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M2-9, or the "Butterfly Nebula" is one of the most iconic outflow sourcesfrom an evolved star. In this paper we present a hydrodynamic model of M2-9 inwhich the nebula is formed and shaped by a steady, low-density ("light"),mildly collimated "spray" of gas injected at 200 km s^-1 that interacts with afar denser, intrinsically simple pre-existing AGB wind has slowly formed all ofthe complex features within M2-9's lobes (including the knot pairs N3/S3 andN4/S4 at their respective leading edges, and the radial gradient of Dopplershifts within 20" of the nucleus). We emphasize that the knot pairs are notejected from the star but formed in situ. In addition, the observed radialspeed of the knots is only indirectly related to the speed of the gas injectedby the star. The model allows us to probe the early history of the windgeometry and lobe formation. We also formulate a new estimate of the nebulardistance D = 1.3 kpc. The physical mechanism that accounts for the linearradial speed gradient in M2-9 applies generally to many other pre planetarynebulae whose hollow lobes exhibit similar gradients along their edges.

Models of the Mass-Ejection Histories of pre Planetary Nebulae. II. The Formation of the Butterfly and its Proboscis in M2-9