Bow shocks, bow waves, and dust waves. IV. Shell shape statistics
  Stellar bow shocks result from relative motions between stars and their
environment. The interaction of the stellar wind and radiation with gas and
dust in the interstellar medium produces curved arcs of emission at optical,
infrared, and radio wavelengths. We recently proposed a new two-dimensional
classification scheme for the shape of such bow shocks, which we here apply to
three very different observational datasets: mid-infrared arcs around hot OB
stars; far-infrared arcs around luminous cool stars; and H alpha emission-line
arcs around proplyds and other young stars in the Orion Nebula. For OB stars,
the average shape is consistent with simple thin-shell models for the
interaction of a spherical wind with a parallel stream, but the diversity of
observed shapes is many times larger than such models predict. We propose that
this may be caused by time-dependent oscillations in the bow shocks, due to
either instabilities or wind variability. Cool star bow shocks have markedly
more closed wings than hot star bow shocks, which may be due to the dust
emission arising in the shocked stellar wind instead of the shocked
interstellar medium. The Orion Nebula arcs, on the other hand, have both
significantly more open wings and significantly flatter apexes than the hot
star bow shocks. We test several possible explanations for this difference
(divergent ambient stream, low Mach number, observational biases, and influence
of collimated jets), but the evidence for each is inconclusive.
