Analysis of Comet Apparitions

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12P/Pons-Brooks

2023/24

After more than 70 years comet 12P/Pons-Brooks (P=71.32a) will reach its 4th observed perihelion. On Apr. 21, 2024 it will pass the Sun at a distance of 0.78 AU. Since it will not get closer than 1.55 AU to Earth during this apparition and will move on the opposite side of the Sun as seen from Earth it is unlikely to become brighter than 4-5 mag. Around perihelion it should also reach its maximum coma diameter (about 10') and tail length (visual: about 2-3°), as indicated by my empiric formulae. The comet should be brighter than 16 mag between July 2023 and December 2024. During this period it moves through the constellations Draco, Hercules, Lyra, Cygnus, Lacerta, Andromeda, Pisces, Aries (perihelion), Taurus, Eridanus, Lepus, Canis Major, Puppis, Vela, Centaur, Lupus and Norma. As seen from mid-European locations it can be spotted quite low above the Northeastern morning horizon between December 2023 and February 2024. However, this comet is primarily visible in the evening sky at this apparition. While it is almost at the zenith at the end of twilight at the beginning of the apparition, it steadily loses altitude during the following months and will disappear above the WNW horizon in the first week of April 2024. The Earth will cross the comet's orbital plane on Dec. 8, 2023 and on June 6, 2024.

Similar to the two preceding perihelion passages the comet showed a very interesting development, displaying several outburst. The first were accompanied by a very interesting coma morphology. The most conspicuous of these outbursts occurred on July 20/21, 2023 with an amplitude of 5 mag (brightening from 16.5 mag to 11.5 mag), which permanently increased its brightness by a factor of 100. During the following months the comet underwent two further major and several smaller temporary outbursts. The larger outbursts resulted in a significant increase of the total magnitude and led to a significant coma expansion. The smaller outbursts caused only a brightening of the central parts of the coma (near the nucleus) and an increase of the degree of condensation, but had only a small or no impact to the total magnitude. During the first outbursts the comet temporarily showed a compact coma of high surface brightness with a tailward appendage that showed a "shadow" behind the nucleus and thus had the shape of two "horns". Until end of April 2024 the following outburst have been noticed: July 20/21, 2023 (total magnitude: amplitude: 5 mag), Sep. 3 (amplitude: ≈0.4 mag), Sep. 23/24 (0.9 mag), Oct. 4/5 (4.0 mag), Oct. 22/23 (≈0.4 mag), Oct. 30/31 (≈0.4 mag), Nov 14/15 (large), Nov. 30/Dec. 1, Dec. 13/14, Dec. 23, Jan. 17/18 (large), Jan. 29, Feb. 29 / Mar. 1 (0.5 mag) and Apr. 2/3 (near nucleus: ≈ 1.0 mag, total magnitude: 0.5 mag). This behavior can be retraced quite well by applying moving weighted 3-days means.

The last outbursts were not as spectacular as those during the second half of 2023, but the very last outburst listed resulted in the comet peaking at 3.9 mag on Apr. 5, 2024.

Based on 1012 observations by 89 observers (until start of May 2024) the brightness development has to be divided into three periods. As the comet showed several large outbursts during the second period (with the brightness subject to strong fluctuations), the corresponding parameter values represent mean values. The estimates published so far after perihelion are significantly below the curve, so it is likely that the development after perihelion represents an additional period. The brightness parameters determined so far are as follows:

The development of coma diameter and degree of condensation also reflect the outburst behavior. Shortly after an outburst the extremely condensed coma was tiny. During the next days/weeks the coma diameter increased noticeably (thereby getting rapidly more diffuse) until the outer parts of the expanding coma reached the surface brightness of the sky background. Shortly afterwards the next outburst occurred with an initially small coma displaying a high surface brightness, which expanded over the following days (while the surface brightness decreased). Due to the comet's approach to Sun and Earth, the apparent coma diameter reached a larger maximum value with each outburst (5' for the first, 7' for the third). The largest apparent coma diameter was reached in mid-March 2024 with 8.5'. In absolute terms the coma reached a diameter of 800,000 km during the first outburst (mid-August) and a maximum value of 900,000 km in mid-November due to the outburst of Oct. 30/31. In contrast, the coma measured only 625,000 km in mid-March 2024.

The degree of condensation also developed in parallel with the brightness outbursts. In each case, the DC value was extremely high at the beginning (up to DC 9), decreasing rapidly during the following days, eventually reaching DC 1 to DC 2. The maximum value and the amplitude of the degree of condensation variations decreased during the following outbursts. Nevertheless, all outbursts are well documented in the weighted 3-day means. The diagram hints towards a further outburst, probably only within the inner coma, around Apr. 25.

Tail sightings were already reported in summer 2023. Starting in September 2023 a dust tail could be observed continuously, reaching a maximum visual length of 1.5° (12 mio. km) around Mar. 27, 2024. The subsequent decline in the reported tail lengths is likely due to the rapidly deteriorating observation conditions at that time. In addition to the dust tail a gas tail appeared at the end of January, whose dynamical behavior and brightness increased significantly as the comet approached perihelion. An image taken by Michael Jäger on the evening of Apr. 2 shows an 8° long gas tail and an 1.5° long dust tail. In summer 2023 the dust tail was orientated towards West. From September 2023 to mid-February 2024 it slowly turned from SE via East towards NNW, thereafter turning back via East to SE until the beginning of June.

Andreas Kammerer

FGK observations