Originally Posted by
Steve Gottlieb
Minkowski's Butterfly is not a typical pre-planetary nebula -- strong evidence points to a symbiotic system. Here's what I wrote in the June '21 issue of Sky & Tel (which also includes the Egg Nebula and the Footprint Nebula, which are true PPNe).
Astronomers usually classify M 2-9 as a symbiotic nebula. In this scenario, the binary central star consists of a hot white dwarf and a Mira-type companion. The latter loses mass via its stellar wind or else transfers mass to the white dwarf when it overflows its Roche lobe. The white dwarf ionizes this gas, thus mimicking a young planetary nebula.
Bruce Balick and collaborators’ recent model of M 2-9 using hydrodynamic simulations showed that a conical spray blasts out at an incredible 200 km/s (450,000 miles/hr) in the polar direction. The gas interacts with the pre-existing AGB wind that is slower and denser, forming and shaping a variety of features.
The Butterfly’s “proboscis” or outer lobe extends 1? in both directions, so the nebula’s total size is 120" x 12?". At the outer tips are blobs of compressed gas. The bright inner lobe is hourglass-shaped and nested with two separate shells: an inner “bulb” and a thin outer “sheath.” Two brighter knots, N3 and S3, are near the tips of the inner lobes and appear flattened in HST images. In Balick’s model, the knots formed in situ and were not ejected by the core.
A 2011 study by Romano Corradi (Instituto de Astrofísica de Canarias) and colleagues compared 12 years’ worth of observations and found dramatic changes in the Butterfly’s inner lobe. H_alpha and O III images showed the knot pairs S1/S2 and N1/N2 marching along the walls of the lobe from east to west. The knots rotate around the symmetry axis (the period is nearly one century) within a “lighthouse” jet from the central star. This pattern points to a symbiotic binary source, whose orbital period matches the rotation period of the knots. The phenomenon results from a collimated jet of high-velocity particles that shocks and excites the bulb walls on impact.
The thin equatorial region has two expanding, ring-shaped structures of molecular gas and dust with diameters of 2.5" and 7". A 2012 investigation headed by Arancha Castro-Carrizo (of the Institut de Radioastronomie Mil- limétrique in France) found these rings move in different directions, suggesting two mass-loss episodes occurred 900 and 1,400 years ago at distinct points in the binary star’s orbit.