Super-sharp imaging confirms that the nearby red giant star L2 Puppis is forming a tiny bipolar nebula as it comes to the end of its life.
Planetary nebulae are strange, intricate, beautifully symmetric denizens of space, and bipolar planetary nebulae are especially so. Take Hubble 12 in Cassiopeia, for example. It formed when an aging, formerly Sun-like star began to blow its outer layers into space. The material funneled out at two poles, creating a double-lobed structure like an hourglass when mapped in three dimensions, or a butterfly as we see it nearly side-on.
Hourglass structures are common among planetaries. But researchers still don’t quite know how these unique nebulae form, as they had yet to see the very beginnings of one.
But maybe now they have.
Using the European Southern Observatory’s Very Large Telescope (VLT), Pierre Kervella (Unidad Mixta Internacional Franco-Chilena de Astronomía) and colleagues confirmed the 3D structure of the tiny nebula surrounding the famous star L2 Puppis. At a distance of 210 light-years, this is one of the three closest “asymptotic giant branch” stars: highly evolved giants nearing the very ends of their lives. (Mira is another.) The team also confirmed the presence of a companion star: a less evolved orange or red giant, whose stellar wind apparently interacts with that of L2 Puppis.
The team used the VLT’s new SPHERE/ZIMPOL instrument, an “extreme adaptive optics” and polarimetry system designed to resolve and study faint objects very close to bright stars. Previous studies had identified the nebula’s presence and signs of a companion star, but were unable to produce images sharp enough to confirm. Now, with confirmation in hand, the researchers think that L2 Puppis and its companion are likely to be in the early stages of forming a bipolar planetary nebula.
How to Make a Butterfly
Theorists have two popular theories for how bipolar planetaries form. Both require a companion star orbiting the aging primary. In one scenario, the stellar wind and radiation pressure from the companion locks the older star’s outflowing dust into a ring-like orbit around the outside of the binary system, with a fraction of the matter funneling out perpendicular to the ring. These perpendicular funnels create the walls of the hourglass — the butterfly’s wings.
In the other theory, the companion star draws the aging star’s expelled matter down toward itself, forming a much smaller disk and jets around the companion only.
Both theories require a companion star, a disk, and streams flowing away from the disk’s poles. The SPHERE/ZIMPOL images allowed Kervella and colleagues to identify all three essential parts, but whether a full-size butterfly will emerge from L2 Puppis’ tiny beginnings of one is still up in the air.
Pierre Kervella et al., “The dust disk and companion of the nearby AGB star L2 Puppis.” Astronomy & Astrophysics, June 10, 2015.
Robert Zimmerman, “Spider Webs in Space.” Sky & Telescope, November 2014 cover story.