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C/2011 L4 (PANSTARRS)


On images taken with the 1.8m-Pan-STARRS 1-telescope on June 6, 2011 a comet was discovered in northernmost Scorpius. The image of comet C/2011 L4 (PANSTARRS) was only about 30% larger than the images of adjacent stars. Long-exposed images confirmed a miniscule coma of magnitude 19.0 and hinted to a faint tail in p.a. 60°. The comet will pass perihelion at the very small distance of 0.30 AU on Mar. 10, 2013 (IAUC 9215 / MPEC 2011-R11). Assuming an average evolution (n=4) yields a maximum brightness of 0.5 mag. If the comet should be a new one, n=3 is more probable, resulting in a maximum brightness of 4.0 mag. According to my empiric formula the visual tail length should reach 18° and 4°, respectively, during the days of perihelion. For mid-European observers the comet will appear in the evening sky shortly after perihelion in the constellation Pisces. During the following weeks the fading comet moves through the constellations Andromeda, Cassiopeia, Cepheus, Ursa Minor and Draco, approaching the North Pole to within 5°.

During winter 2012/13 the comet brightened in an ever accelerating pace, but not as rapid as had been hoped for. Every passing week showed more obviously, that the absolute brightness as well as the activity parameter were not up to the high expectations. My analysis at the beginning of February 2013 indicated a maximum brightness of 2.0 mag around perihelion.

In the end comet C/2011 L4 (PANSTARRS) didn't make it into the illustrious circle of the Great Comets. First, because it was not a bright and obvious object for the unaided eye (mainly due to its low altitude in the dusk and the notoriously bad weather worldwide in the weeks around perihelion). Second, because its tail fell much short compared to the average length a comet of its brightness develops. However, it still will make it into the books due to its maximum brightness of 1.2 mag at perihelion and its most interesting tail evolution. The 86 observations by 10 members of the German Comet Section and 550 international observations used for the analysis indicate that the activity of the comet declined more rapidly after perihelion than it increased pre-perihelion. This is unusual, indicating that the comet either run out of material or the main region of activity shifted towards a less-favorable illumination by the sun. The brightness evolution can be described by the following formulae:

Pre-perihelion: m = 5.4m + 5×log D + 7.4×log r
Post-perihelion: m = 5.5m + 5×log D + 9.1×log r

The evolution of the coma diameter has some uncertainties, mainly during the weeks around perihelion while the comet remained in the twilight and a clear differentiation between coma and the bright tail was difficult. The apparent coma diameter increased from 0.8' at the start of the apparition to 2.0' in mid-September 2012, where it remained until the first conjunction with the Sun. At the beginning of 2013 it was estimated at 3.5', increasing until mid-February to its maximum value of 6'. This value remained constant until mid-May. Thereafter it decreased rapidly, measuring 4' at the beginning of June and only 2' at the start of September. In absolute dimensions the coma had a diameter of 140.000 km at the start of the apparition, increasing until mid-September to 325.000 km and to its maximum value of 425.000 km at the start of January. Thereafter the strong solar wind caused it to shrink, reaching 275.000 km during the first week of March. However, the implied large decrease could have been exaggerated by the uncomfortable observing circumstances (low altitude, twilight). During the following weeks the coma increased again, reaching 425.000 km at the end of May. Thereafter it eventually began to shrink slowly, measuring still 325.000 km in early September.

Total Brightness and Coma Diameter

During most of the apparition the coma was strongly condensed. During the first weeks the degree of condensation was estimated to be DC 5-6, followed by an increase to DC 6-7, starting in mid-July. During the second week of February 2013 it condensed further, reaching DC 8 at perihelion. Immediately thereafter the coma began to become more diffuse, reaching DC 4-5 at the end of April. Then the decrease of the degree of condensation slowed, with the coma estimated as DC 2-3 in early September. The coma itself showed a prominent central condensation during the weeks around perihelion, incorporating a stellar false nucleus, which was still visible at the start of June 2013.

Tail Length

The most noteworthy fact of this comet was its tail and its evolution. It was expected that the tail would reach a maximum length of at least 10°. Instead comet PANSTARRS developed a tail which could not followed by more than 2.5░ visually. Assuming a linear antisolar pointing tail - which is quite unlikely for this comet - the absolute length was only about 10 mio. km. First visual observations were reported in mid-January. Until perihelion the tail increased in surface brightness, but only marginally in length, which at that time was attributed to the increasing sky background caused by the twilight and low altitude. Post-perihelion the tail showed a high surface brightness, but remained short. It was noticeably bent counter-clockwise, exhibiting a sharply defined western edge and a diffuse, fan-shaped eastern part. During the first two weeks the tail could only be seen in its whole glory in photographs: besides the unremarkable ion tail - located close to the bright western edge of the dust tail - the dust tail showed a fan spanning more than 90° eastward, with the easternmost part even directed slightly in the direction of the sun. In the photographs the dust tail showed structures typical for brighter comets, but they remained invisible for visual observers. Not until the start of April did the comet reach comfortable altitudes, which allowed detailed visual inspection of the tail. However, by then it had already faded considerably. Surprisingly the length of the dust tail did not change significantly during more than 4 weeks - it just grew fainter and became ever more difficult visually.

Position Angle of the brightest Part of the Tail

Photographically the unfolding amazing development of the tail was recorded in detail, documented by numerous impressive pictures. While the surface brightness of the dust fan decreased, the brightness of the 10Ám syndyne - positioned at the eastern edge - increased, attracting the curiosity of the visual observers as well. Earth crossed the comet's orbital plane on May 27. Ten days in advance it became clear, that the 10Ám syndyne would develop into an anti-tail - which happened in due course. During the days around May 27 even visual observers could discern the anti-tail, despite the fact that the tail as a whole already experienced a low surface brightness (visually the main part was already very difficult to spot). Under a dark sky the anti-tail could be followed for more than 1° as a thin, linear feature of moderate surface brightness. Photographically it could be traced for 13°, indicating a huge amount of large dust particles ejected by the nucleus during the weeks and months before. Following the crossing of the orbital plane the anti-tail rotated back into the direction of the main tail, thereby growing fainter and more diffuse. The diagram of the position angle shows that the tail was oriented in all directions during the apparition.

Narrow-band photometry with the 0.6m-TRAPPIST-telescope at La Silla on Feb. 8 (r = 0.86 AE) yielded the following production rates (molecules/s): OH: 4.0×10^28, NH: 4.4×10^26, CN: 1.7×10^26, C_2: 2.8×10^26, C_3: 1.0×10^26, Staub: 12.300 cm. This indicates a very dust-rich comet (CBET 3433).

Andreas Kammerer

FGK observations


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