These images are different enhancements of a 500 second exposure of Comet C/1996 B2 (Hyakutake) taken on March 11, 1996 (11:40 UT) through a narrowband H2O+ filter (central wavelength 6200A, FWHM of 15A). The initial image was taken by Dr. Walt Harris from the Space Astronomy Lab of the University of Wisconsin using the 3.5m WIYN telescope on Kitt Peak with a 2048x2048 CCD chip with a plate scale of 0.2"/pixel, and was reduced by Dr. Beatrice Mueller from NOAO.
These images were taken at the 3.5m WIYN telescope during queue-mode observing, for the collaborative project between Dr. Walt Harris (Wisconsin) and Dr. Beatrice Mueller (NOAO). There is one image from each of the nights of Tuesday March 19th and Wednesday March 20th. These frames show a much smoother tail coming from the head of the comet, as compared to the March 11th image (see above) probably due to increased, no longer patchy, activity. Different physical components making up the comet are represented by different colors in these false color composites: the dusty material is shown as red, water and ice are shown as green, and carbon emission (basically, soot) is shown as blue. Color gradients therefore show the different distributions of these components due to their varying densities and level of activity. The image from March 20th has more exaggerated color gradients than the picture from the day before: this probably does not reflect a physical difference, simply the way the picture was created. The comet is moving very rapidly across the sky, and stars appear as faint streaks in only one color: by the time the image of the next comet component is taken, the stars from the previous field are already out of the frame. There are also some colored "flecks" visible: these are due to random events triggering the very sensitive CDD detector and are quite spurious. These `cosmic rays' are usually edited out of images before final use, but that rather time-consuming step has not yet been performed. The reductions of these images were done by Dr. Nigel Sharp at NOAO.
Here we show two observations from the WIYN telescope from the nights of Sunday March 24th and Tuesday March 26th (UT of observation 25/03/96 12:10 and 27/03/96 11:35). These images show the nucleus of Comet Hyakutake, revealing apparent `chunks' moving back along the tail, as well as a strong `fan-like' structure in the coma, mostly in front of the nucleus. The frames are paired, with the earlier observation to the left, and are presented in black and white and in false color. The images were then enhanced, by dividing by a smoothed model in order to show the structures more clearly, and the result is also shown in false color. These observations were made by Dr. Walter Harris from the Space Astronomy Laboratory (SAL) at the University of Wisconsin, Dr. Kent Honeycutt from Indiana University, and Dr. Beatrice Mueller from NOAO. These observations were only possible because of special non-sidereal tracking software written especially for this observation by Jeff Percival (also of SAL). Conditions on Kitt Peak were better for the first observation, which shows smaller scale structure which is more blurred in the later data, but the feature in the tail is clearly moving out behind the nucleus, and the structure in the `fan' also changes with time.
These images were created from observations made on the night of Monday March 25th, 1996. The first frame is a composite of two, two-minute exposures using a blue continuum filter (4875/38 Angstroms), and shows similar structure to other images taken of Comet Hyakutake and available at this site. This picture is shown using a similar false color scheme. The second frame, however, is a summation of three, two-minute exposures taken using a filter (at 3919/199 Angstroms) that isolates emission coming from a bright set of CN lines (the cyanogen molecular complex). Two pictures are shown derived from this frame, firstly a false color representation of the summed image, and secondly a false color version of the image after enhancement by dividing by a smoothed model of the image. A shaded 3D plot is also shown of the summed image.
The images comprising the first frame were taken before and after the observations making up the second frame, so that the differences are not simply a change in time but represent a definite difference in the comet's structure between different molecular species. In the CN frame, a bright structure is observed extending in the anti-sunward direction from the comet, appearing as an arc crossing the tail with a faint conical feature reaching as far as the nucleus itself. The conical structure is only detectable in bright resonant emission lines from neutral gas such as CN or OH. For example, there no trailing features apparent in the continuum filter image, which looks mainly at dust. The shape of this feature is most reminiscent of a 3-dimensional analog to a ship's wake, and that may indeed be what it is, although considerable study is still required before we will know for sure the causes of this structure or its composition.
These observations were performed at the 3.5m WIYN telescope by Dr. Walter Harris from the Space Astronomy Laboratory (SAL) at the University of Wisconsin and Dr. Kent Honeycutt from Indiana University with the assistance of Alex Macdonald ( NOAO).
Note: All image enhancements were done by Dr. Nigel Sharp (520-318-8273) at NOAO.