Envelope density pattern around wide binary AGB stars: A dynamical model

Context. Various morphologies such as multi-concentric shells and spiral-like patterns have been observed around Proto-Planetary Nebulae and AGB stars. It is widely argued that the regular density patterns are produced by binary systems. Aims. The goal is to build up a simple dynamical model for the out-flowing circumstellar envelope around AGB stars in a wide binary system to explore the parameter dependence of the geometrical characteristics of column density patterns. Methods. For an AGB star in a wide binary system, the orbital motion of the star can be approximated as a series of pistons that simultaneously push out dust and gas along radial directions, but work in different oscillation phases. The piston model can fast produce column density patterns with high enough spatial resolutions for parameter dependence exploration. Results. The formation of 3-D quasi-spherical density structure is induced by orbital motion of the AGB star. The column density pattern only depends on two parameters: eccentricity of the orbit e and the terminal outflow velocity to mean orbital velocity ratio γ. When viewed perpendicular to the orbital plane, spiral, broken spiral, and incomplete concentric shell patterns can be seen, while when viewed along the orbital plane, alternative concentric half-shell, egg-shell, and half-shell half-gap patterns will develop. Nonzero eccentricity causes asymmetry, while larger γ makes a weaker pattern and helps bring out asymmetry. A spiral pattern may becomes broken when e > 0.4. The spiral center is always less than 12% of spiral pitch away from the orbit center. One should have more chances (∼ 80%) seeing spiral-like patterns than seeing concentric shells (∼ 20%) in the circumstellar envelope of wide binary AGB stars.