Why do massive stars form bow shocks? Bulk ISM motion as the main driver of bow shock formation and geometry
Bow shocks are one the most commonly observed impact sites of massive star feedback. Their existence is often taken as evidence of the supersonic motion or even runaway nature of the massive star. Bow shock geometry is also used to infer the properties of the stellar wind. However, some bow shocks have been reported around sub-sonic massive stars or are orientated differently than the stellar direction of movement, suggesting a substantial influence of the interstellar medium (ISM). In this work, we quantitatively investigate the dominant cause of bow shock formation around massive stars. Starting from an infrared sample of known bow shocks, we measure the stellar peculiar velocities and degree of alignment of the bow shocks. We find that only half ($52\pm2\%$) of the bow shocks is driven by a supersonic star and that $70\pm2\%$ of bow shocks is substantially misaligned (>30 degrees). Of our sample, only $21\pm2\%$ of systems can be regarded as classical bow shocks, driven by a supersonic star in the direction of stellar movement. For the remaining $79\pm2\%$ of systems, the role of bulk ISM movement in creating the shock is equal to or dominant over the stellar movement. We then introduce a new statistical method to derive the magnitude of the ISM motions required to play this dominant role. We find that ISM motion of the order $10-15$ km s$^{-1}$, oriented randomly within the local standard of rest, suffices to explain the existence and orientations of the bow shocks in our sample. For the subset of systems facing HII regions, we find evidence that outflows at $>25$ km s$^{-1}$ from those regions drive the nearby shocks. We conclude that ISM motion is the dominant factor in creating massive star bow shocks and discuss how ignoring these motions can lead to underestimates of the stellar wind's mass loss rate by more than an order of magnitude.