Why does this nebula have spiral arms?

Here’s how astronomers might have discovered a stellar companion to LL Pegasi — without even seeing it.

Graham Doskoch
Look Upwards
5 min readJun 20, 2019

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When I say the word “nebula”, what comes to mind? Perhaps a colorful star-forming region, where pillars of gas and dust enshroud newly formed protostars; perhaps a supernova remnant, a thin shell of gas expanding into interstellar space, marking the grave of a massive star. These types of nebulae have taken center stage in images from Hubble and other space telescopes for decades, tapestries of the universe.

A composite (optical/near-infrared) image of LL Pegasi, taken by the ACS camera on the Hubble Space Telescope. The horizontal line is an artefact, a diffraction spike from a nearby star. Image credit: ESA/NASA & R. Sahai, CC BY 4.0.

Stars can form spectacular nebulae when they’re born and when they die, but what about in between? It turns out that a number of intermediate-aged stars also produce nebulae, and in a wide range of shapes and sizes. Today’s blog post focuses on a very specific example: the case of LL Pegasi, a star with a dusty outflow shaped like a spiral. This sort of thing had been seen around other systems, but LL Pegasi was the first to reveal a possible culprit behind the delicate structure: a hidden companion.

Even without its nebula, LL Pegasi is interesting for a few reasons. It’s a Mira variable, an old red giant slowly pulsating over the course of hundreds of days. Mira variables are amazing because their pulsations in brightness come from slowly expanding and contracting, throwing off enormous quantities of dust in the process. Most of these stars bring large quantities of carbon to their surfaces through convection, but Mira variables more than about four times the mass of the Sun destroy it in a special type of nuclear fusion called hot bottom burning.

Mira, the namesake of Mira variables, in ultraviolet and optical images, as seen by the Galaxy Evolution Explorer. Its pulsations have left a long train flowing behind it, visible only at ultraviolet wavelengths. Image credit: NASA/JPL-Caltech/POSS-II/DSS/C. Martin (Caltech)/M. Seibert(OCIW). Public domain.

LL Pegasi, though, is light enough that the carbon it dredges up won’t be fused, and so we call it a carbon star, because the dust it expels is rich with carbon-based compounds. This produces a “sooty” nebula, which in the case of LL Pegasi completely hides the star; only infrared light can penetrate it. This can make it exceedingly difficult to study Mira variables at most wavelengths. It turns out to be this dense cocoon that makes LL Pegasi all the more fascinating.

Arcs in the Egg

Mira variables are a subclass of stars called asymptotic branch (AGB) stars, which are nearly at the planetary nebula phase of their lives. Not all AGB stars pulsate the way Mira variables do, but most exhibit the same sort of drastic mass loss that LL Pegasi does. Interestingly enough, similar expanding arcs of material had been seen several other AGB stars, notably, the central star of the Egg Nebula, CRL 2688.

A composite (visible/infrared) image of the Egg Nebula, taken by the Wide Field Camera 3 on the Hubble Space Telescope. The bipolar jets were what first caught the astronomers’ eyes, but the layers of the nebula might hold additional secrets. Image credit: ESA/Hubble & NASA, CC BY 3.0.

The team of astronomers who first studied the Egg Nebula arcs in detail with Hubble, Sahai et al. 1998, noticed them only as a side feature in their overall investigation of the dusty cocoon around the star. They suggested three broad classes of mechanisms to explain the arcs:

  • Instabilities in the outer rotating layers of the central star
  • Instabilities in the region beyond the star where dust condenses and is accelerated outward
  • A binary companion or some other external influence

Sahai et al. actually thought the third class — the binary model — was unlikely to be correct. In particular, the group was skeptical of the model put forth by Harpaz et al. 1997, who postulated an unseen companion with a period of 100–500 years in an eccentric orbit. Harpaz et al. suggested that the wind from the AGB star was isotropic, as might be expected. However, when the star grows large enough, some of its extended atmosphere overflows its Roche lobe, and some of the wind is diverted towards the system’s equatorial plane. When the companion makes it closest approach, it disrupts the isotropic flow, forming shells. Sahai et al. took issue with the Harpaz proposal because it implied that the arcs should be much less irregular than they were observed to be, and felt that the classes of instabilities provided much more room for developing irregularities.

Figure 1, Sahai et al. 1998. One of the original Hubble images of the Egg Nebula, taken using the Wide Field Camera at 606 nm.

Another reason for arguing against the binary model was simply that no companion star had been found. A discovery of something lurking in the dust cloud — well, that would convince the skeptics. Unfortunately, additional observations showed that while the Egg Nebula has a rich structure, CRL 2688 seems to be all alone.

What makes our story interesting is that LL Pegasi is not.

A new paradigm for AGB outflows?

The key observations that showed that the theorists might be right — at least in the case of LL Pegasi— were performed by Morris et al. 2006, a few years later. As you might have guessed, their measurements were made in the infrared, using near-infrared filters on Hubble’s Advanced Camera for Surveys (ACS) and the Keck II Telescope’s Near Infrared Camera 2 (NIRC2).

Figure 2, Morris et al. 2006. The key near-infrared images taken at the Keck II telescope that showed clear evidence of a binary companion around LL Pegasi.

The results were twofold: a companion star of unknown type, orbiting at roughly 109 astronomical units, and a spiral shell that wrapped around the system at least four times. The spiral arms, looking a bit like a spiral galaxy, appeared to have been formed 710 years apart, which is close to the derived orbital period of the binary, 810 years. Unlike the Egg Nebula, LL Pegasi made a much better case for Harpaz et al.’s theory.

The outflows of AGB stars are exciting because they allow us to better understand strange phenomena like late thermal pulses and superwinds, which are characteristic features of these surprisingly low-mass stars. If we can develop even more detailed theoretical models of binary interactions, we can come that much closer to a fuller understanding of nebula morphologies. Harpaz et al. estimated that only about 0.3% of AGB stars should display these spiral shells from binary interactions, but even that gives us a large population of stars to search — and our catalogue of these outflows grows larger every year.

ALMA images of LL Pegasi have revealed a more intricate three-dimensional of the nebula, but they also show the concentric rings in even starker contrast. Image credit: ALMA (ESO/NAOJ/NRAO)/H. Kim et al., CC BY 4.0

Inspiration for this post came from a tweet by Judy Schmidt about LL Pegasi.

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Graham Doskoch
Look Upwards

PhD student in radio astronomy. Pulsars, pulsar timing, radio transients, gravitational waves, and the history of astronomy.