2011A&A...527A..62B

This paper is part of our ongoing study of star formation at the borders of Galactic HII regions. In this paper, we report our observations and analysis of Sharpless 217 (Sh 2-217). We study the stars and gas in and around HII regions to see if the various physical parameters derived from the data (such as column densities, masses, sizes, and timescales) are consistent with the predictions of a simple model of the collect-and-collapse mechanism. This should indicate whether stars forming in molecular gas at the borders of the HII regions could have been triggered by the expansion of the ionized gas. We observed the emission of various molecules and transitions towards Sh 2-217, and obtained both near-infrared photometry in the H and K bands, and near-infrared images in [FeII] and H₂ narrow-band filters of the stars in a molecular condensation at the edge of the HII region, where an UC HII region is also located. For the atomic and ionized hydrogen gas, we used literature data. Several molecular condensations are found on the borders of Sh 2-217 and both behind and in front of the ionized emission. We find signs of star formation (an UC Hii region, outflows, and water masers). The masses of the larger molecular condensations, derived from ¹³CO-data, are >330-1100M_☉, while smaller clumps or cores within them have up to several tens of solar masses. The morphology of the atomic and molecular gas associated with Sh 2-217, especially the condensations of molecular gas on its border, and the presence of star forming activity within them, is strongly indicative of it being the result of star formation triggered by the expansion of the ionized region, following the collect-and-collapse scenario. Application of a simple model illustrates that the present radii of both Sh 2-217 and the UC HII region, the masses of the condensations, and the timescales needed to sweep up these amounts of gas and allow massive stars to form in them, are consistent with the model predictions. We show that it is highly plausible that the expansion of Sh 2-217 has swept-up the ambient interstellar medium, and that it has taken about 4Myr to reach its present radius. About 1Myr ago, the most massive gas condensation became unstable and produced a cluster of stars. The most massive of these stars gave rise to the UC HII region, which took about 0.5Myr to reach its present size.