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C/2011 W3 (Lovejoy)


On Apr. 10, 2010 R.E. Hill discovered a 18.5 mag comet in the constellation Hercules, near the border to Ophiuchus, on Catalina Sky Survey images. Comet C/2010 G2 (Hill) showed a diffuse 6-8" coma and a 20" fan-shaped tail in p.a. 200°. It will reach perihelion at the start of September 2011, predicted to peak at 12 mag. It will be brighter than 14 mag during the period March 2011 and the end of 2011, during which it will move through Draco, Cepheus, Cassiopeia, Camelopardalis, Lynx, Auriga, Gemini/Taurus, Orion into Eridanus. On May 15/16 it will pass the North Pole at an distance of just 1.5° thereby passing Polaris at an distance of only 0.7°. Most of the time it will be observable in the morning sky, becoming an evening object near the end of the mid-European visibility. The comet rounds the sun with a period of about 950 years (IAUC 9134 / MPEC 2010-L69). The Australian comet observer Terry Lovejoy discovered a comet on CCD images, taken in the course of his search program Nov. 27 and 29, 2011 in the constellation Centaurus. Comet C/2011 W3 (Lovejoy) showed a 1' coma with central condensation of magnitude 16, but no tail. On Dec. 1 he noted a 1.2' coma that had brightened by one full magnitude. Images taken at Cerro Tololo Observatory on Dec. 2 showed a broad 50" tail at p.a. 327°. It was soon recognized that Lovejoy's comet was a member of the Kreutz group, which would approach the Sun to a distance of only 0.006 AU on Dec. 16. Assuming a typical activity factor for Kreutz group members (n=4), a maximum brightness between -5 mag and -8 mag was predicted, with the comet very close to the Sun at that time (IAUC 9245 / CBET 2931 / MPEC 2011-Y19). From mid-European locations it would not be possible to see the comet in twilight, because it approached the Sun from the south, then circled the Sun, only to head south again. Furthermore it would be very unlikely to see any tail in the morning sky, because this would point at an angle of only 25° above the horizon.

Due to its very faint absolute magnitude the empiric formula by John Bortle did not make hope that the comet would survive the extreme solar heat near perihelion. But comet C/2011 W3 (Lovejoy) surprised the astronomical community, not only surviving perihelion but presenting a spectacular display in the morning sky for southern hemisphere observers.

The small number of pre-perihelion estimates indicated a steady brightness increase from 12.0 mag on Dec. 1 to 8.5 mag on Dec. 9. However, because the comet was only visible in twilight at low altitudes on that days the significance of the estimates was unclear, thus they did not seem to qualify for any reliable prediction.

In the course of its approach, the comet entered the SOHO field of view in the morning of Dec. 14, already displaying a short dust tail. With decreasing solar distance the comet brightened, as did the tail, which in addition became longer and broader and showed an ever increasing curvature. At 20hUT, only 4 hours prior to perihelion, a narrow gas tail emerged. The brightness was estimated as -3.0 mag at this time.

Between Dec. 15, 21hUT and Dec. 16, 3hUT the comet was occulted by the central obstruction of the SOHO field, while the tail remained at constant, regarding position and appearance! At 3hUT the comet reappeared behind the obstruction as a bright but tiny object without a tail (which may have been covered by the spike). Starting at 10hUT a short dust tail reappeared while the pre-perihelion dust tail became fainter and more diffuse. At 20hUT the gas tail was remarked again, while the pre-perihelion dust tail had almost vanished. The Solar Dynamics Observatory (SDO) was able to take many images of the comet near perihelion. Cropped in a video sequence these images give a vivid impression of the high dynamics during the passage, showing the comet as a strongly vaporizing object with a distorted and twisting tail. Almost all attempts to observe the comet in daylight failed. Only Terry Lovejoy was able to image it (with low S/N ratio) Dec. 16 around 18hUT with his CCD equipment, estimating it as -1.0 mag. Rick Baldridge and Brian Day reported a visual sighting in daylight. At Foothill College Observatory (California) they observed the comet on Dec. 16, 19:48 UT with a 16"SC telescope stopped down to 3", only 4° beside the Sun as a -1 mag star-like object with a faint fan-shaped, diffuse 20" coma or tail.

