Analysis of past comet apparitions

6P/d'Arrest

2008


During summer 2008 comet 6P/d'Arrest should be a rather well observable evening comet. Positioned in Aquila at the beginning of July it will move southward in an accelerating manner, entering Microscopium at the end of August when it will disappear above the southern horizon for mid-European observers. Applying the brightness formulae I derived from the 1995 apparition the comet should reach 8.5 mag before disappearing. Probably it could be 1 mag brighter, because it was surprisingly faint during the 1995 apparition. The ICQ gives a maximum brightness of 10.5 mag. However, this comet is prone of its very diffuse coma.

Until the end of 2008 only 7 observations by 4 members of the German Comet Section were received. For the analysis 75 international observations were taken into account. These show an asymmetric brightness evolution, which is a well-known feature of this comet.

t < +5d: m = 11.7 mag + 5×log D - 0.146×(t-T)
+5d < t < +55d: m = 11.2 mag + 5×log D - 0.035×(t-T)
t > +55d: m = 8.4 mag + 5×log + 0.017×(t-T)D

resulting in a maximum brightness of 8.6 mag in mid-October.

Total Brightness and Coma Diameter

During the first weeks of the apparition the apparent coma diameter increased rapidly from 2' at the end of July to 13' at the first days of August. The following decrease was more slowly. The coma measured still 9' at the end of September, 5' in mid-November and 2' at the end of December 2008. The absolute coma diameter increased from 45.000 km at the end of July to 250.000 km at the start of September. This value was constant until mid-November. Thereafter the coma shrunk slowly, measuring 150.000 km at the end of December. The degree of condensation was DC 3 at the beginning of the apparition, decreasing only marginally to DC 2-3 until end of September. Thereafter the coma started to become more diffuse, measuring DC 1 at the end of December.

Andreas Kammerer

FGK observations


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