Photometric observations of distant active comets^*

¹ Department of Experimental Physics & Astronomical Observatory, University of Szeged, 6720 Szeged, Dóm tér 9, Hungary
² Department of Optics & Quantum Electronics & Astronomical Observatory, University of Szeged, POB 406, 6701 Szeged, Hungary
³ Department of Physical Geography, ELTE University, 1088 Budapest, Ludovika tér 2, Hungary

Corresponding author: L. L. Kiss, l.kiss@physx.u-szeged.hu

Received: 21 September 2000
Accepted: 17 April 2001

Abstract

We present CCD VR_C observations of 6 distant comets located at heliocentric distances of 3.4-7.2 AU. Time-series data were obtained on three nights in July, 2000 covering 16 hours. Each comet was observed after the perihelion, when a lower activity was expected. Contrary to expectation, we found well-defined circular comae and extended tails visible out to a few (3-5) arcmin. We detected a quasi-cyclic light variation of C/1999 J2, while C/1999 N4 showed some hints of a more complex variation. C/2000 K1 was constant to mag during the observing run. The standard V and R_C data were used to estimate nuclear diameters, while the colour indices implied a slighly reddish (, C/2000 K1), a neutral (, C/1999 N4) and a slightly bluish (, C/1999 J2) coma. Simple fits of the surface brightness distributions are also presented enabling order of magnitude estimates of nuclear radii. Beside the time-series observations, further single-shot observations of three faint comets are briefly described.