On May 27, 2009 R.H McNaught discovered a comet of magnitude 17 near the border of the constellations Pavo/Octant. Comet C/2009 K5 (McNaught) showed a significantly condensed 10" coma. It will pass perihelion at the end of April 2010, at a predicted maximum brightness of 9.5 mag (IAUC 9050 / MPEC 2009-L37). For mid-European observers it will appear above the southeastern morning horizon at the beginning of February (constellation Scutum). During the following weeks it will move northward, passing Cygnus and Cepheus at the time of maximum brightness.
At the time it reappeared in the moring sky in mid-February 2010 the comet was already 2 mag brighter than expected. This positive trend continued during the next weeks.
This comet showed an interesting brightness evolution, according to 21 observations by 6 members of the German Comet Section and 215 international observations. The comet brightened rapidly until 30 days prior to perihelion. Surprisingly the heliocentric magnitude didn't peak at perihelion. Instead it slowly inceased further for two additional months. As a result the total magnitude peaked at 8.1 mag at perihelion, fading very slowly for the following two months. But then, starting in July, the comet began to fade rapidly. The three evolutionary periods can be described by the following formulae:
t < -30d: m = 4.3 mag + 5×log D + 19×log r -30d < t < +55d: m = 7.5 mag + 5×log D - 0.009×(t-T) t > +55d: m = 4.8 mag + 5×log D + 11.2×log rNo visual observation were published after December 2010 indicating an even more rapid fading. This suggestion was supported by CCD-observations which placed the comet at 15 mag at the end of January 2011.
The apparent coma diameter measured 2' at the start of the apparition, increasing to the maximum of 7' at the end of March. This value was kept over a long period. The decline started not until the final days of May. In the following weeks it shrunk steadily, reaching 3.5' by August. Thereafter the decline slowed with the coma still measuring 1.5' in mid-October. The absolute coma diameter measured 200.000 km at the beginning of the apparition. It increased to 425.000 km until the end of March. This value was kept until perihelion. It was followed by a second increase of the coma, which reached 525.000 km at the start of June. Thereafter it shrunk steadily, reaching 275.000 km in mid-September and 150.000 km in mid-October. Pre-perihelion the coma was moderately condensed (constant at DC 4), showing a small increase to DC 5 during the weeks following perihelion. Thereafter it grew more and more diffuse with the degree of condensation estimated as DC 3-4 in mid-July and as DC 3 in mid-October. Between mid-April and the end of June a tail was seen visually, reaching a maximum length of 15' (1.3 Mio km), pointing constantly westwards.
Total Brightness and Coma Diameter
Andreas Kammerer