Brazilian amateur Alexandre Amorim succeeded in first observing the comet post-perihelion. On Dec. 17, at 8:06 UT (Sun only 1° below horizon) he estimated the comet, using 10x50 binoculars, as -2.9 mag with a 0.2° tail in p.a. 240°. The first post-perihelion image was secured by Jakub Czerny and Jan Ebr on Dec. 17, 9hUT with the FRAM-telescope in Argentinia: the comet showed a very bright central condensation and a parabola-shaped tail with a darker central section; the brightness was estimated as -1 mag. The first images which showed the complete, then 10° long tail (but without the coma which was then still below the horizon) were taken, independently, on the morning of Dec. 21 by Colin Legg and Vello Tabur in Western Australia. These show two superimposed tails with a multitude of structures, namely in the midst of the tail, which however remained unnoticed visually (CBET 9246 / Comet's Mailing List).

At the time the comet's head emerged out of the bright twilight on Dec. 22 it appeared surprisingly unremarkable compared to the tail with its high surface brightness. In the following days it grew - in parallel to the tail - ever fainter, giving the impression of a tail with no head. However, the parabola-shaped coma could always be identified as the starting point of the tail. In large telescopes a faint spike could be seen, starting from the apex of that parabola. The tail itself was very prominent at first, showing a high surface brightness (with the brightest parts situated in its central section), so that Comet Lovejoy was referred to as the Great Comet 2011 or the Christmas Comet of 2011. The overall as well as the surface brightness of the tail faded during the following days, while it lengthened. The maximum length of 40° was observed on the last two days of 2011. After the Holidays the comet was no longer prominent to the unaided eye, with the effect being increased by the fact that the comet was passing through the Milky Way. During the first days of January 2012 the comet was last seen with unaided eye from dark locations. Accordingly the tail length estimated visually decreased. Actually it continued to lengthen, as was indicated by extremely contrast enhanced images.

Z. Sekanina modelled the following scenario: within a very short time interval several massive outbursts happened, peaking at Dec: 17.6 UT and ending on Dec. 20.3 UT (when the central condensation began to fade). During these outbursts the comet experienced a massive mass loss, which however did not destroy the nucleus, as indicated by the spike still discernible on Jan. 3. The largest parts shed may have had the size of boulders. Comet Lovejoy shows similarities to the headless sungrazer C/1887 B1. However, the nucleus of this comet dissolved completely with the tail remaining visually observable for 19 days (CBET 2967).

Only 65 international observations can be used for the analysis. This astonishingly small number could be due to the difficulties in estimating this comet (coma could not be distinguished well from tail, coma and the tail only visible in dawn or from dark locations) In the consequence many observers may just have looked up in awe or tried to capture the appearance in images. Based on the rather small data basis, the absolute brightness is very different pre- and post-perihelion, while the activity parameter seems to have been constant:

pre-perihelion: m = 15.5 mag + 5×log D + 15×log r
post-perihelion: m = 10.0 mag + 5×log D + 15×log

Applying the formulae to the time of perihelion yields a maximum brightness between -15 mag and -20 mag. However, SOHO images clearly show that the comet not only lost its tail at closest proximity to the Sun, but also faded during several hours around perihelion. I have compared the brightness pre- and post-perihelion at the same distances from the SOHO image center to account for an assumed filter gradient. The comet seems to have reached its maximum brightness 4 hours prior to perihelion and 12 hours past perihelion. At both points of time the formulae yield a brightness of -8 mag.

Total Brightness and Coma Diameter

The coma diameter could only be estimated under a reasonably dark sky some days after the appearance of the tail. These circumstances plus the decreasing solar wind contributed to the fact that the maximum coma diameter of 15' (400.000 km) was not observed until Dec. 27. The degree of condensation was at DC 3-4 before perihelion. Just after the reappearance it was estimated at DC 8-9, but decreased to DC 1 until Dec. 24 and to DC 0 after the Holidays. The greatest tail length, which was visually observed, was about 40° (appx. 60 mio. km) in the days around Dec. 29.

Tail Length

Andreas Kammerer


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