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Aims. We have started a program of spectroscopic and photometric investigations of distant active comets in the optical domain. The comets with a significant level of activity-in particular, extended tails-are the objects of our observations. Methods. The observations were made at the 6-m telescope BTA (SAO RAS, Russia), with the focal reducer SCORPIO attached to the prime focus of the telescope. Long-slit and photometry modes were turned to perform our first observations. The spectral resolution in the spectroscopy mode was 10 Å. Results. The result of the spectrum analysis of comet C/2002 VQ94 (LINEAR) is of particular interest. The comet observed at the heliocentric distance of 6.8 AU clearly shows a rich molecular spectrum. We identified 14 vibrational bands of CO + (Comet Tail system), emissions of C 3 , and some tentatively assigned to N + 2 and CN. It should be emphasized that, for now, CO + and tentatively assigned N + 2 emissions are detected at a record heliocentric distance.

We investigated three comets, which are active at large heliocentric distances, using observations obtained at the 6-m BTA telescope (SAO RAS, Russia) in the photometric mode of the focal reducer SCORPIO. The three comets, 29P/Schwassmann-Wachmann 1, C/2003 WT42 (LINEAR) and C/2002 VQ94 (LINEAR), were observed after their perihelion passages at heliocentric distances between 5.5 and 7.08 AU. The dust production rates in terms of Af was measured for these comets. Using these retrieved values, an average dust production rate was derived under different model assumptions. A tentative calculation of the total mass loss of the cometary nucleus within a certain observation period was executed. We calculated the corresponding thickness of the depleted uppermost layer where highvolatile ice completely sublimed. The results obtained in our study strongly support the idea that the observed activity of the comet SW1 needs a permanent demolition of the upper surface layers.

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Context. The study of small bodies that have a cometary activity at a large heliocentric distance (larger than 5 au) is important for a better understanding of the physical properties of comets and their origin. Such studies require long-term monitoring and a significant observational effort. Aims. Our goal is to monitor the cometary activity and search for possible emission lines of C/2006 S3 (LONEOS), which is a distant comet that was discovered in 2006 and passed perihelion at 5.13 au in April 2012. Methods. We performed different observing runs with telescopes, ranging from 2 m to 6 m both in imaging and spectroscopic modes in the optical range, between 2006 and 2014. Results. The comet C/2006 S3 (LONEOS) was a very active object with A f ρ ∼ 4000 cm at its maximum of activity, corresponding to a dust production rate of 82 kg s −1 , considering a mean geometric albedo of 0.1 and grain outflow velocities in the range 2−22 m s −1 . It appeared to be more active after its perihelion than before and we did not manage to detect any emission lines. Upper limits for the main cometary species that have emission bands in the optical range are provided. Our dust environment modeling is in agreement with a collimated source of dust that appeared in 2013.

Abstract. CCD observations of comet C/1999 J2 (Skiff) were made at the Pik Terskol Observatory on September 15, 1999. The 2-m telescope equipped with the two-channel focal reducer of the Max-Plank-Institute for Aeronomy was used to study the cometary environment. In spite of the large heliocentric distance, 7.24 AU, a straight dust tail with fairly well defined boundaries was recorded. These data provide an opportunity to study the peculiarity of dust, that is not driven by water vapor. To fit the dust tail, a Monte Carlo model was developed. We trace the trajectories of about 10 7 sample grains to construct the detailed brightness distribution in the comet tail. The simulated isophote field and the observed one agree very well. In our model we also take into account the heliocentric dependence of the dust production rate and ejection velocity, and the dust ejection anisotropy. To transform the particle population to brightness in the modelled tail we calculate the scattering cross section of a separate particle using Mie theory. The age, ejection velocity, dust size distribution, minimum and maximum size of the involved dust particles have been derived from the model giving the best fit. The intensity map is in agreement with a flow of the slowly travelling icy grains. It has been determined that the age of the dust tail of comet C/1999 J2 (Skiff) was, at the moment of the observations, about 540 days and that the main reason for its appearance is likely phase transition from amorphous to crystalline water ice activated at the heliocentric distance of 8.6 AU.