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SPHERE reveals earliest stage of planetary nebula formation

These observations of the red giant star L2 Puppis also clearly show a close stellar companion.
VLT/SPHERE image of the star L2 Puppis and its surroundings.
VLT/SPHERE image of the star L2 Puppis and its surroundings.
ESO/P. Kervella
Some of the sharpest images ever made with the European Southern Observatory’s Very Large Telescope have, for the first time, revealed what appears to be an aging star giving birth to a butterfly-like planetary nebula. These observations of the red giant star L2 Puppis, from the ZIMPOL mode of the newly installed SPHERE instrument, also clearly show a close stellar companion. The dying stages of stars continue to pose astronomers with many riddles, and the origin of such bipolar nebulae, with their complex and alluring hourglass figures, doubly so.

At about 200 light-years away, L2 Puppis is one of the closest red giants, stars known to be entering their final stages of life. The new observations with the ZIMPOL mode of SPHERE were made in visible light using extreme adaptive optics, which correct images to a much higher degree than standard adaptive optics, allowing faint objects and structures close to bright sources of light to be seen in greater detail. They are the first published results from this mode and the most detailed of such a star.

ZIMPOL can produce images that are three times sharper than those from the NASA/ESA Hubble Space Telescope, and the new observations show the dust that surrounds L2 Puppis in exquisite detail. They confirm earlier findings, made using NACO, of the dust being arranged in a disk, which from Earth is seen almost completely edge-on but provide a much more detailed view. The polarization information from ZIMPOL also allowed the team to construct a 3-D model of the dust structures.

The astronomers found the dust disk to begin about 560 million miles (900 million kilometers) from the star — slightly farther than the distance from the Sun to Jupiter — and discovered that it flares outward, creating a symmetrical funnel-like shape surrounding the star. The team also observed a second source of light about 186 million miles (300 million km) — twice the distance from Earth to the Sun — from L2 Puppis. This close companion star is likely to be another red giant of a similar mass, but younger.

The combination of a large amount of dust surrounding a slowly dying star, along with the presence of a companion star, means that this is exactly the type of system expected to create a bipolar planetary nebula. These three elements seem to be necessary, but a considerable amount of good fortune is also still required if they are to lead to the subsequent emergence of a celestial butterfly from this dusty chrysalis.

“The origin of bipolar planetary nebulae is one of the great classic problems of modern astrophysics, especially the question of how exactly stars return their valuable payload of metals back into space — an important process because it is this material that will be used to produce later generations of planetary systems,” said Pierre Kervella.

In addition to L2 Puppis’ flared disk, the team found two cones of material, which rise out perpendicularly to the disk. Importantly, within these cones they found two long, slowly curving plumes of material. From the origin points of these plumes, the team deduces that one is likely to be the product of the interaction between the material from L2 Puppis and the companion star’s wind and radiation pressure, while the other is likely to have arisen from a collision between the stellar winds from the two stars or be the result of an accretion disk around the companion star.

Although much is still to be understood, there are two leading theories of bipolar planetary nebulae, both relying on the existence of a binary star system. The new observations suggest that both of these processes are in action around L2 Puppis, making it appear probable that the pair of stars will in time give birth to a butterfly.

“With the companion star orbiting L2 Puppis only every few years, we expect to see how the companion star shapes the red giant’s disk,” said Kervella. “It will be possible to follow the evolution of the dust features around the star in real time — an extremely rare and exciting prospect.”
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