The Light Curve of Comet C/2002 C1 (Ikeya-Zhang) IV Mark Kidger & Graeme Waddington


The TA data archive for this comet now comprises of some 511 visual estimates made up to April 30th. Unfortunately, due to the huge quantities of data involved it is not possible for the editor to supply the May data before the magazine deadline. The light curve to April 30th is shown below. Note a change of slope around 30 days after perihelion indicates either a small outburst or a significant perihelion asymmetry.


The existence of an asymmetry of the light curve around perihelion has important implications for the non-gravitational terms in the orbit. An asymmetry indicates that the sublimation of water vapour is asymmetric about perihelion and thus the simplest form of the non-gravitational terms is invalid. At present work is in progress to investigate this effect.


Since the last update there has been a major change in the database for the orbital solution as 3 images of C/2002 C1 have been found in NEAT images from 2001 Aug. 25. These have increased the available arc by more than 5 months and have led to a large decrease in the size of the non-gravitational terms that must be applied (A1 decreases from 3 to 0.58). At the same time the linkage to the 1661 becomes very much improved. Nakano's solution gives the previous perihelion pass as 1660 Dec. 3, although the exact date is heavily dependent on how many outliers are eliminated; the MPC gets a date of 1660 Oct. 3 on eliminating just a very few more points.


Note that the TA database now confirms that the comet has been significantly brighter post-perihelion than pre-perihelion. The fit to the light curve (above) shows that the best fit to the data has a brightening rate that is very similar, but with the post-perihelion data systematically 0.4 magnitudes brighter. Some caution must be applied to the rate of fade post-perihelion as the brightening event observed in the light curve in late April significantly flattens the apparent rate of fade. When these data are ignored the rate of brightening pre-perihelion is indistinguishable from the rate of fade post-perihelion. Spanish data and an advance of TA data kindly forwarded by the editor shows that the comet showed a 0.5 magnitude brightening in late April that continued for some 3 weeks. At the peak of this event the comet was actually brightening in real terms as the heliocentric distance increased.

Investigation of the data shows that the small perihelion asymmetry in the date of maximum brightness may be due to the phase angle term. In other words, shadowing of dust grains in the coma. The plot right was prepared as a test from on-line ICQ data that was made to assess the possible size of the effects of perihelion asymmetry on the non-gravitational terms. The residuals from the best light curve fit are shown. Note that in the top plot there is a significant trend to brighter magnitudes post-perihelion. When a phase angle term is included the residuals flatten out although two interesting effects are seen. Note that the comet brightens quite significantly from mid-April. There is also a possible sinusoid in the residuals that has been noted in other data sets. This shows the characteristic signature expected from a precession of the nucleus with a period of approximately 6 weeks. However, to confirm this feature will require a very much longer data set probably at least double the one shown here. More detailed investigation of the phase angle effect is currently under way but the size of the TA archive is slowing its completion.


It is of interest to note that recent data shows that the morphological behaviour of the comet is reproducing an effect seen in Hevelius's comet of 1661. As commented in the March TA, Hevelius's comet became very large and diffuse and faded out rapidly. This effect has been present in C/2002 C1 (Ikeya-Zhang). Recent visual observations have shown that the coma is half a degree or more across and extremely diffuse, with DC=0-1 such that it is now difficult to observe despite remaining a relatively bright object. The coma diameter estimates give a consistent linear diameter of 250 000km around perigee in late April, but that by June 10th the coma now measures 1.3-1.5 million kilometres in diameter. This makes it seem unlikely that visual observers will be unable to follow the comet much beyond the early summer if the trend continues.


The addition of the 2001 data has apparently resolved the problems with the linkage to the 1661 apparition. Nakano has also provided a possible linkage to the comets of February-March 877 (observed in Japan and Europe) and February 1273 (observed in Japan and Korea). At present this linkage seems to be somewhat speculative, as there is considerable doubt about the position and dates of observation of the 877 object, which is regarded by some as a possible nova. The addition of the 2001 data and the resultant improvement of the linkage with 1661 should allow the veracity of further linkages to be tested.