FLIERs: Cosmic bullets or evaporating gas?

These fast-moving knots remain one of the many mysteries of planetary nebulae

When a low-mass stars reaches the end of its life, it sheds its outer layers and forms a planetary nebula, a shell of hot, ionized gas. As seen from far away, a planetary nebula appears looks like a piece of cosmic art, with brilliant colors standing in stark contrast to the inky reaches of space. These objects have long been targets of study for astronomers because of their complexity and beauty.

MyCn18, the Engraved Hourglass Nebula, as seen by the Hubble Space Telescope’s Wide Field and Planetary Camera 2. This is a false-color image, showing [N II] emission in red, hydrogen (possibly Hα) emission in green, and [O III] emission in blue. Image credit: NASA, R. Sahai, J. Trauger (JPL), and The WFPC2 Science Team. Public domain.

If you flip through any catalog of planetary nebulae, you’ll notice that they vary wildly in shape, color and size. Some patterns and features, however, crop up again and again, variations on a theme. An example of this is a group of tiny structures with the mundane moniker of fast, low-ionization emission regions — FLIERs, for short. These dense clumps of relatively neutral gas appear to be moving away from the center of the nebula at high speeds — usually several times the speed of sound in the nebula!

FLIERs were first studied as part of a larger group of kinematic structures in planetary nebulae. After most low-mass stars leave the main sequence, they spend several million years as red giants, gradually shaking off their outer layers. When these stars enter the planetary nebula phase of their lives, their stellar winds may speed up dramatically, driving matter into the remains of the red giant envelope. This forms a shock wave, creating a colorful ring or shell around the star.

A group of astronomers (Balick et al. 1987) were interested in testing out this “interacting winds” model, and decided to perform a series of spectroscopic observations of a set of elliptical planetary nebulae. They chose to map the Hα and [N II] lines; Hα is usually one of the strongest spectral lines in a planetary nebula, and [N II], singly-ionized nitrogen, lies right next to it. Using a method known as long-slit spectroscopy, the team observed eight elliptical planetary nebulae on the Mayall 4-meter telescope at Kitt Peak National Observatory, with the intent of mapping gaseous outflows and studying their motion.

Figure 1, Balick et al. 1998. Four well-known examples of planetary nebulae containing FLIERs, as seen by the Hubble Space Telescope’s Wide Field and Planetary Camera 2. [N II] emission is shown in red, [O III] emission is shown in green, and He II λ4686 emission is shown in blue. Given the wavelengths of these three lines, these are effectively true-color images.

All eight nebulae contained interesting structures, and three of them — NGC 3242, NGC 6826, and NGC 7662 — were found to include the knots we now refer to as FLIERs; a fourth nebula, in the study, NGC 7009 would later be confirmed to have them, too. The knots appeared in pairs, on opposite sides of each nebula, and appeared to be quickly moving away from the central star.

Figure 9, Balick et al. 1987. The authors proposed that many of the microfeatures observed around planetary nebulae were the result of the interactions between the old red giant envelope and shock waves moving outward from the star.

The authors suggested that the knots, as well as the other microfeatures they observed, were the result of inhomogeneities in the red giant envelope. If the envelope was denser along its equatorial plane, the geometry of the shock would take on an elliptical shape. A reverse shock could also form, traveling towards the star and collimating the outflow into a more focused shape, leading to features like these knots. The knots would be photoionized by ultraviolet light from the central star on one side, and heated by collisions with the ambient gas on the other. Still, this explanation remained vague, and astronomers needed a deeper understanding of the physical processes leading to the formation of these structures.

FLIERs were heavily studied throughout the next decade, and many groups attempted to explain them. Any theory behind their formation would have to account for several key properties:

• Their tendency to appear in pairs, lying opposite one another along a nebula’s symmetry axis
• Their high rate of incidence (FLIERs showed up in roughly half of all planetary nebulae studied)
• Their extreme velocities, often up to five times the speed of sound in the surrounding gas

Broadly speaking, as another group of astronomers (also led by Bruce Balick) put it in a 1998 paper, FLIER models fall into two distinct classes. The first are the “bullet” models, involving small, low-ionized fast-moving clumps of gas traveling through the surrounding gas of the nebula. They could be formed by interacting winds in a binary system or some other sort of stellar behavior. Unfortunately, observations show that both the morphology and ionization structure of FLIERs cannot be reproduced by bullet models; they decrease in ionization with increased radius from the star, rather than having heavily ionized heads.

Figure 1, Mellema et al. 1998. In this photoevaporation model of FLIERs, ionizing photons cause evaporating gas to move backwards toward the star, driving a shock front in the opposite direction and propelling the rest of the clump forward.

The other set of models involve photoevaporation, where ambient gas in the nebula is ionized by high-energy radiation from the central star. Astronomers had suggested that dense knots could then be accelerated to high speeds by either stellar winds or so-called exhaust gas released by the aforementioned photoevaporation. Balick et al. argued that these models, too, should be ruled out, because the acceleration mechanisms are too weak to ensure that the knots reach those high speeds.

Normally, I’d end a blog post of this sort by talking about the theory or model that ended up accepted by the astronomical community. Unfortunately, I can’t do that in here, because despite the ever-increasing sample size of planetary nebulae and the rise of hydrodynamic modeling codes, we don’t have a firm explanation of how FLIERs are formed. Therefore, I’m simply going to wrap things up with a gallery of some of the most striking examples of planetary nebulae that contain FLIERs. I hope you enjoy this cosmic art gallery.

NGC 6826, the Blinking Eye Nebula, displays a prominent pair of FLIERs. Lying 2000 light-years away, it was one of the first nebulae known to contain these structures, and has been heavily studied over the past three decades. This magnificent image from Hubble’s WFPC2 was featured as the Astronomy Picture of the Day over two decades ago. Image credit: B. Balick, J. Alexander, et al., NASA. Public domain.

NGC 3242 was discovered by William Herschel in the 18th century. Thanks to its haunting, translucent appearance, it’s often called the Ghost of Jupiter. FLIERs are visible at the bottom left and top right. Image credit: Judy Schmidt, CC BY 2.0.

The symmetric pair of FLIERs observed around IC 4593 ([N II], in red) are notable for lying beyond the edge of the observed [O III] and Hα emission. There are also similar features along the upper half of the nebula, though apparently without counterparts on the other side. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA). Public domain.

NGC 2392, the Eskimo Nebula, might deserve a blog post itself for the variety of complicated structures that lie around its edges. The thick, dense regions on the top and bottom edges are thought to be FLIERs, but the other “cometary” streaks on the right and left might be unrelated. These globules are light-years in length, but their origin is still unknown. Image credit: NASA, ESA, Andrew Fruchter (STScI), and the ERO team (STScI + ST-ECF). Public domain.

Not all images of planetary nebulae are crisp. These observations of the nebula ETHOS 1 by the Very Large Telescope show that it’s difficult to observe — but rewarding. At the center lies a binary star, making ETHOS 1 much different than most of the nebulae we’ve discussed above, and a possible candidate for the now-rejected interacting winds model. (a) shows a color composite of [N II] + Hα (red), [O III] (green), and [O II] (blue). The [N II] and Hα emission is highlighted in (b) and (e), the [O III] emission is shown on the rightmost two panels, and the [O II] emission — a rarity so far — appears in panel (d). Figure 5, Miszalski et al. 2011.