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On Dec. 28, 2019 the ATLAS team discovered a comet in the southern part of the constellation Ursa Major, near the borders to the constellations Leo and Coma Berenice. Comet C/2019 Y4 (ATLAS) showed an highly condensed 8" coma of total magnitude 18.5 and a 15" tail in p.a. 280°. It will approach the Sun to within 0.25 AU on May 30, 2020 (CBET 4713). The comet should reach magnitude 16 at the start of March and should peak at magnitude 8 (n=3) or even 6 (n=4) at perihelion. For mid-European locations it will drop below the evening horizon around May 25. Between March and end of May it will move through the constellations Ursa Major, Camelopardalis and Perseus. However, the comet is rather small with an absolute magnitude 2-3 mag below the Bortle limit. Thus it is very likely that the comet will disintegrate well before reaching perihelion. Maik Meyer, Germany, noted the similarity of the orbit of the Great Comet C/1844 Y1 = 1844 III and comet C/2019 Y4, adding that the intrinsic brightness of the latter is much lower than that of the former. Thus comet C/2019 Y4 can be regarded as a large fragment of the former.
This comet showed an unusual (but for comet fragments typical) development and became the most interesting comet in spring 2020. After a very rapid brightening during the first weeks of the apparition the comet experienced a great fragmentation, which showed up in the light curve starting on Mar. 18 (73 days prior to perihelion). During the following 36 days (until Apr. 23) the expected fading took place, however, not in a rapid manner. Thereafter the two larger surviving fragments started to release larger amounts of gas and dust due to the increasing solar radiance. Thus the heliocentric brightness remained constant, resulting in a minor increase of the total magnitude until the last observation (on May 16). The comet peaked at 8.0 mag on Mar. 18, but the brightness remained nearly constant for the following 10 days. Until Apr. 23 the total brightness decreased to 9.3 mag, but increased slightly to 9.0 mag until May 16. The appropriate formulae are based on 498 observations from 58 observers:t < -73d: m = -4.5 mag + 5×log D + 52.5×log r -73d < t < -37d: m = 9.6 mag + 5×log D – 7.5×log r t > -37d: m = 9.5 mag + 5×log D
The development of the coma diameter and the tail length can only be described with some uncertainty. This uncertainty is caused by the fact that the coma grew ever more elongated after the fragmentation took place and it is thus not clear if the observers reported the minor axis (as would be correct), an average between minor and major axis or if they even reported parts of it as a tail. For example, the author observed the following values for the coma: Apr. 11.90 UT: 1.5x6' (major axis: 85-265°), 14.87: 3.5x7' (85-265°), 20.88: 2.5x8' (75-255°), 22.88: 2.7x9' (85-265°) and 25.91: 3.5x11' (85-265°). According to the reports the coma diameter increased rapidly from 1' (50.000 km) at the start of the apparition to 6' (225.000 km) on Mar.10. During the following days it literally exploded – due to the fragmentation (see CBETs 4744/51/63 for details) – reaching the maximum of 14' (675.000 km) on Mar. 18. This diameter was constantly reported during the next 10 days. Thereafter it decreased, measuring 8' (350.000 km) on the opening of April and short of 3' (90.000 km)in mid-May.Total Brightness and Coma Diameter
The coma was rather diffuse. Until Mar. 18 the comet showed a degree of condensation of DC 3, which then increased shortly to DC 4. Between the start of April and the opening of May it decreased from DC 4 to DC 1. However around May 10, DC 2-3 was reported, decreasing to DC 1 in mid-May.
A tail was reported between the start of April and mid-May, which reached a length of 15' (550.000 km) around May 10. Its orientation changed from ESE towards NE. Here again it is unclear if the observations are related to the tail or to the elongated coma.