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 and are typically interpreted as evidence for supersonic stellar motion. However, some bow shocks have been reported around subsonic massive stars or orientated differently than the stellar direction of movement, suggesting a substantial influence of the interstellar medium (ISM). Here, we quantitatively investigate the dominant cause of bow shock formation around massive stars, combining infrared bow shock samples with Gaia DR3. We find that only half (⁠52 ± 2 per cent⁠) of the bow shocks is driven by a supersonic star and that 70 ± 2 per cent is substantially misaligned (⁠> 30°⁠). Only 21 ± 2 per cent of the studies systems can be regarded as classical bow shocks, driven by a supersonic star in the direction of stellar movement. For the remaining 79 ± 2 per cent of systems, the role of bulk ISM movement in creating the shock is equal to or dominant over the stellar movement. We also introduce a statistical method to quantify the ISM motions required to play this dominant role. We find that random ISM motions at 10-15 km s^-1 suffice to drive the existence and orientations of massive star bow shocks. For systems facing H ii regions, we find evidence that outflows at > 25 km s^-1 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 an order of magnitude.