If you examined images of planetary nebulae, you would find that many of them have an appearance that is too “messy” to be accounted for in the standard model of how planetary nebulae form. So what causes these structures?
A Range of Looks
At the end of a star’s lifetime, in the red-giant phase, strong stellar winds can expel the outer layers of the star. The hot, luminous core then radiates in ultraviolet, ionizing the gas of the ejected stellar layers and causing them to shine as a brightly colored “planetary nebula” for a few tens of thousands of years.
Planetary nebulae come in a wide variety of morphologies. Some are approximately spherical, but others can be elliptical, bipolar, quadrupolar, or even more complex.
It’s been suggested that non-spherical planetary nebulae might be shaped by the presence of a second star in a binary system with the source of the nebula — but even this scenario should still produce a structure with axial or mirror symmetry.
A pair of scientists from Technion — Israel Institute of Technology, Ealeal Bear and Noam Soker, argue that planetary nebulae with especially messy morphologies — those without clear axial or point symmetries — may have been shaped by an interacting triple stellar system instead.
Departures from Symmetry
To examine this possibility more closely, Bear and Soker look at a sample of thousands planetary nebulae and qualitatively classify each of them into one of four categories, based on the degree to which they show signs of having been shaped by a triple stellar progenitor. The primary signs the authors look for are:
If a planetary nebula has a strong axisymmetric or point-symmetric structure (i.e., it’s bipolar, elliptical, spherical, etc.), it was likely not shaped by a triple progenitor. If clear symmetries are missing, however, or if there is a departure from symmetry in specific regions, the morphology of the planetary nebula may have been shaped by the presence of stars in a close triple system.
- Interaction with the interstellar medium
Some asymmetries, especially local ones, can be explained by interaction of the planetary nebula with the interstellar medium. The authors look for signs of such an interaction, which decreases the likelihood that a triple stellar system need be involved to produce the morphology we observe.
From the images in two planetary nebulae catalogs — the Planetary Nebula Image Catelog and the HASH catalog — Bear and Soker find that 275 and 372 planetary nebulae are categorizable, respectively. By assigning crude probabilities to their categories, the authors estimate that the total fraction of planetary nebulae shaped by three stars in a close system is around 13–21%.
The authors argue that in some cases, all three stars might survive. This means that we may be able to find direct evidence of these triple stellar systems lying in the hearts of especially messy planetary nebulae.
Ealeal Bear and Noam Soker 2017 ApJL 837 L10. doi:10.3847/2041-8213/aa611c