Terry Jay Jones, Professor of Astronomy
Office: Physics 355, Phone 612-624-8009, Fax 612-626-2029 email: tjj@astro.spa.umn.edu

IR Imaging Polarimetry Comet Hale-Bopp


After two observing runs in 1996 with no measurable polarization, Hale-Bopp finally decided to cooperate in April of 1997. Below is a series of panels showing the intensity, edge filtered and polarization of Hale-Bopp at K (2.2 microns).


Intensity Image

At right is a normal intensity image of the comet at K. Note how the combination of the jet activity and the rotation of the comet creates a clear 'sprinkler hose' pattern on the side of the nucleus facing the Sun (SW). If you look carefully, you will see three jet arcs, each one rotation of the nucleus apart from the other. Also note that the jet is not a very high contrast feature in the intensity image, sticking up only 20-30% above the underlying continuum of the coma.

Edge Filtered

This is the same image run through a Sobel edge filter. The filter just takes the derivative of the image and is brightest where the slope in the intensity is greatest. The bright arcs to the SW are at the leading edges of the jets.

Percent Polarization

Here is the percent polarization image. The brightest spots in the second jet arc are about 11% at 2.2 microns. The darkest areas to the NW are about 7%. The polarization for the whole comet and the nucles is about 9.5%. If all of the grains in the coma were the same everywhere, then the polarization would be the same everywhere as well. The clear presence of the jet in the polarization image indicates that the grain population in the jet material is different is some ways than the general grain population lifting off of the nucleus. Most likely the grains in the jet are significantly smaller than the grains in the rest of the coma.

Polarization vs. Phase

Normally, the polarization characterisitcs of comets as a function of phase (scattering) angle are similar to asteroids. They show a 'negative' branch (polarization in the plane of scattering) at small phase angles. In asteroids, this is interpreted as due to a rough surface. In comets this is usually attributed to large fluffy aggregates of small grains. At 2.2 microns, Hale-Bopp shows a polarization that lies somewhat in between the normal behavior of comets in the visual and pure Rayleigh scattering. This is probably due either to 1) a smaller grain population size in Hale-Bopp than in most comets 2) the effects of observing at a much longer wavelength where the fluffy aggregates are begining to become comparable to the wavelength or 3) a combination of the two.

AAS Poster Text


I. Phase Curve


1. Hale-Bopp shows no negative branch at the 0.5% level at 2.2um.
2. The comet is more highly polarized in the near infrared at intermediate phase angles than comets observed in the visual.

II. Intensity Image


1. The jet is a low contrast feature.
2. The radial intensity profile of the jet remained unchanged over a period of 3 hours.

III. Polarization Image


1. The spiral jet is clearly more highly polarized than the surrounding coma.
2. The polarization in the jet has a much higher contrast to the rest of the coma than the intensity.
3. The polarization is highest on the leading edge of the jet, at least at this particular epoch.
4. The percent polarization is not constant in the jet from one rotation period to the next.
5. The more distant jet arc is more highly polarized than the inner one.

Conclusions

1. At near infrared wavelengths, the dust grains are probably intermediate in behavior between fluffy aggregates and Rayleigh particles. This should place constraints on the size of the aggregates.
2. The grains in the jet are significantly smaller than in the surrounding coma.
3. The grain size distribution in the jet material varies on the time scale of hours.
4. The grains or grain aggregates may still be breaking up as far out as 30" from the nucleus.
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