• Title: Two short mass-loss events that unveil the binary heart of Minkowski’s Butterfly Nebula
• Authors: A. Castro-Carrizo, R. Neri, V. Bujarrabal, O. Chesneau, P. Cox, and R. Bachiller
• First Author’s Institution: Institut de Radioastronomie Millimétrique, France
When asymptotic giant branch (AGB) stars cease their final rounds of nuclear fusion and ‘die’, they transition to become planetary nebulae (PNe). However, the details of this transition from an AGB star to a PNe are not well understood. One particularly troublesome mystery associated with the transition is the appearance of winds along the poles of some of these PNe, which disturb the expected spherical expansion of the gas, instead forming elongated structures such as those shown in Figure 1.
Minkowski’s Butterfly Nebula is a young planetary nebula with a very elongated structure. Optical images of M 2–9 show two concentric sets of twin lobes of material, an inner and an outer shell, which expand outwards together. Until now, they were assumed to have been formed at the same time. M 2–9 is suspected of containing a binary system due to a changing pattern of knots that rotate about the nebula’s symmetry axis (see here for an animated gif) and could be explained by, for instance, a jet emitted by one member of the binary.The authors of this paper use high-angular resolution interferometry to study carbon monoxide (CO) line emission within the obscured core of M 2–9, probing the mass distribution near this core. While this is not a new approach, the study in this paper was done at a higher angular resolution than previous studies.
The authors found that the core of the nebula contains two concentric rings of gas and dust (see Figure 2). They rest in the plane of the equator of the nebula, and the inner and outer rings appear to be completely separated — which strongly suggests they were formed as a result of two entirely separate mass-expelling events. The authors postulate that the two concentric sets of optical lobes are counterparts to the concentric rings seen in CO emission, suggesting that each set of twin optical lobes was stimulated by a mass-loss event which also created an equatorial ring. This would imply that the lobe sets were created at different times, rather than both from the same event.
Despite the limitations on what can be directly imaged at the center of planetary nebulae, these authors have demonstrated that it is still possible to learn a lot about what’s happening there indirectly. They’ve shown reasonably conclusively that a binary stellar system is present at the center of M 2–9, and they’ve determined many of the system’s characteristics using clever modeling. This work puts in place an interesting piece of the puzzle of PNe formation, and helps us to further understand the process of how these structures are formed and evolve.