Analysis of past comet apparitions

C/2007 N3 (LULIN)

An apparently asteroidal object of magnitude 19, discovered on July 11, 2007 in the course of the Lulin Sky Survey (Taiwan) in the constellation Aquarius showed its cometary nature by closer inspection. Comet C/2007 N3 (LULIN) displayed a 2-3" coma with a central condensation. It will pass perihelion in mid-January 2009 and could get as bright as 6 mag (assuming n=3 it still could reach magnitude 7.5) (IAUC 8857 / MPEC 2007-Q09). Mid-European observers can glimpse the comet for the first time at the end of 2008 with the comet situated above the southeastern morning horizon in Libra. Until end of March 2009 it will move towards Gemini, then being best visible in the evening sky.

During the days of perigee comet C/2007 N3 (LULIN) reached 'naked eye visibility'. While being an inconspicuous object to the naked eye and even a difficult one under a suboptimal sky, in binoculars it showed an interesting tail development. The following analysis is based on 84 observations by 13 members of the German Comet Section and 375 international observations. Neglecting the observations shortly before conjuntion, which are too bright, the comet showed a very smooth brightness evolution, which can be described by the formula

m = 5.4m + 5×log D + 10.3×log r

The maximum brightness of 5.0 mag was reached on Feb. 25. After perigee the brightness decreased rapidly due to the increasing distances to both the Earth and the Sun.

During the first weeks of its apparition an apparent coma diameter of 1.0' was observed, increasing to 4.5' during the second half of August where it remained until conjunction. Thereafter it increased steadily, reaching 11' in the first week of February. Until perigee the increase accelerated further, reaching a maximum of 24' around Feb. 25. Thereafter it decreased at a similar pace, reaching 9' in mid-March, 4' at the end of April and only 1.5' in mid-May. The evolution of the absolute coma diameter went differently. During the first weeks it was 100.000 km large, but increased rapidly to 500.000 km between mid-August and the end of October 2008. After the conjunction with the sun the coma measured 525.000 km until mid-March 2009. Thereafter it shrunk significantly, measuring 400.000 km in mid-April and only 200.000 km in mid-May. However, due to the deteriorating observing conditions at that time, the last value could be too small. Before the conjunction with the sun the coma was significantly condensed (constant at DC 5-6). After the conjunction with the sun the degree of condensation decreased from DC 5-6 to DC 3 at the end of April 2009. The false nucleus, which was well recognizable at times, reached a maximum brightness of about 9.5 mag.

Total Brightness and Coma Diameter

Visual tail reports were received during the period mid-January to end of March 2009. The comet showed a gas tail as well as a dust tail, with the latter - due to the special geometry (the Earth was close to the comet's orbital plane for several weeks) - appearing as an antitail. The thin gas tail was visually observed between mid-January and Feb. 20. At first it was oriented westward, later to WNW, reaching a maximum length of 1.5°. The absolute length can only be estimated roughly to 3 Mill. km since the tail was temporarily significantly shortened due to the perspective. The wide, at times cone shaped dust tail showed a high surface brightness and was observed between the end of January and the end of March. It pointed eastward and reached a length of 1.5° as well. Due to the geometric situation and the curvature of the dust tail its absolute length is even more difficult to estimate, but it should have been a bit longer than the gas tail.

Andreas Kammerer

FGK observations