4 Discussion

The presented observations revealed some basic parameters of three interesting distant comets. Despite the large heliocentric distances of 5.5-7.2 AU, we could observe extended tails and fairly bright comae. The canonical view of cometary activity at such distances excludes the possibility of water ice sublimation as the inner "engine'' of the gas and dust production, because the heat from the Sun is not sufficient. Usually, it is assumed that a CO- or CO₂-dominated nucleus may be the source of this activity (Houpis & Mendis 1981; Luu 1993). Bar-Nun & Prialnik (1988) discussed the possible hydrogen comae in distant comets as a further mechanism beyond $\sim$4 AU. Based on a very extensive database of comet observations ($\sim$50 comets over a range of 1 to $\sim$30 AU), Meech & Hainaut (1997) have clearly shown that dynamically young comets are intrinsically brighter, exhibiting dust comae and activity at large distances in the region where water ice sublimation is not possible. Our observations fit very well the description above.

Because of the unexpectedly strong coma contamination, the presented results may suffer from large systematic errors, as the applied coma subtraction is likely an oversimplification of the real case. Thus, the calculated nuclear diameters should be considered as well-educated guesses with possibly large systematic uncertainties (up to 5-10 km). The recorded surface brightness profiles with logarithmic derivatives -1.6...-1.8 are characteristic for isotropic steady-state outflows affected by the radiation pressure alone (Jewitt 1991; Luu 1993).

Finally, the main observational results can be summarized as follows:

1. CCD observation of three distant comets taken on three night in July, 2000 are presented. The time-series data revealed short-term variations in two comets, while the third one was constant at a level of $\pm$0 $.\!\!^{\rm m}$05. As the time span is quite short, no firm conclusion was drawn on the possible rotation.

2. The absolute photometry was corrected for the presence of comae and nuclear radii were determined assuming a 0.04 albedo. The results are: C/1999 J2 - 10 km, C/1999 N4 - 3 km, C/2000 K1 - 11 km.

3. The observed V-R colours imply a slightly reddish coma for C/2000 K1, a normal solar reflected light for C/1999 N4 and a slightly bluish coma for C/1999 J2. The latter one may be caused by the C₂ emission decreasing the V-magnitude. The ion tail of C/1999 J2 was also detected.

4. We note that our results are strongly affected by selection effects, as the target list was formed to include the brightest and most distant active comets visible in July, 2000.

5. As a by-product of this project, coma and tail characteristics of three other comets have been also examined. The results are also discussed, although two of these targets were not further than 4 AU.

Acknowledgements

This research was supported by Pro Renovanda Cultura Hungariae Grants DT 2000. máj./43., DT 2000. máj./44., DT 2000. máj./48. and DT 1999. ápr./23, FKFP Grant 0010/2001, OTKA Grant #T032258, "Bolyai János'' Research Scholarship of LLK from the Hungarian Academy of Sciences and Szeged Observatory Foundation. The referee, Dr. Olivier Hainaut, helped to improve the paper with his comments and suggestions. The warm hospitality and helpful assistance of the staff of Calar Alto Observatory is gratefully acknowledged. The NASA ADS Abstract Service was used to access data and references.