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Comet Hyakutake's Tail Structure

4 mars 1996

This is the most detailed image so far obtained of the complex tail system of Comet Hyakutake. It was obtained with the ESO 1-m Schmidt telescope at La Silla on February 28.36 UT under good observing conditions by ESO night assistant Oscar Pizarro. The exposure lasted 60 min and was made on sensitized Kodak Pan 4415 film behind a GG385 filtre. This emulsion/filtre combination allows to record a broad spectral interval (3900-7000 A) whereby faint structures are better seen. However, detailed spectral information, by which the emitters may be identified, is lost for the same reason.

In order to show the faintest possible details, a photographically amplified reproduction of the film was made by Hans-Hermann Heyer in the photographic laboratory at the ESO Headquarters in Garching. Due to this process, small variations in emulsion sensitivity are greatly enhanced also, resulting in an uneven background. However, this procedure allows to characterize in some detail the very complex tail system.

The field shown on this photo measures approx. 2.25 x 1.39 degrees. North is up and East is to the left. In what follows, the position angles (p.a.) are measured in degrees, counterclockwise from North over East.

The coma measures approx. 20 arcmin across. A broad and featureless dust tail is seen extending at p.a. 280. It can be followed until 40 arcmin from the centre of the coma.

A rather narrow ion tail extends to about 20 arcmin at p.a. 289. Here it branches into two main components. The most conspicous is the broader one at p.a. 289. It has a very complex structure and there are strong kinks, 35 and 96 arcmin from the center. They are caused by the comet's passage through magnetic boundaries in the interplanetary magnetic field. The total length of this tail exceeds 2.7 degrees on the original plate. There is another, rather narrow ion tail with sharp borders between p.a. 300 and 301 that can be followed to about 100 arcmin distance. Finally, there are several streamers in the interval between p.a. 305 - 315, some reaching to about 25 arcmin.

The tail structure is therefore quite complicated and reflects the comet's increasing production of gas and dust as it moves closer towards the Sun. All of the indicated features are reasonably normal for a large comet at the given heliocentric distance (230 million kilometres).

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