Triggered star formation at the borders of the H ii region Sh 2-217

Abstract

Context. This paper is part of our ongoing study of star formation at the borders of Galactic H ii regions. In this paper, we report our observations and analysis of Sharpless 217 (Sh 2-217). Aims. We study the stars and gas in and around H ii 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 H ii regions could have been triggered by the expansion of the ionized gas. Methods. 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 [Fe ii] and H 2 narrow-band filters of the stars in a molecular condensation at the edge of the H ii region, where an UC H ii region is also located. For the atomic and ionized hydrogen gas, we used literature data. Results. 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 H ii region, outflows, and water masers). The masses of the larger molecular condensations, derived from 13 CO-data, are > ∼ 330−1100 M , 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 H ii 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. Conclusions. 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 4 Myr to reach its present radius. About 1 Myr 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 H ii region, which took about 0.5 Myr to reach its present size.

Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System

“…Observational evidence for triggered star formation has been found at the boundaries of wind-bubbles around massive O and B stars (Deharveng et al 2003(Deharveng et al , 2005Zavagno et al 2007;Watson et al 2009;Brand et al 2011). Further evidence arises from the statistical correlation of young stellar objects with wind bubbles.…”

Triggered massive star formation associated with the bubble Hii region Sh2-39 (N5)

“…In recent years, the triggered star formation process, especially the C&C mechanism, has been studied extensively at the edges of many bubble-shaped Hii regions such as Sh2-104, RCW 79, Sh2-212, RCW 120, Sh2-217, Sh2-90, Gum 31, and S 24 (Deharveng et al 2003Zavagno et al 2006Zavagno et al , 2010Brand et al 2011;Samal et al 2014;Duronea et al 2015;Cappa et al 2016). Dense molecular clumps placed along the border of Galactic bubble Hii regions are therefore among the most likely sites for stellar births, and hence, where to look for early stages of massive star formation.…”

NGC 6334 and NGC 6357: Hαkinematics and the nature of the H II regions

“…Deharveng et al (2010) and Thompson et al (2012) show that H ii regions can display an accumulation of dense material at their edges and, sometimes, contain condensations in which subsequent star formation can occur (e.g. Brand et al 2011). Observations made with Herschel confirm that supernova remnant etc.)…”