Two-wind interactions in binaries with two orbiting giant stars
  Some red giant envelopes present spiral structures (seen either in dust
scattered stellar continuum or in molecular line emission), the most striking
example probably being AFGL 3068. This object has been modeled (both
analytically and numerically) in terms of a wind ejected from a star in orbit
around a binary companion. We revisit both analytical models and 3D simulations
of a wind from an orbiting red giant star, and extend the numerical simulations
to the case of a binary with two red giants with strong winds. We find that
most two-wind models on the orbital plane show a "double spiral" structure
close to the binary source, and that these two arms merge into a single spiral
structure at larger distances. However, for the case of a binary with two
identical winds the two spiral arms are still present at large distances from
the binary source. We also find that for models of two (not identical)
dynamically important winds, a region close to the orbital plane has material
from both winds. Also, an approximately conical region centered on the orbital
axis is filled exclusively by the wind with larger momentum rate. These two
structures lead to morphologies reminiscent of the so-called "hour glass"
planetary nebulae. Finally, we find that increasing wind velocity disparities
lead to the formation of clumpy structures along the spiral amrs. Observations
of "clumpy spirals" are therefore likely to indicate the presence of two strong
winds from the stars in the central binary system.
