On Sep. 9, 2009 R.H. McNaught discovered a comet near the border of the constellations Microscopium/Piscis Austrinus. Comet C/2009 R1 (McNaught) showed an 8" coma of magnitude 17. It will pass perihelion in July 2010 in a distance of only 0.4 AU from the sun and could reach magnitude 5. However, during this days it will be positioned near the sun (IAUC 9071). Mid-European observers could follow the evolution from about May 15 (at predicted magnitude 9.5) until June 20 (5.5 mag), however rather low above the northern morning horizon, moving from Andromeda into Auriga.
In spring 2010 the comet was brighter than expected. Even the first observations in early April showed the comet to be about 1.5 mag brighter. This was due to a brighter absolute magnitude rather than an activity above average.
Despite the unfavorable circumstances the analysis can be based on 27 observations by 6 members of the German Comet Section and 140 international observations. They clearly show a significant change in the brightness evolution on June 7, 2010. During the first weeks the comet's brightness evolution was similar to that of a new one. Then, 25 days prior to the perihelion passage the activity level dropped significantly. Therefore, the comet could not reach a maximum brightness of 4.2 mag as predicted by me, but peaked at only 5.4 mag around perihelion. The brightness evolution can be well described by the following formulae:
t < -25d: m = 6.7 mag + 5×log D + 9.2×log r t > -25d: m = 5.9 mag + 5×log D + 2.5×log rEvolution of the heliocentric magnitude
Due to the unfavorable circumstances the evolution of the coma can only be defined with greater uncertainty. It seems that it increased from 1.5' (100.000 km) in mid-April to about 7' (425.000 km) at the end of May, only to decrease to 4' (200.000 km) at the end of June - likely a value affected by the deteriorating observing conditions. During the first five weeks (until May 20) the degree of condensation was constant at DC 4-5. During the following weeks it increased to a maximum of DC 7-8 around June 20, getting more diffuse during the last days of the apparition. A visual tail was sighted between May 25 and June 25 with a maximum length of 2° (7 Mio. km) in mid-June. The decrease during the following days was most likely caused by the deteriorating observing conditions.
Total Brightness and Coma Diameter
Andreas Kammerer
