An apparently asteroidal object of magnitude 18, discovered on July 17, 2006 near the border of the constellations Capricornus/Microscopium by the amateur astronomer J. Broughton showed its cometary nature by closer inspection. On Sep. 17, 2006 comet C/2006 OF2 (Broughton) displayed a condensed 7" coma without a tail, which was elongated towards northeast on Sep. 26. The comet will pass perihelion in fall 2008 and could reach 11 mag (IAUC 8756). From the end of 2007 to mid-2009 it would be brighter than 14 mag. During this period it moves from Aquarius through Pegasus, Andromeda, Perseus into the northern parts of Lynx. Thereafter it moves southward through Auriga and Gemini.
The following analysis incorporates 28 observations by 5 members of the German Comet Section and 190 international observations. These indicate an extraordinary brightness evolution. The heliocentric magnitude increased until 130 days after perihelion (until about Jan. 20, 2009)! Probably the most active region raised its activity until this time (maybe because of an increase of the altitude of the sun). Thereafter it decreased rapidly, probably because the active region had exhausted its gas and dust reservoir. The whole brightness evolution can be described very well by the formulae
t < 130d: m = 9.5 mag + 5×log D - 0.004×(t-T) t > 130d: m = 8.0 mag + 5×log D + 0.008×(t-T)which indicate a maximum brightness of 10.3 mag around Year's end of 2008. The apparent coma diameter measured 0.5' during the second half of 2007. In mid-July 2008, with the comet reappearing, it was in the order of 1.0'. The maximum of 2.5' was reached in December 2008. Thereafter it shrunk, measuring about 1.5' at the end of March 2009 and only 1' in mid-May. The absolute coma diameter measured 75.000 km in mid-2007. In mid-2008 it had increased to 150.000 km. Thereafter it continued to increase, but at a much slower pace until mid-October 2008, when it peaked at 200.000 km. This value was still valid at the end of May 2009. The coma was significantly condensed for much of the apparition. In mid-2007, and again in mid-2008, the degree of condensation was about DC 4-5. Then it condensed slightly, until January 2009, reaching DC 5-6. Thereafter it grew more diffuse, reaching DC 2-3 at the end of May 2009. Visual tail sightings were rare and limited to August 2008, not exceeding 4'.
Total Brightness and Coma Diameter
Andreas Kammerer
