C/2007 T1 (McNaught)

On Oct. 9, 2007 Rob NcNaught discovered a 13 mag comet on images taken in the course of the Siding Spring Survey near the border of the constellations Ophiuchus/Serpens. Comet C/2007 T1 (McNaught) showed a 2' coma with a significant central condensation, as well as a 75" long tail in p.a. 80°. According to the first orbital elements the comet should have passed the Sun at 0.3 AU at the end of November, with a proposed maximum brightness of 6 mag. Actually the comet will pass its perihelion in mid-December at 1.0 AU with a proposed maximum brightness of only 12 mag (IAUC 8877 / MPEC 2007-V53). It disappeared above the evening southwestern horizon at the end of October for mid-European observers. During the first week of March it will reappear above the southern evening horizon. It will move from Columba into Monoceros until May 2008.

Probably it brightened significantly shortly before its discovery, since it should have been brighter than magnitude 16 since summer (assuming an average evolution), thereby being favourably positioned.

For the analysis 75 international observations could be used. They show a significantly different brightness evolution pre- and post-perihelion, according to the formulae:

what means that the comet reached a maximum brightness of 8.6 mag during the Christmas days 2007.

Total Brightness and Coma Diameter

The coma diameter increased from 2.5' (200.000 km) at the beginning of the apparition to a maximum of 4.5' (250.000 km) during January and the first half of February 2008. The rapid decrease at the end of March is most probably due to the then deteriorating observing conditions. The coma was medium condensed, with the degree of condensation nearly constant around DC 3-4.

Andreas Kammerer