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Astron. Astrophys. 341, 912-917 (1999)

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3. Results

In the following, we describe for each comet separately the results obtained from our CCD image analysis and photometry. The (equivalent) radius was calculated from the mean (averaged over the available measurements) filter brightness of the comets following the standard equation as given for instance in Huebner (1992; Eq. (2.1), Sect. 2.2.1). The albedo was assumed to be 0.04, for the (linear) phase angle correction of the radius value a darkening coefficient of 0.03 mag/deg was used. The summary of the results is given in Table 2. R(1,1,0) and V(1,1,0) are the R and V magnitudes of the comets at 1 AU Sun and Earth distance and for 0 deg phase angle. The V-R colour of the Sun is 0.55 mag.


Table 2. Photometry, colour, radius, rotation and activity status of 26P/GS and 73P/SW3. Errors are 1 sigma.

3.1. 26P/Grigg-Skjellerup

At the time of our observations (i.e. when the comet was at about 3.8 AU from the Sun) 26P/GS was inactive when moving outbound after the 1992 encounter with the GIOTTO spaceprobe of ESA. This is concluded from the star-like appearance of the comet and from the absence of a weak coma in our coadded image (Fig. 1). The equivalent radius of the nucleus is 1.45-1.50 km with a good agreement in both measured filters R and V (Table 2). The V-R colour is slightly bluish as compared to the Sun (Table 2). The lightcurve of the nucleus does not seem to be properly sampled by our observations (Fig. 4). However, a minor variability of (at least) 0.1 mag peak-to-peak is noticeable. This would correspond to a small-to-large-axis ratio of 0.9. The detected variability of the nucleus brightness seems to be real since it is larger than the 1-sigma uncertainty of the photometric magnitudes (and it is very similar in both the absolute and the relative photometry data). From the temporal development of the observed brightness variation we conclude that a rotation period of less than 0.5 days would hardly match our (partial) lightcurve of the nucleus.

[FIGURE] Fig. 4. The R filter lightcurve of Comet 26P/Grigg-Skjellerup during the observing period. Time zero point is 15/09/1993 0UT. Error bars for 1 sigma.

3.2. 73P/Schwassmann-Wachmann 3

In late December 1994 73P/SW3 was at 3 AU solar distance moving inbound to its next perihelion passage in autumn 1995. It had already developed a weak dust coma and tail as can be seen in our images (Figs. 2 and 3). Hence, the photometry of the comet may no longer give the light reflected at the surface of the nucleus alone, but it is contaminated by some light from the coma (which was estimated not to exceed about 20 percent of the total light from the object). Therefore, the radius value of 1.1 km (which was determined using the uncorrected flux) represents an upper limit for the nucleus size (almost identical in R and V; Table 2). The V-R colour of 73P/SW3 was solar (within the errors of our photometry; Table 2). The R filter brightness increased by about 0.25 mag within 1 day (Fig. 5). However, our observations do not indicate whether this was a secular (due to a general brightness increase) or a periodic (due to nucleus rotation) effect. The coadded images of the comet show a weak coma and a short tail (Figs. 2 and 3). The estimated radial diameter of the coma was 11800 km, the lateral one was 8300 km. The tail had a length of 13200 km and pointed to a position angle of 281 [FORMULA] 2 deg (measured North over East) which is in agreement with the position angle of the extended radius vector from the Sun (282 deg). Apart from this tail structure, the comet showed a very symmetric and smooth coma, and no coma structures typical for insulated active regions on a rotating nucleus (like jets, fans, shells) could be detected. Although no direct proof by simultaneous spectroscopy exists, we can assume - based upon the similarity of the cometary images in R and V and upon the weakness of coma gas emission bands in these wavelength ranges - that the observed coma and tail represented mostly the dust distribution around the comet.

[FIGURE] Fig. 5. The R filter lightcurve of Comet 73P/Schwassmann-Wachmann 3 during the observing period. Time zero point is 28/12/1994 0UT. Error bars for 1 sigma.

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998