Analysis of Comet Apparitions

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C/2019 Y1 (ATLAS)

On images, taken in the course of the "Asteroid Terrestrial-Impact Last Alert Systems" (ATLAS) on Dec. 16, 2019, J. Robinson discovered a comet near the border of the constellations Phoenix/Sculptor. Comet C/2019 Y1 (ATLAS) showed a 20" coma, elongated in p.a. 80°, of total magnitude 17.9. On Dec. 17 H. Sato observed an highly condensed inner coma and a 45" outer coma of magnitude 16.5. During the final days of December several observers reported a coma of diameter 1.5' and of total magnitude 15.0. The comet will pass perihelion in the solar distance of 0.84 AU on Mar. 15, 2020, expected to reach a maximum brightness of 12.0 mag during these days (CBET 4708). It should have faded to magnitude 16 by mid-June. During this interval it will move through the constellations Aquarius, Pisces, Pegasus, Andromeda (perihelion), Cassiopeia, Camelopardalis and Ursa Major. For mid-European observers it will be positioned low above the western to northwestern evening horizon until the start of April. Thereafter it will gain altitude, reaching 60° by mid-May, but sinking towards horizon during the following weeks. However, this comet has an absolute magnitude very near the Bortle-limit. Thus it is possible that the comet could disintegrate before reaching perihelion. On Apr. 21 Earth will cross the orbital plane of the comet.

The comet surprised in two ways: it became significantly brighter than expected and it showed three development phases, which can all be described with standard formulae. Pre-perihelion it developed very rapidly, and did so again post-perihelion, starting 37 days after perihelion (on Apr. 21, 2020), however with an absolute magnitude 2 mag above. During the 31 days following perihelion the heliocentric magnitude decreased very slowly. The fact that during this period the comet approached Earth resulted in a near constant total magnitude of 8.3 mag. Between Apr. 15 and 18 the comet experienced a short-term outburst with an amplitude of 0.6 mag (well discernible in the moving weighted 3-days means), with the comet peaking at 7.8 mag on Apr. 18, fading out during the following days. Based on 451 observations from 49 observers (until mid-July) the appropriate formulae are as follows:

Between end of January and Apr. 15 the coma diameter increased rather rapidly from short of 2' (130.000 km) to short of 6' (310.000 km), During the short-term outburst it increased to short of 7' (340.000 km) for several days. Between mid-May and mid-July it shrunk from short of 6' (290.000 km) to 1' (75.000 km). Between end of January and the opening of April the degree of condensation increased from DC 2-3 to DC 5. The following decrease was interrupted by a short-term increase from DC 4 to DC 5 during the small outburst (with the increase of the DC-value occurring two days before the brightness increased). Thereafter the coma grew steadily more and more diffuse, reaching DC 1-2 in mid-June and DC 1 in mid-July.

A tail could be observed between Apr. 10 and end of May, reaching a length of 15' (1.25 Mill. km) shortly after the small outburst. Until end of April the tail was oriented towards North, after mid-May towards East.

Andreas Kammerer

FGK observations