Comet LINEAR 1999 S4
Imaging Results from Before Breakup

Our imaging results consist of measurements obtained with both broadband filters (V and R) and narrowband filters that isolate different moecular species (see the photometry page for more information). Data obtained with the narrowband filters allow us to look at the morphology of the gas species in the coma. So far, our analysis has concentrated on the broadband images, mainly because the give the highest signal-to-noise.

We have images, obtained from McDonald Observatory and Lowell Observatory, on 28 nights between November 2, 1999 and July 9 2000. Typically, the observations consist of approximately monthly coverage (when the comet was not too close to the sun), however, in January and July we have extensive coverage.

Typical Appearance and Morphology

From the time it was discovered through March 2000, when it went into solar conjunction, LINEAR had a typical round or teardrop shape to its coma. There were no obvious features or significant changes in the coma or tail.

After LINEAR come out of solar conjunction in May, however, it had changed markedly. It frequently displayed unusual morphologies in its inner coma and experienced a number of outbursts. Various observers recorded large outbursts on June 2, June 16, July 5 and July 18, with a number of smaller outbursts in between.

The outburst of July 5, 2000

From July 5-9, 2000, we obtained images of comet LINEAR. On July 5, an outburst occurred, in which the comet brightened by at least 1.5 magnitudes and was accompanied by very dramatic changes in the appearance of the coma. Furthermore, the Hubble Space Telescope observed a fragment that had broken off the nucleus and was drifting down the tail.

July 5-9, 2000 outburst sequence:

July 5, 2000	July 6, 2000	July 7, 2000
July 8, 2000	July 9, 2000

This sequence of images shows the evolution of the coma after the outburst on July 5, 2000. The images were obtained with an R-band filter and have been processed to remove the bright central peak and to enhance the low contrast features. The nucleus is at the center of each image, and the sun is to the left. The bright triangle extending to the right is the plasma tail, over-enhanced by the processing. The inset images show, at the same scale, the shape of the inner third of the larger frame, with no processing.

The inset images show the coma getting brighter on July 5 and 6, and then fading again over the next few days. The coma also broadens out perpendicular to the sun-comet line, producing a rounder shape than before. After the comet fades again, the coma is more elongated than it was before the outburst.

The larger images show the dramatic changes in the coma morphology. On July 5, there is little structure in the coma other than the plasma tail. On July 6, "wings" start to form perpendicular to the comet-sun line, reaching their brightest point on July 7 before they start to fade again.
Closeup of jet in July 8 image
On July 8, a 5000-km (3000 mile) long jet was seen
pointing upward from the nucleus. A blow-up
of the region showing the jet is shown here.

What is causing the changes in appearance?

The wings and jet are formed by dust being emitted from an active region or vent on the surface of the comet. As the dust moves outward, solar radiation pressure pushes it away from the sun, making the wings appear to curve.

It is unlikely that the wings are formed by two separate vents, because they appear so much alike from night to night and they appear concurrently with outbursts (the same features were seen after at least two other outbursts). So the most likely explanation is that there is only one active region, but the nucleus is spinning, causing the vent to point in different directions. When the vent is near the plane of the sky, the emission forms the wings that we see. On July 8, emission from the vent is actually seen as the 5000-km (3000 mile) long jet.

Rotation of the Nucleus

The previous interpretation of the coma morphology allows us constrain some of the properties of the nucleus' rotation state:

Rotation period: Because the wings on opposite sides look so much alike in each image, the rotation period must be relatively short (less than 12 hours). This allows the material in the wings to be replenished fairly frequently, maintaining the uniform symmetry. If it took longer than 12 hours for one rotation, then one wing would be dissipating while the other is being replenished, producing an asymmetric appearance.
Orientation of the spin axis: We can use the line of symmetry (LOS) of the wings to figure out where the spin axis is pointing, and where on the nucleus the active region is located. The LOS is simply the projection of the spin axis onto the sky, which means the spin axis lies in the plane defined by the LOS and the Earth. With this information from at least two dates, the spin axis can be defined in three dimensions (the intersection of two planes is a line, which is the axis). We find that the rotation axis points to a right ascension of 109^o and a declination of 22^o. There is not enough information to determine whether the rotation is prograde or retrograde, however.
Location of the Active region: As can be seen from the above images, the wings are emitted from the nucleus at and angle of about 70-80^o from the LOS. This means that the active region is located at a latitude of about 10-20^o from the equator.

Go to the Imaging Results from After Breakup.