Analysis of past comet apparitions

C/2006 A1 (Pojmanski)


The first comet of the year 2006 was already discovered on Jan. 4. Grzegorz Pojmanski of the Warshaw University Observatory found the comet on images taken by the Automated All Sky Survey (ASAS) on Jan. 1 and Jan. 4, resprectively, near the border of the constellations Indus and Pavo. Browsing the archive showed the object as far back as Dec. 29, 2005. Then K. Cernis reported a comet found on SWAN images as far back as Dec. 25, which turned out to be identical with the first one. On a CCD image taken on Jan. 5 with a 0.46 m telescope the comet presented itself as an object of magnitude 14.5, displaying a 1' coma. This lead to the prediction of a maximum brightness of about 10.0 mag at perihelion, which will be passed at the end of February 2006 (IAUC 8653/54, MPEC 2006-A40). For mid-European observers comet C/2006 A1 (Pojmanski) will appear over the eastern morning horizon at just that time, situated in the constellation Aquila. Until the beginning of April the fading comet will move through the constellations Delphinus, Vulpecula, Cygnus and Lacerta, thereby reaching altitudes of almost 40°.

After the Moon did no longer interfere with observations, the comet was observed visually. On Jan. 23 it surprised the comet observers community with a brightness of 7.5 mag and coma diameter of 4'. Thereafter the comet brightened steadily until end of February/beginning of March, as is documented by 14 observations of 7 members of the German Comet Section and by 185 international observations. These demonstrate different brightness evolutions prior and after perihelion, according to the formulae

pre: m = 8.5m + 5×log D + 11.5×log r
post: m = 7.8m + 5×log D + 8.8×log r

Total Brightness and Coma diameter

These yield a maximum brightness of 5.1 mag during the last days of February / the first days of March, just at the time when the comet appeared above the southeastern horizon for mid-European observers. The situation in May is unclear. The brighter estimates are supported by the above formulae, but an observation by Stefan Beck hint to the fainter ones. He could not find a trace of the comet brighter than 18 mag on his CCD-image of July 3.

The apparent coma diameter increased from 3' at discovery to 5.5' in mid-March. Thereafter it shrunk continuously to 1.5' until the start of May, where it hovered during the following weeks. The absolute coma diameter measured 180.000 km at discovery, but decreased slightly to a minimum of 160.000 km at the beginning of March, due to the increasing solar wind. Thereafter it swallowed significantly, reaching a maximum of 240.000 km at the beginning of April. Until mid-May it decreased to 140.000 km. The compression of the coma resulted in an increase of the degree of condensation from DC 3 at discovery to DC 7 at the beginning of March. Thereafter it decreased, reaching DC 1-2 at the beginning of May. Visual sightings of a false nucleus were not reported. A visual tail was first reported around Feb. 10. It reached 1.0° (2 Mill. km) at the start of March. During the first week of April it was last sighted. In February the tail was oriented SSW, turning steadily to NW until the start of April.

Andreas Kammerer

FGK observations


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