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Analysis of Comet Apparitions


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141P/Machholz

2020/21


On Aug. 13, 2020, comet 141P/Machholz (P=5.34a) was recovered as a 21.5 mag stellar object in the constellation Ophiuchus. This periodic comet, which was discovered in 1994, showed up one or several fragments in most apparitions. Every fragment, however, was only observed during one apparition. The current perihelion passage will happen at a solar distance of 0.81 AU on Dec. 16, 2020, followed by a perigee of 0.53 AU on Jan. 19, 2021 (CBET 4834). The brightness development is uncertain, but so far the comet showed a rapid increase in activity pre-perihelion with a distinctly slower decrease post-perihelion. During its last apparition in 2015 the derived parameters were m0=15.0 mag / n=10 pre-perihelion and m0=13.0m / n=6 post-perihelion. Applying these values the comet should be brighter than 16 mag between early November 2020 and mid-March 2021. In December the comet should peak at magnitude 11. However, the CBAT utilizes the parameters m0=12.0m / n=12, indicating a maximum brightness of 8.5 mag. During the period mentioned above the comet moves through the constellations Scutum, Aquila, Aquarius, Cetus, Eridanus and Orion and is therefore comfortably placed for mid-European locations at the end of evening twilight.

The comet did not get as bright as expected by CBAT (8.5 mag), but brighter than predicted by myself (11.0 mag). Based on 81 observations from 26 observers the comet peaked at magnitude 10.2 on Jan. 2, 2021. Until 17 days past perihelion its brightness increased rapidly and can be described best by a time-dependent formula, matching its typical high state of activity while approaching the Sun. Thereafter its brightness – again typical for this comet – decreased much slower. The formulae are as follows:

t < +17d: m = 13.5 mag + 5×log D + 0.12×|t-T|
t > +17d: m = 12.0 mag + 5×log D + 9.5 ×log r

The coma diameter increased from 2' (75.000 km) at the start of December 2020 to the maximum value of 6' (175.000 km) during the first days of January 2021, but immediately thereafter decreased surprisingly fast, measuring short of 0.5' (20.000 km) at the beginning of March. The coma remained rather diffuse. The degree of condensation decreased from DC 2-3 to DC 1-2 throughout the apparition. No tail was observed.

Total Brightness and Coma Diameter

On March 4, 2021 the innermost part of the coma brightened up to 3 mag, with the area of maximum brightness shifted towards WSW. Probably this brightening heralds another fragmentation of the nucleus and we will see additional fragments during the coming days/weeks (CBET 4940).

Andreas Kammerer

FGK observations


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