Binary stars drive planetary nebula

Planetary nebulae, which represent one of the final stages in the life of low and intermediate mass stars, are some the most spectacular sights in our Galaxy.  After exhausting their central supply of hydrogen, stars move from the main-sequence (which is the core hydrogen burning phase) to the red giant branch of the HR diagram, where their energy comes from hydrogen burning in a thin shell surrounding their inert helium core. Eventually the core becomes hot enough to start burning helium, and the star enters the horizontal branch phase of its evolution. Once the supply of core helium has been used up (producing a carbon and oxygen core), the star then goes through the asymptotic giant branch phase, in which the inert core is surrounded by a helium-burning shell and a  hydrogen-burning shell. This situation is unstable and the star experiences thermal pulses. During these pulses the star sheds rings of gas into the surrounding space. Dust also forms in the cool outer envelope of the star and is effected by radiation pressure, which pushes the dust outwards and helps drive a strong stellar wind of gas and dust.

Hertzsprung-Russell(HR) diagram, showing the evolutionary phase of a low mass star towards the planetary nebulae phase. (Credit: Swinburne)

The remaining stellar core lights up the shell of surrounding gas and dust in a dazzling display of colour. Planetary nebulae (PNe) come in a wide variety of shapes, with complex  morphologies including asymmetries, bipolar jets and high-velocity outflows. The origin of their complex structure has been debated for many years – how can a spherical star produce such an asymmetric structure?  The resulting asymmetry is  likely due to processes that occurred during the main mass-loss phase of the star and are thought to result from a central binary system, or structure in the local stellar magnetic field, or both.

The Cat’s Eye Nebula (or NGC 6543) seen in X-ray (left) and optical+X-ray composite (right). The optical image shows a complex structure, while the X-ray reveals a bipolar expansions of hot gas. (Credit: Left – NASA/UIUC/Y.Chu et al. Right: NASA/UIUC/Y.Chu et al. and NASA/HST)

(For more amazing images of PNe, see the HST/WFC3 Compact Galactic Planetary Nebulae image database and the Macquarie/AAO/Strasbourg Hα Planetary Galactic Catalog.)

While these is a lot of evidence suggesting that the most extreme PNe morphologies arise from a central binary system, there has as yet been no clear-cut examples of interacting binaries shaping the resulting PNe. (Though there are a number of PNe, including NGC 1514, which are known to host a central binary.)

Planetary nebula NGC 1514 is know to host a binary star. Left: ground-based optical image. Right: infrared image from WISE. (Credit: NASA/JPL-Caltech/UCLA/DSS)

In this week’s edition of Science, Henri Boffin and collaborators present new results using the ESO Very Large Telescope to study the PNe Fleming 1, renowned for its spectacular bipolar jets. The jet contain a series of high-speed knots which follow a curved path. Boffin et al. report on finding two white dwarfs at the centre of the PNe on a circle orbit of period 1.1953 days, and explain the knotted, S-shaped jet of Fleming 1 as resulting from a precessing outflow of compact star (in this case a white dwarf surrounded by an accretion disk) in a circular orbit.

Planetary nebula Fleming 1, imaged with the ESO Very Large Telescope. A white dwarf binary pair at the centre of the PNe produce the symmetric structures of the jets seen in the surrounding nebula gas. (Credit: ESO/H. Boffin)

For more information, see

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