Observer: Hal Weaver
Location: Hubble Space Telescope, Earth Orbit
Date: August 27, 1997 - February 19, 1998
The above figure shows the post-perihelion images of Hale-Bopp obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). Each frame is only 1.25 x 1.25 arcsec in size (25 x 25 CCD pixels), which means that we are focussing here on the innermost regions of the coma. The horizontal bar in each case subtends a distance of 500 km at the comet.
All three images have been normalized to the same peak brightness and displayed using the same logarithmic intensity stretch. There has been no image processing to enhance, for example, any coma jets or shell structures that might be present. The images have not been rotated to a common orientation because that would involve resampling the images, which leads to degradation of the resolution. Instead, the compasses show the celestial orientation of each image.
Inspection of the above images does not reveal any obvious companion(s) to the main nucleus of Hale-Bopp. The peak pixel stands out more clearly in the Aug97 image than in the other two images, which could be interpreted as the effect of two unresolved nuclei. But such an effect could also be produced if the light from a single nucleus falls near the border of two pixels. Detailed modeling must be performed before anything more can be said.
If there is a companion, then either it has remained within 1 STIS pixel (0.05 arcsec) of the primary for more than 3 months, or we were unlucky and observed near the time of an orbital transit event (i.e., when the two objects appear to move across each other) in both Nov97 and Feb98.
As you may know, Zdenek Sekanina (JPL) claims to detect at least one, and possibly several, companions to Hale-Bopp based on a detailed analysis of the pre-perihelion HST images taken with the WFPC2. I point out that these companions, if they exist, are also not obviously present in the HST images. Their presence is deduced only after comparing model images with the observed images. Thus, the apparent lack of obvious companions in the STIS images discussed above does not rule out the possibility that Hale-Bopp has companions of the type described by Sekanina.
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