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

TL;DR

This paper presents a hydrodynamic model explaining the formation of the complex structures in the Butterfly Nebula (M2-9) through interaction of a steady, mildly collimated gas spray with a pre-existing AGB wind, revealing insights into nebular shaping.

Contribution

The study introduces a new hydrodynamic model that accounts for the formation of M2-9's features without ejection from the star, and provides a novel distance estimate of 1.3 kpc.

Findings

Knot pairs form in situ, not ejected from the star.

The model reproduces the nebula's complex features and velocity gradients.

A new estimate of the nebula's distance as 1.3 kpc.

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.