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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 became the most interesting, but not the brightest comet in spring 2020. It showed a rapid increase in brightness during the first weeks of 2020, which is rather common for comet fragments, followed by a break in the brightness development with a fading afterwards – also typical for fragments. However, due to the comet's surprisingly slow fading it is possible that the comet could show a certain brightening near perihelion, if the two major fragments observed by the HST maintain a sufficient activity.
Based on 448 observations from 57 observers (until May 1, 2020) the brightness development can be well described by the following formulae, which now indicate that the break took place on Mar. 18 (my first analysis indicated Mar. 17).t < -73d: m = -5.7 mag + 5×log D + 58×log r t > -73d: m = 9.6 mag + 5×log D – 7.5×log r
An intermediate phase between Mar. 18 and 28 with the parameters m0=7.9 mag / n=0 could be applied, but with a rather low significance. The larger scatter in the weeks following the break in brightness development can be explained by the fact that the coma got ever more diffuse and elongated, making estimates difficult. The comet peaked at 8.0 mag on Mar. 18, 2020.Total Brightness and Coma Diameter
The coma diameter increased rapidly from 1' (65.000 km) to 6' (225.000 km) until 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. 21. Thereafter the diameter decreased comparatively fast, measuring 8' (350.000 km) on Mar. 31 and only 4' (175.000 km) on May 1. However, it has to be considered that the coma became more and more elongated and that the values reported could be either the small axis or an average between the small and large axis. 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°).
The coma was rather diffuse. Until Mar. 15 the comet showed a degree of condensation of DC 3, which increased to DC 4 until Mar. 27. Thereafter it decreased steadily, reaching DC 1 on Apr. 25.
A tail was reported between early March and the end of April, with a maximum length of 9' (500.000 km) during the last days of April. Here again it is unclear if the observations are related to the tail or to the elongated coma. The position angle of the tail changed very constantly from p.a. 190° in early March to p.a. 60° in late April.