Unexplainable Fading of Nuclear Magnitude of Comet C/2001 OG108
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Updated on December 12, 2005 |
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Comet C/2001 OG108 ( LONEOS ) is an old periodic comet with a period of 49 years and a perihelion distance of 1.0 A.U. It is being exhausted, changing from a Halley-type comet, originated from the Oort cloud, to a Damocloid asteroid, an extinct comet. It was discovered at 18.7 mag on 2001 July 28 by M. E. Van Ness in the course of the Lowell Observatory Near-Earth Object Search (LONEOS). It had been observed for one year until 2002 July. It was originally regarded as an asteroid, but it suddenly started cometary activity in 2002 January. Around the perihelion passage in 2002 March, the cometary activity continued for four months until 2002 May. Then it turned to be asteroidal again. No observations were reported after that because it became conjunction with the sun and moved towards the southern sky. When the comet looked asteroidal, the nucleus itself was observed. The brightness suggests the size of the comet's nucleus. Therefore, the absolute nuclear magnitude must be the same brightness both before and after the perihelion passage. However, unexpected difference of the nuclear magnitude was observed between the pre-cometary activity and post-cometary activity. Only one observer, Akimasa Nakamura, observed the nuclear magnitude both before and after the perihelion passage when the comet looked asteroidal. Nakamura observed it twice before the perihelion passage, and twice after the perihelion passage. The absolute nuclear magnitude faded by 1 mag from 12.9 mag to 13.9 mag during the cometary activity around the perihelion passage. Here is the light curve. The green curve shows the absolute magnitude of H = 12.9 mag. The blue curve shows the absolute magnitude of H = 13.9 mag. The red curve shows the visual observations of its cometary activity. Nuclear magnitude by Nakamura are plotted as black dots. CCD observations in late April by Ken-ichi Kadota and Yusuke Ezaki are also plotted as pink dots, which are consistent with Nakamura's photometry. 
The fading of the nuclear magnitude is unexplainable. Fernandez et al. confirmed the rotational variation with an amplitude of 0.3 mag and a period of 57.2 hours before the perihelion passage. But the difference is much larger than the amplitude. The phase angle was around 20 degree before the perihelion passage, and around 40 degree after the perihelion passage. So the fading was not an apparent effect caused by the phase angle. If the fading was due to the decrease of the size, the diameter of the nucleus must have been reduced to 63% through only one perihelion passage. But no nuclear split or collapse were confirmed. Maybe the color or albedo of the nuclear surface was darkened. The nuclear surface was observed using large telescopes only before the cometary activity started. So the change of the surface was not confirmed. Maybe the nuclear surface is not homogeneous and the darker side became visible towards the earth after the perihelion passage. But no sign of the precession was detected by the nuclear observations before the perihelion passage. Fernandez et al. estimated the nuclear size as 10.1 x 7.9 km, which is much larger than a typical comet. Therefore, drastic physical change of the nucleus in a short time is less likely. The comet will return again in 2050. The measurement of the nuclear magnitude and observations of the surface are encouraged.
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