Observer: Olivier Hainaut, Richard West, Manuel Pizarro, Vicente Reyes
Location: European Southern Observatory, La Silla, Chile
Date: March 19, 1996
The most recent ESO observations of bright Comet Hyakutake have shown rapid changes in the innermost coma, within a few hundred kilometres from the cometary nucleus. This result has only become possible because of the unusual combination of a bright comet being near the Earth, together with the excellent imaging quality of the ESO 3.5-metre New Technology Telescope (NTT).
The images shown here are based on a unique series of fourty-four 2-sec exposures, obtained under excellent conditions (seeing 0.7 - 0.8 arcsec) in the morning of March 19, 1996 with the SUSI camera at the ESO NTT at La Silla, Chile. The observers, Olivier Hainaut (Institute of Astronomy, Honolulu, Hawaii) and Richard West (ESO), were assisted by Manuel Pizarro (ESO La Silla) and Vicente Reyes (ESO Garching).
These images were obtained during an observing run in remote control from the ESO Headquarters in Garching (Germany). In order to better show the interesting features, they are all reproduced in false colours.
The comet was 0.222 AU (33.2 million km) from the Earth and 1.165 AU (174 million km) from the Sun at the time of these observations. At this geocentric distance, 1 arcsec corresponds to 161 km. The nominal angular resolution, as indicated by the seeing, was about 110 kilometres projected at the distance of the comet.
The comet was moving at a speed of about 600 arcsec/hour in the sky and it was not possible to use the automatic guider. Instead, the NTT was set to perform blind tracking at the rate of the cometary motion. During each exposure, i.e. in 2 seconds, the comet moved 0.5 arcsec in the sky. The advantage of using a large telescope like the NTT for this type of observation is not only its intrinsically good imaging quality, but in particular that enough light to show even faint details may be collected in very short time, thereby avoiding additional image smearing because of the motion during the exposure.
The first image, ESO Press Photo 25a/96 , shows a combined image, based on 24 R-frames with a total exposure of 48 seconds. It shows the asymmetric shape of the coma contours (isophotes), indicating the presence of much structural detail in the coma. The frame measures 145 x 145 arcsec; 1 pixel = 0.128 arcsec. North is up and East is to the left. The distance between the two components of a close double star, best seen in the upper right part of the image, is 1.9 arcsec. The multiple images of the same star is caused by the combination of several frames - this also shows the comet's very large rate of motion.
For the further analysis, a comparison was made of two series of 12 R-frames each, obtained at mean epochs 19.342 and 19.363, that is with a 30-min interval.
ESO Press Photo 25b/96 [GIF,43k] and ESO Press Photo 25c/96 [GIF,43k] show the asymmetric components of these two, combined frames. They have been produced by subtracting the corresponding radially averaged frames, thus leaving only asymmetrical features behind. The field shown in these frames is 60 x 60 arcsec, i.e. about 10,000 km square at the distance of the comet. On these frames, 1 pixel = 0.064 arcsec.
In both frames, a straight ion tail is seen at p.a. 276 (towards West, i.e. right); it is noticeable at only 0.7 arcsec (115 km projected) from the peak of light (within the nucleus is presumed to be located).
Strong dust jets are also visible. They trace the ejection of dust particles from the surface of the cometary nucleus. Of particular interest is the fact that there are substantial morphological changes on arcsec scale within the short time interval between these exposures. This is well brought out on a `differential' frame that has been obtained by subtracting the frame from the later epoch from that of the earlier one (ESO Press Photo 25d/96 [GIF,54k]).
On this false-colour photo, black areas are where the intensity is strongest in the first image (25b-96), and red where it is so in the second image (25c-96). In addition to a well indicated clockwise rotation of approx. 20 deg, there are also apparent changes in the jet structure; for instance, there seems to be more such structures in the first frame than in the second. By careful comparison, these differences can be perceived directly on the combined frames also.
This is the caption to ESO PR Photos 25a-d/96. They may be reproduced, if credit is given to the European Southern Observatory.
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