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The Tail of Comet C/1996 B2 (Hyakutake)

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At the 2. International Workshop on Cometary Astronomy (IWCA II), held in Cambridge, England in August 1999, I presented a poster in which I demonstrated that the problematic tail length estimates (reporting a tail length greater than the phase angle) can only be explained thoroughly by assuming physiological effects. In response to IAUC 7395 IAUC 7395 I want to present this poster here. As can be read in the following text the results obtained by Ulysses still cannot provide a plausible explanation of these observations. The derived deviation from the anti-solar-direction of 60 at perihelion (at 0.2 A.U.) cannot be extrapolated to the situation in March. Because of the much smaller velocity of the nucleus and the neglecible curvature of the orbit at 1. A.U. the deviation must have been much smaller near earth. However, the most extreme estimates require a deviation from the anti-solar-direction of up to 40! Furthermore, as can be seen in the diagram, the tail of comet Hyakutake during earth passage (no matter how great a curvature is assumed) had to rotate in step with the changing position of the earth to properly explain the problematic estimates. A rather geocentric (or even anthropocentric) view! Eventually, it is by no means realistic to compare a tail length derived via a magnetometer to the visual tail length! Conclusions: with the Ulysses data in mind the estimates near the phase angle are becoming a bit more plausible, but in no means is this the case for the extreme ones, as Geraint Jones, one of the authors of the Nature article, agreed in a private communication. Andreas Kammerer.

There is still some discussion about the tail of comet Hyakutake. Several observers, including experienced ones, claimed having seen visual tails approaching or exceeding the phase angle. Most of these estimates were made during the days of the earth passage.

Soon after this problem was recognized, a debate arose about the reality of these estimates. The critics suggested that these estimates should be regarded as a result of physiological effects of the eye-brain-system (which tends to lengthen linear features at the threshold of detection) and of the influence the rapid publication of observations via the internet has on observers.

The defenders of the reality of these observations suggested that a thorough tail modeling should be made to support them. However, until now the author knows of no successful investigation.

In principle, there are three possible approaches to explain the problematic observations: an extraordinarily long tail or a tail deviating from the antisolar direction or a combination of both effects. A comet with a heliocentric magnitude around 5m normally displays a visual tail of around 20 million km. But even the assumption of a visual tail of 100 million km is not enough to explain all the problematic estimates! All of them can only be explained by the assumption of a 100 million km tail plus a deviation from the anti-solar direction of up to 40 toward the earth! What makes these assumptions even more unrealistic is the fact, that the tail would have to rotate in step with the changing position of the earth!!

The diagram demonstrates in which way the tail has to be deviating from the anti-solar direction (assuming a 100 million km visual tail). Regarding the average tail length of a comet with a heliocentric magnitude around 5 mag the depicted deviation angle must be regarded as the smallest required. Interestingly, the observers reporting problematic estimates noted position angles in accordance with a tail in anti-solar direction!

Conclusion: it is much more convincing to state that the problematic tail estimates were caused by physiological effects of the eye-brain-system, which should be taken in account in the future. The visual tail length of comet Hyakutake therefore did not exceed 75 (perhaps 80) at maximum.

Ettlingen, June 6th, 2000.