On Apr. 4, 2009 R.D. Matson reported the discovery of a comet in data aquired by the satellite SWAN. By Apr. 6 one visual observer and several using CCD equipment confirmed the comet. Hereupon H. Yamaoka, of Kyushu University, reported to have received an email from the Korean Dae-am Yi, in which he informed him about the discovery of a 12 mag comet on two CCD images taken on Mar. 26 in the constellation of Lacerta. On one image taken with a digital camera on Mar. 25 the comet could be identified as a 10.7 mag object. Visual observations of comet C/2009 F6 (Yi-SWAN) from Apr. 6 showed it as a 8.5 mag object with a condensed coma of diameter 4' (IAUC 9034/35).
The comet showed an unusual brightness evolution as is indicated by 14 observations by 7 members of the German Comet Section and 65 international observations. According to these the heliocentric brightness is decreasing since discovery, indicating an outburst during which the comet was detected. Accordingly the maximum brightness of 8.3 mag was reached at the date of discovery. The brightness evolution can be described by the formula
m = 7.7 mag + 5×log D + 0.018×(t-T)in which the time dependency also indicates an outburst. In parallel the coma shrunk during the mentioned period from 5' (425.000 km) to 3' (300.000 km). In addition, the coma got more diffuse. Starting at a degree of condensation of DC 4 at discovery it decreased to DC 2 at the end of May.
Total Brightness and Coma Diameter
Andreas Kammerer
