Title: JWST/MIRI Study of the Enigmatic Mid-Infrared Rings in the Planetary Nebula NGC 1514
Authors: Michael E. Ressler, Alba Aller, David Jones, Ryan M. Lau, Luis F. Miranda, Karen Willacy
First Author’s Institution: Jet Propulsion Laboratory, California Institute of Technology, CA, USA
Status: Published in The Astronomical Journal [open access]
Planetary nebulae are possibly the greatest misnomer in all of astrophysics. Named because the first few observed with now-dated telescopes resembled planets, these nebulae are anything but. Low-to-intermediate mass stars in the throes of death eject most of their material into the interstellar medium through binary interactions and/or a relatively brief period of intense winds. The compact and hot stellar core at the scene of the crime then irradiates the material, making it glow in one final light show before the long goodnight.
These planetary nebulae (PNe) have long been the subject of awe and science alike. Interactions of binaries (or higher order systems) during the planetary nebula (PN) phase can sculpt the ejected material into complex geometries resembling rings, lemons, cat’s eyes, and everything in between. As with many astronomical objects, though, there is more than meets the (visible) eye: looking at these nebulae in different wavelengths – especially in the infrared, which emphasises dust emission – can reveal additional rich structure.
The authors of today’s paper look now to NGC 1514. In visible light, this nebula presents as an asymmetric, but mostly circular, cloud of ionised gas. When imaged in the mid-infrared with the James Webb Space Telescope, however, the bright thermal emission of two expanding rings of dust is revealed (Figure 1). These rings extend to a much larger radius around the central stars of the nebula compared to the blobby nebula observable in the visible and mid-infrared.
Figure 1: The three monochromatic infrared images taken by JWST of NGC 1514 (top row, bottom left), and the colour composite (bottom right) made by assigning each grayscale image to a colour. Source: Figure 3 in the paper.
NGC 1514, like many asymmetric PNe, is the complex product of interactions between the two stars at its centre. One of these is a 90,000 K subdwarf O (sdO) star who has lost most of its mass (which led to the planetary nebula to begin with) and is now ionising that gas to produce the nebula’s bright glow. This star’s neighbour is a comparatively much more massive ~10,000 K star who has left the main-sequence and will soon share a similar fate to its sdO companion.
Identifying the scenario in which the rings are produced by the stars remains an open question, but today’s authors have some ideas. In particular, they cite a particular model from a 2018 study modelling the shaping of PNe via the ‘common envelope’ phase – a phenomenon where one star is engulfed by the swelling of another, and they mutually orbit within an ellipsoid of shared material. At some stage within this phase, the authors suggest that there is a period of intense mass loss from this common envelope, where subsequent fast polar winds from the dying star carve out regions to leave behind two expanding rings.
To help discern the nature of the bipolar rings, the authors investigated their expansion velocity away from the central stars. If the rings are expanding uniformly and radially away from the central stars, their velocity profile from our perspective should appear as in Figure 2. With spectra taken at five locations across the two rings, they were able to conclude that the rings’ expansion is consistent with a ~5.5km/s speed uniformly away from the central binary – a valuable measurement for future modelling of how these rings came to be!
Figure 2: The left plot shows the simple geometric double-ring model, where each ring is coloured according to the expected velocity along the line-of-sight from our perspective (more red means receding fast, more blue means approaching faster). The right plot shows the recovered radial velocities of the five orange points (from the left figure) compared to what we’d expect from the model (black curve). Source: Figures 12 and 13 in the paper.
Planetary nebulae are usually synonymous with gas ionised by the central hot stars, so why are these rings special? The proof is in the pudd-ring: since these rings are only visible in the mid-infrared and their spectra resemble a blackbody profile, they are indicative of the thermal emission of cool dust and not ionised gas as is typical for PNe. The authors argue that the rings are composed of a distribution of dust grain sizes, with temperatures ranging from roughly 110K up to 200K which could be marginally heated by the inner stars.
While JWST did not discover these rings, it has offered the most detailed view into a strikingly unique planetary nebula. Today’s study presents essential observational clues from this nebula and investigates the fleeting stage of low-to-intermediate mass stellar evolution. Such studies with JWST are only just beginning, and further examination of this nebula and the stars at its heart — as well as others like it — will give us deeper insight into how they enrich the Milky Way with complex chemistry and beautiful vistas.
Astrobite edited by Cesiley King
Featured image credit: [NGC 1514:] NASA, ESA, CSA, STScI, Michael Ressler (NASA-JPL), Dave Jones (IAC), [White dwarf artist impression:] ESO/L. Calçada, [Illustration:] Ryan White
