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On PanSTARRS images, taken on Sep. 7, 2016, a comet was discovered in the southern part of Eridanus. Comet C/2016 R2 (PanSTARRS) showed a very diffuse, miniscule coma of magnitude 18.0. It will pass perihelion (solar distance: 2.6 AU) at the start of May 2018 and should peak at about 12.0 mag around the turn of the years 2017/18 (CBET 4318 / MPEC 2016-U07). Between mid-2017 and mid-2019 it should be brighter than 16.0 mag. During this period it will move through the constellations Eridanus, Orion, Taurus, Auriga, Lynx, Ursa Major, Canes Venatici and Bootes. Thus it should be a well-placed object during the most-interesting months.
The comet got brighter than expected and exhibited an astonishingly dynamic tail - taking into account the distance and brightness of the comet - during the last weeks of 2017 and the first weeks of 2018. Based on 321 observations by 37 observers the brightness developed differently pre- and post-perihelion, according to the formulaepre-perihelion: m = 2.8 mag + 5×log D + 14.8×log r post-perihelion: m = 4.6 mag + 5×log D + 10.5×log r
The resulting confidence belt is rather large. The reason seems to be a period of enhanced activity between mid-November 2017 and mid-January 2018, which correlates well with the appearance of the extremely dynamic tail. Neglecting those weeks, the above pre-perihelion brightness formula describes the brightness development well. Considering the period of enhanced activity the comet peaked at 10.7 mag in mid-January 2018. During the following weeks the total brightness of the comet decreased, because the distance Earth-comet increased significantly. After perihelion the distances to Earth and Sun increased only slowly, stretching the period of slow fading. Thus the comet had faded by only 0.5 mag until mid-April, and was still of magnitude 12.5 at the start of October. However, at the end of March 2019 the comet had faded to 14.5 mag and to 15.0 mag at the end of June.Total Brightness and Coma Diameter
The coma diameter showed a very similar development. During the first two months it stayed constant at 1.0' (150.000 km), only to increase rapidly to the maximum of 3.7' (350.000 km) during the following four weeks. The apparent coma diameter remained at this value for four additional weeks, thereafter decreasing steadily, reaching 1.7' in mid-April, 1.0' at the end of 2018 and 0.6' at the end of March 2019. The maximum absolute coma diameter was hold for about four weeks too, decreasing only slowly thereafter, measuring still 250.000 km at the end of April, 150.000 km at the end of 2018 and 110.000 km at the end of March 2019. The coma was rather diffuse. The degree of condensation decreased from DC 3 to DC 2 until perihelion, then was constant at this degree.
In parallel to the period of enhanced activity the comet developed an eastward pointing, very dynamic tail. It exhibited structural changes within hours and developed streamers - especially in January - which seemed to rotate or "wave". However, this development could only be recorded photographically. Visually only a faint 15' long tail was discernible during the two months, representing an actual length of at least 3 Mio. km. Tail reportings ceased at the end of April.
Observations with the 10m-Submillimeter telescope on Mt. Graham on Dec. 22 and 23, 2017 (r = 2.9 AE) yield a CO-production rate of 4.7×1028 molecules/s. This is nearly one third of the production rate of comet Hale-Bopp in the same solar distance, indicating an extremely CO-rich object (CBET 4464).