Pinwheel outflow of extreme carbon star AFGL 3068

This amazing discovery is a few years old now and had escaped my attention. I remember there was some discussion in the literature some years ago as to whether the central star of another protoplanetary nebula, the Red Rectangle (HD 44179) was also binary* and whether this could give rise to its unique structure. The origin of the structures seen in planetary nebulae is an emerging field of study.

This remarkable picture from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope shows one of the most perfect geometrical forms created in space. It captures the formation of an unusual pre-planetary nebula, known as IRAS 23166+1655, around the star LL Pegasi (also known as AFGL 3068) in the constellation of Pegasus (the Winged Horse).

The striking picture shows what appears to be a thin spiral pattern of astonishingly regularity winding around the star, which is itself hidden behind thick dust. The spiral pattern suggests a regular periodic origin for the nebula’s shape. The material forming the spiral is moving outwards a speed of about 50 000 km/hour and, by combining this speed with the distance between layers, astronomers calculate that the shells are each separated by about 800 years.

The spiral is thought to arise because LL Pegasi is a binary system, with the star that is losing material and a companion star orbiting each other. The spacing between layers in the spiral is expected to directly reflect the orbital period of the binary, which is indeed estimated to be also about 800 years.

Image source

Full 2006 paper ‘A binary-induced pinwheel outflow from the extreme carbon star AFGL 3068’

* A recent paper discusses the geometry of binary, accretion disk, jet and outflow of HD 44179.