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

Abstract

M2-9, or the "Butterfly Nebula" is one of the most iconic outflow sources from an evolved star. In this paper we present a hydrodynamic model of M2-9 in which 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 a far denser, intrinsically simple pre-existing AGB wind has slowly formed all of the complex features within M2-9's lobes (including the knot pairs N3/S3 and N4/S4 at their respective leading edges, and the radial gradient of Doppler shifts within 20" of the nucleus). We emphasize that the knot pairs are not ejected from the star but formed in situ. In addition, the observed radial speed of the knots is only indirectly related to the speed of the gas injected by the star. The model allows us to probe the early history of the wind geometry and lobe formation. We also formulate a new estimate of the nebular distance D = 1.3 kpc. The physical mechanism that accounts for the linear radial speed gradient in M2-9 applies generally to many other pre planetary nebulae whose hollow lobes exhibit similar gradients along their edges.

15 pages, 3 figures, accepted by ApJ 11 Jan 2018

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• Panchromatic HST/WFC3 Imaging Studies of Young, Rapidly Evolving Planetary Nebulae. I. NGC 6302
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• The symbiotic and bipolar nebula M2-9: Morphological variability of the collimated ionized wind arising from the core
• Uncovering the structure and kinematics of the ionized core of M\,2-9 with ALMA
• [Fe II] 1.644$\mu$m imaging survey of planetary nebulae with low-ionisation structures