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Stellar bow shocks result from relative motions between stars and theirenvironment. The interaction of the stellar wind and radiation with gas anddust in the interstellar medium produces curved arcs of emission at optical,infrared, and radio wavelengths. We recently proposed a new two-dimensionalclassification scheme for the shape of such bow shocks, which we here apply tothree very different observational datasets: mid-infrared arcs around hot OBstars; far-infrared arcs around luminous cool stars; and H alpha emission-linearcs around proplyds and other young stars in the Orion Nebula. For OB stars,the average shape is consistent with simple thin-shell models for theinteraction of a spherical wind with a parallel stream, but the diversity ofobserved shapes is many times larger than such models predict. We propose thatthis may be caused by time-dependent oscillations in the bow shocks, due toeither instabilities or wind variability. Cool star bow shocks have markedlymore closed wings than hot star bow shocks, which may be due to the dustemission arising in the shocked stellar wind instead of the shockedinterstellar medium. The Orion Nebula arcs, on the other hand, have bothsignificantly more open wings and significantly flatter apexes than the hotstar bow shocks. We test several possible explanations for this difference(divergent ambient stream, low Mach number, observational biases, and influenceof collimated jets), but the evidence for each is inconclusive.

Bow shocks, bow waves, and dust waves. IV. Shell shape statistics