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Comet Hale-Bopp Light Curve Update

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Mark Kidger
May 7, 1997
mrk@ll.iac.es

The best part of the big show is now over and the comet is very definitely fainter than magnitude 0 and dipping deeper and deeper into twilight.

As near as can be estimated from the light curve the magnitude finally faded below 0 on April 26th, although there were still some estimations being made at negative magnitude in early May. This gives a final total period as a negative magnitude object of around 50 days (7 weeks), slightly less than we might have hoped, but still really exceptional.

Hale-Bopp is made more exceptional by the fact that most of the time when it was brightest it was visible in a dark sky after the end of twilight. Only a very few comets over the last few centuries have ever been magnitude zero or brighter in a dark sky and I can find no really clear-cut case where the observations (not the theoretical light curve) seem to imply that a comet was significantly brighter than Hale-Bopp in a dark sky.

With the large dispersion in the light curve, it is dificult to judge the exact date of maximum, particulaarly as this maximum was very flat and severely affected by the lack of suitable comparison stars. The best estimate is that maximum was about 2-3 days before perihelion, in other words, around March 29th-30th. This is a little later than was expected because the geocentric distance was already increasing at that time. This may indicate that fresh activity was appearing on the surface of the nucleus even at perihelion with the absolute magnitude still brightening slightly.

The fade in brightness post-perihelion is rather slower than the pre-perihelion rise. There is a dip over the last few days, but that may be due as much to poor observing conditions (twilight and low altitude) than to a real large fade in the comet's brightness. The indications are that the comet will still be a bright object for several weeks to come allowing southern hemisphere observers a brief taste of what they have missed.


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