Blowing in the wind: The dust wave surrounding $\sigma$ Ori AB
  Observations with the Spitzer Space Telescope and the WISE satellite have
revealed a prominent arc-like structure at 50" ($\simeq0.1$ pc) from the
O9.5V/B0.5V system $\sigma$ Ori AB. We attribute this dust structure to the
interaction of radiation pressure from the star with dust carried along by the
IC 434 photo-evaporative flow of ionized gas from the dark cloud L1630. We have
developed a quantitative model for the interaction of a dusty ionized flow with
nearby (massive) stars where radiation pressure stalls dust, piling it up at an
appreciable distance (> 0.1 pc), and force it to flow around the star. The
model demonstrates that for the conditions in IC 434, the gas will decouple
from the dust and will keep its original flow lines. We argue that this dust
structure is the first example of a dust wave created by a massive star moving
through the interstellar medium. Dust waves (and bow waves) stratify dust
grains according to their radiation pressure opacity, which reflects the size
distribution and composition of the grain material. Comparison of our model
with observations implies that dust-gas coupling through Coulomb interaction is
less important than previously thought, challenging our understanding of grain
dynamics in hot, ionized regions of space. We describe the difference between
dust (and bow) waves and classical bow shocks. We conclude that dust waves and
bow waves should be common around stars showing the weak-wind phenomenon and
that these structures are best observed at mid-IR to FIR wavelengths. In
particular, dust waves and bow waves are most efficiently formed around
weak-wind stars moving through a high density medium. Moreover, they provide a
unique opportunity to study the direct interaction between a (massive) star and
its immediate surroundings.
