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Some red giant envelopes present spiral structures (seen either in dustscattered stellar continuum or in molecular line emission), the most strikingexample probably being AFGL 3068. This object has been modeled (bothanalytically and numerically) in terms of a wind ejected from a star in orbitaround a binary companion. We revisit both analytical models and 3D simulationsof a wind from an orbiting red giant star, and extend the numerical simulationsto the case of a binary with two red giants with strong winds. We find thatmost two-wind models on the orbital plane show a "double spiral" structureclose to the binary source, and that these two arms merge into a single spiralstructure at larger distances. However, for the case of a binary with twoidentical winds the two spiral arms are still present at large distances fromthe binary source. We also find that for models of two (not identical)dynamically important winds, a region close to the orbital plane has materialfrom both winds. Also, an approximately conical region centered on the orbitalaxis is filled exclusively by the wind with larger momentum rate. These twostructures lead to morphologies reminiscent of the so-called "hour glass"planetary nebulae. Finally, we find that increasing wind velocity disparitieslead to the formation of clumpy structures along the spiral amrs. Observationsof "clumpy spirals" are therefore likely to indicate the presence of two strongwinds from the stars in the central binary system.

Two-wind interactions in binaries with two orbiting giant stars