of this region obtained at the IRAM 30-m telescope to investigate the distribution of associated molecular material. We also use deep observations obtained at the CFHT to study the stellar content of the region, and radio observations obtained at the VLA to look for the presence of an ultra-compact (UC) H ii region and for maser emission.Results. In the optical, Sh2-212 is spherically symmetric around its central exciting cluster. This H ii region is located along a molecular filament. A thin, well-defined half ring of molecular material surrounds the brightest part of the H ii region at the rear and is fragmented. The most massive fragment (~200 ) contains a massive young stellar object displaying a near-IR excess; its spectral energy distribution indicates a high-mass (~14 ), high-temperature (~30 000 K), and high-luminosity (~17 000 ) source. This object ionizes a UC H ii region.Conclusions. Sh2-212 is a good example of massive-star formation triggered via the collect and collapse process. The massive YSO observed at its periphery is a good candidate for a massive star formed in isolation. © 2008 ESO.