Candidate bow shock stars: Spectral analysis and environment

Aims . Infrared bow shocks are arc-shaped structures located ahead of a star and generally observed at mid- to far-IR wavelengths. They are thought to result from the interaction of the stellar wind with the ambient interstellar medium and are typically (but not always) related to runaway stars. However, the formation of bow shocks seems to be dominated by local environmental factors rather than stellar motion. In this context, we aim to probe the links between bow-shock driving stars and their environment. Methods . We observed 47 bow shock driving star candidates with the Multi-purpose InSTRument for Astronomy at Low-resolution (MISTRAL) spectro-imager at Haute-Provence Observatory (OHP) in the 420–800 nm range to perform spectral classification of the candidate stars. In parallel, we evaluated the transverse motion of stars from GAIA DR3 in order to determine whether they are runaways. We then characterised the bow shock environmental conditions. Results . We find that among the 47 candidates we have 3 unclassifiable stars (suspected to be G- or K-type stars), 3 M- or K-type stars, 2 A-type stars, 10 O stars, and 29 B (mainly giant and supergiant) stars. We find that 17 stars (among the 37 with determined transverse velocity) are runaways, among which only 7 have their transverse velocity aligned to the bow-shock axis. This suggests that runaway is not the only origin for bow shock formation. We note the diversity of environments where bow shocks are observed: stellar associations, a cluster, and H II regions. For most stars, the origin of the bow shock is not clear; however, the 11 bow shocks observed in the Cygnus OB stellar association suggest that the ISM conditions in such regions favour bow shock observability. We also identify that the bow shock ahead of the star ionising the H II region Sh2-135 could be produced by a photoevaporated flow of about 16 km/s coming from the H II region molecular cloud’s interface. Finally, for six stars we were able to identify the cluster from which they were ejected and determine the ejection process (dynamical ejection from star cluster or binary supernovae scenarios). Conclusions . The formation of bow shocks seems to be dominated by local environmental factors rather than stellar motion.