Welcome to the F L I E R s home page!

F L I E R S: WHAT ARE THEY, AND WHO CARES? F L I E R s are pairs of low ionization knots which lie near the symmetry axis of many planetary nebulae (PNs). Their name, Fast Low-Ionization Emission Region, is based on observations which show that they have substantially higher outflow speeds as wel l as much lower ionizations than the nebulae in which they are embedded. (They may also have much different checmial abundances.)

Their high speeds suggest that F L I E R s are much younger than their parent nebula. Their low ionizations mean that the hard stellar ultraviolet photons that ionize the gas all aropund them don't penetrate into the knots - which in turn suggests that these newly ejected knots from a very hot star are formed in a neutral state - quite a trick!

The anamolous chemical abundances of F L I E R s argue that they were dredged from highly processed material deep inside a star, and then flung outward as small spitballs - an astonishing feat for a nearly solid, tightly bound, and rapidly aging hot white dwarf...

No model of stellar or nebular evolution accounts for either the formation or the longevity of FLIERs. You can read the details in an article by Bruce Balick in American Scientist Magazine, July-August 1996, vol 84, page 342.

In order to get a better handle on the appearance ("morphology") of F L I E R s and to provide further observational hints about their history, Bruce Balick & Jay Alexander (U Washington), Arsen Hajian (U.S. Naval Obs), Yervant Terzian (Cornell U), Mario Perinotto (U Firenze), and P Patriarchi (Obs. Arcetri) undertook imaging observations using the Hubble Space Telescope (HST).

Ground-based images of the targets that we have observed.

Here (red,green,blue) is used for the [N I I], [O I I I], He I I images, respectively. ([N I I], [O I I I], He I I) is a (low,moderate,high)-ionization ion found where the gas is mostly (singly,doubly,triply) ionized.

F L I E R s can be identified in these images as small red knots, generally lying near the nebular symmetry axis and on opposite sides of the central star.


Now for the HST images...

The color images are made through the same filters as the ground-based images immediately above, but using the Wide-Field and Planetary Camera 2. The WFPC2 reveal far more detail in the nebular structure, especially the red FLIERs.

Check out the color images and details of
Bigger Color Picture of NGC3 2 4 2 .......Click for details about N 3 2 4 2.
Bigger Color Picture of NGC 6 8 2 6 ....... Click for details about N 6 8 2 6.
Bigger Color Picture of NGC 7 0 0 9 ....... Click for details about N 7 0 0 9.
Bigger Color Picture of NGC 7 6 6 2 ....... Click for details about N 7 6 6 2.

If you are viewing the image on a SUN or other high-contrast monitor, I suggest that you browse
tiff images using xview with a gamma setting of 2. Macs should have no trouble.

Note that the orientation of the spacecraft, and hence that of the pictures above, is arbitrary.
In other words, north is not at the top of these images!


The Halpha and [NII] WFPC2 images of NGC 7662 have been color coded to indicate their internal motions. In these "Doppler images" red indicates red shift, blue=blueshifts, yellow is null shift. The original ground-based doppler image is shown as an inset. North is up and east is to the right.

The Halpha image is fascinating. The bright ellipse covers the inner halfof the object. Its upper left region is blue shifted, and the lower right part is redshifted. More detailed analysis shows that a good 3-D model for the ellipse is that of an expanding football ("prolate ellipsoid") with rounded tips. The football is tilted by about 45 degress to the line of sight, the upper left tip is approaching us, the lower right end is receding.

The nebular edge mimicks the shape of the football in its center. Also, its kinematics (internal motions) are similar. Material at the far upper left is approaching, and the opposite side is receding.

However, between the inner ellipsoid and the nebula's outer edge is a more subtle and mysterious structure. You will see it as a faint, extended red feature surrounding the blue part of the ellisoid. There is a corresponding extednded blue region near the red end of the ellipsoid. The reversal of the velocity's sign along its long axis and the shape of the feature suggest that it is a thin, nearly round expanding (but not rotating) disk that surrounds the football's waist. Becuase the football is tilted, the disk around its waist is tilted too. The lower right portion of the disk is in front of the ellispoid and approaches us. The rest of the disk is behind the ellipsoid and recedes.

This is an interpretive model, not necessarily the truth. It is the simplest model consistent with the data. Other planetary nebulae show the same sort of structure. The best knowmn case is NGC 6543, the "Cat's Eye" nebula. In the latter case the tilt of the ellipsoid and its surrounding disk are less.

The [NII] image shows these same features. However, they are far less conspicuous than in Halpha. Much more prominent are a series of jets and comet-like features, called FLIERs. One's visual impressionis of strong outward mortions of the jets and the comet tails. Their high red and blue shifts confirm this impression. Since stars lose mass, the idea is noit altogether zany.

However, all that glitters in not gold. There are fundamental problems. How did the disk, the FLIERs (knots and jets) form? What is pushing them outward at speeds of about 50 km/sec (30 miles/sec)? How long do they last? Why are they so common? And most poignantly, why are answers to these questions so elusive?

The way in which Doppler images are made is very complex. If you are interested, the method is descibed by Balick et al 1987, AJ, 94, 1641.

Doppler images of the other nebulae are under construction. They promise to be very exciting. Watch this space.

NEW!! NGC7009's Halpha and [NII] doppler images have just arrived. The color coding is the same as for NGC 7662. The ansae (the small features seen furtherst from the star in the [NII] image) have low velocities. Inisde the main body of the nebula the gas has large red and blue shifts, especially near the top and the bottom. See the insets which showthe original ground-based data for the "big picture".

Fast-moving material is seen all of the small and very bright structure found most easily in the [NII] image. These are FLIERs which, as usual, lie along the major axis of the nebula. The long and slightly curved football-shaped outline of the bubble is seen best in the Halpha image. It and the gas outside of it are quiescent. They seem to act as a "pipe" through which the fast-moving material is moving radially, more-or-less toward the ansae.

A physical model for the system has not yet occurred to us. Suggestions are welcome.


Next we take a look at some closeups of the FLIERs using other combinations of pictures taken with WFPC2.


For most of the images the details are 20 times sharper than in the best ground based pictures (0."045 / pixel). The images are laid out as follows:

Two upper left pair of frames are the F L I E R s in N G C 3 2 4 2.
The lower left pair of frames are the F L I E R s in N G C 6 8 2 6.
The upper right pair of frames are the F L I E R s in N G C 7 0 0 9.
The lower right frame is N G C 7 6 6 2. (The image of N G C 7 6 6 2 has been reduced by a factor of three.)

Red is [O I]630nm, Green is [N II]658nm, Blue is Halpha. Pure blue regions are twice ionized gas generally the edge of the inner, bright core of the parent nebula. The red structure consists of neutral gas, and the green structure is once ionized.

Insofar as the FLIERs are concerned, the images show small but easily measurable displacements between the neutral, singly ionized, and doubly ionizaed gas. The importnace of this is that the gas is sufficiently dense to stop the ionizing agent, presumably stellar photons. This requires that the gas is very dense (at least 10,000 atoms per cubic centimeter). How do thse neutral knots of gas form? Why are there comet tails behind many of them? What is the relationship betweentheir structure and their velocities as revealed in the Doppler images? We don't yet have good answers.

The red and green emission arises largely in the FLIERs. Note that the red [OI] is frequently displaced outward (from the star) relative to [NII] and/or Halpha, but not always. This could be the result of a shock or an ionization front.