STARS IN THE DUST? Signs of Previously Unknown Companions in the Southern Ring Nebula, revealed by JWST

Title: The messy death of a multiple star system and the resulting planetary nebula as observed by JWST

Authors: Orsola De Marco et al.

First Author’s Institution: School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia.

Status: Published in Nature Astronomy [closed access]

Some stars end their lives with a bang; others not with a whimper, but a glow. Stars of “intermediate mass” (between 1–8 solar masses — not too small, and not too large!), when reaching the end of the red giant stage of their lives, will eject their outermost layers as an expanding cloud of ionized gas, pushed outwards by stellar winds from the slowly dying star. These structures are called “planetary nebulae,” though they have nothing to do with planets — the name is historical, as the astronomers in the 1700s who first observed these objects thought they looked like planets. Examples of well-known planetary nebulae include the Dumbbell Nebula, the Ring Nebula, and the Southern Ring Nebula, also known as NGC 3132. It is this last example on which today’s paper is focused.

NGC 3132 is a bright planetary nebula (hereafter, PN) located about 2000 light years from Earth. Within the nebula, two stars have been observed. One is a white dwarf, at the center of the PN, and is likely the progenitor of the nebula; the other is an A-type, main-sequence star orbiting the white dwarf at a radius of over 1200 AU, making this pair a wide binary. In 2022, infrared images of NGC 3132 taken by JWST were released as part of its Early Release Observations program. The authors of today’s paper claim that within these images, there is evidence that NGC 3132’s progenitor system was not merely a binary, but a system of at least four or five stars.

Hidden in the Halo

A typical PN consists of a hot wind bubble, surrounded by a shell of ionized gas from the red giant’s atmosphere. Often that shell is itself surrounded by stellar ejecta of molecular gas and dust. When PNe originate from a lone star, they  are generally spherical. If the giant interacted gravitationally with a stellar companion (or more than one!) during its later years, however, then one would expect the PN to contain evidence of these interactions. These signs are seen in departures from spherical symmetry in the shape of the nebula, as well as structures such as rings, arcs, spirals, and jets in the gas.

Figure 1 shows colored images of JWST’s observations of NGC 3132 in different infrared bands. In particular, distinction is made between the emission bands of H-II (ionized hydrogen, which traces the PN’s ionized gas) and H2 (molecular hydrogen, tracing the molecular gas).

Figure 1: Colored JWST images of NGC 3132. Upper left: H-II emission, tracing the ionized gas region. Lower left: H2 emission, tracing the molecular gas halo. Right: greyscale, single-filter zoomed-in image of the molecular halo to show detail. Note the extension of the halo way further than the ionized gas reaches, and the concentric arc features present within it. Figure 1 in the paper.

The molecular gas halo is seen in unprecedented detail by JWST’s images. Regular structures such as those described previously can be seen in the halo, which is much clumpier and more irregular than the ionized gas. Of the many features, one of particular interest is a series of concentric arcs similar to those observed in other PNe; the most accepted explanation for these is the stellar wind being influenced by a stellar or sub-stellar companion. Should this be the case, the average size and separation of the arcs indicates such a companion would orbit the central white dwarf at a radius of 40-60 AU; the bright A-star at >1200 AU cannot be responsible, which implies NGC 3132 is at least a triple star system. Given the fact that the hypothetical new star is not directly visible in the glare of the dwarf, the authors place an upper limit of 0.2 solar masses on what would presumably be a main-sequence companion.

Four (or Five)’s a Crowd?

The hypothesis of a close binary companion is reinforced by the shape of the innermost cavity and ionized gas shell. Combining the JWST images with spectroscopic observations of the PN, the authors were able to reconstruct a 3D visualization of this component of the nebula, as can be seen in Figure 2.

Figure 2: Author’s reconstruction of the ionized cavity of NGC 3132. Color-coding indicates Doppler shift as seen from Earth, with bluer regions approaching the observers and redder regions moving away. Figure 3 in the paper.

In the 3D reconstruction, the shape of this cavity is very clearly not smooth — it is covered in numerous protuberances, which could be due to intermittent jets. If this is the case, these jets are being generated over a huge range of axes — too many for a single close binary to cause. This leads the authors to conjecture that the central dwarf is not a member of a close binary, but at least a close triple. If there is indeed another companion, then combined with the wide-orbiting A-star, this brings the total number of gravitationally bound stars in the system to four…

…or possibly even more! JWST also discovered a disk of hot dust around the central white dwarf. The presence of this disk also favors the existence of a close binary companion, which would have donated a significant amount of material and angular momentum to the disk. One could think that this companion could be the same as the first new hypothesized companion: the one at 40-60 AU with a maximum mass of 0.2 solar masses.

However, when the authors attempted to use 2D hydrodynamic simulations to replicate the structures found in the larger molecular halo (as seen in Figure 1), they found that the arches in the gas — those produced by the first hypothesized companion — were much sharper in the JWST images than in their geometric model. This would possibly indicate that the arches are aligned close to the plane of the sky from our point of view, and thus that said hypothetical companion is closely aligned to the waist of the elliptical-shaped PN. If this is true, then this companion cannot be the one partaking in the formation of the hot dust disk. Furthermore, it cannot be the one launching strong jets into the ionized region, as the accretion rate at these orbital distances would not be large enough to do so. Combined, these could be signs of yet another stellar companion, bringing the total number of stars in the system to five!

A Tale of Stars Revealed

All in all, the sensitivity and precision of JWST’s instruments can help shed great light into the underlying history of NGC 3132, and other PNe. The observed structures in the nebula help paint a picture of this multiple-star system. Before it became the central white dwarf, there was a red giant in a wide orbit with a main sequence A-star. Alongside these, a smaller, closer companion at 40-60 AU, which left its presence in the form of arcs in the molecular gas; another close companion, the source of the dusty disk, which either escapes detection or perished in an interaction with the giant; and, if the jets and protuberances are as complex as they seem, a possible third close companion, similarly undetected. While speculation about the nature of the origin of these features still remains, the authors are confident in claiming that the original star system that became NGC 3132 was at least a quartet (if not a quintet) — something rare, but not impossible, given the range of masses of the stars at interest.

Discoveries like these are only the start for the era of JWST. With complementary observations from other instruments, JWST has the potential to revolutionize what we know in many fields of astronomy; among them the histories and evolution of PNe, and the role of stellar companions in shaping these impressive structures of our universe.

Astrobite edited by Mark Dodici

Featured image credit: NASA, ESA, CSA, STScI, and the Webb ERO Production Team

About Aldo Panfichi

Hello! I'm currently finishing up my Master's degree in Physics at the Pontificia Universidad Católica del Peru in Lima, Peru, writing a thesis project related to asteroids. I previously got my BSc in Astronomy and Astrophysics at the University of Chicago. In my free time, I like spending time with my friends (and my dogs!), going swimming in the summer, and cozying up inside in the winter, playing games or reading science fiction.

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