I. Kulyk

Monitoring of the cometary activity of distant comet C/2006 S3 (LONEOS)

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 Afρ ∼ 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.

High-resolution spectra of comet C/2013 R1 (Lovejoy)

Context. High-resolution spectra of comets permit deriving the physical properties of the coma. In the optical range, relative production rates can be computed, and information about isotopic ratios and the origin of oxygen atoms can be obtained. Aims. The main objective of the work presented here was to obtain information about the chemical composition of comet C/2013 R1 (Lovejoy), a bright and long-period comet that passed perihelion (0.81 au) on 22 December 2013. Methods. We used the HARPS-North echelle spectrograph at the 3.5 m telescope TNG to obtain high-resolution spectra of comet C/2013 R1 (Lovejoy) in the optical range immediately after its perihelion passage during four consecutive nights in the period December 23 to 26, 2013. Results. Our results demonstrate the ability of HARPS-North to efficiently obtain cometary spectra. Very faint emission lines, such as those of 15 NH2, have been detected, leading to a rough estimate of the 14 N/ 15 N ratio in NH2. The 12 C/ 13 C ratio was measured in the C2 lines and is equal to 80±30. The oxygen lines were studied as well (green to red line intensity ratios and widths), confirming that H2O is the main parent molecule that photodissociates to produce oxygen atoms. This suggests that this comet has a high CO2 abundance. Relative production rates for C2 and NH2 were computed, but we found no significant deviation from a typical NH2/C2 ratio.

Optical spectrophotometric monitoring of comet C/2006 W3 (Christensen) before perihelion

Context. Long period distant comet C/2006 W3 (Christensen) was discovered in 2006 and passed perihelion at 3.13 au in July 2009. It was a target for the Herschel Space Observatory, Spitzer Space Telescope, and ground-based radio telescopes. We enlarge the observational database with optical spectrophotometric ground base data. Aims. We performed optical monitoring of cometary activity and searched for possible emission lines in the C/2006 W3 (Christensen) spectrum in a wide range of heliocentric distances. Methods. We carried out pre-perihelion observations of the comet at heliocentric distances from 8.49 to 3.13 au with the 2 m and 6 m telescopes, both in imaging and spectroscopic modes, during the period 2006 to 2009. Results. No emissions were detected in the spectra at heliocentric distances of 8.49 and 6.25 au. Rich molecular emission spectra were obtained, when the comet was at a distance of 3.74 au from the Sun. The CN, C3, C2, CH, NH2, CO, and, probably, CH emissions were assigned in the 3800–6800 Å spectral range. Gas production rates or corresponding upper limits in the emissionless case were calculated using a Haser model. We derived the gas production rates of 1.03 × 1026 mol s−1 for CN, 4.12 × 1024 mol s−1 for C3, 6.54 × 1025 mol s−1 for C2, and 3.12 × 1025 mol s−1 for NH2, which is matched with the CN:C3:C2:NH2 = 1.0:0.04:0.63:0.30 gas production ratios at a heliocentric distance of 3.74 au. The CO production rate of the comet, which was equal to 2 × 1024 mol s−1, was also obtained. We quantified the evolution of the comet activity in a wide range of heliocentric distances with the A fρ technique. Heliocentric dependence of the A fρ parameters indicates that steep growth of the comet activity occurred around 3.96 au, almost simultaneously with the appearance of molecular emissions in the comet spectrum. The dust production rate was estimated from the A fρ reaching a maximal value of 380 kg s−1 at a distance of 3.13 au before perihelion passage.

Comet C/2010X1 (Elenin). Unrealized expectations

We present the analysis of the photometric and spectroscopic data obtained for comet C/2010 X1 (Elenin) when it was at a distance of 2.92 AU from the Sun. The observations were made at the prime focus of the 6-m BTA telescope with the SCORPIO focal reducer. The magnitude of the comet, measured in the Rc-band with an 9″ aperture radius amounted to 16ṃ8 ± 0ṃ1. The computed dust production rate was estimated to be about 6 kg/s. The cometary coma manifested the emissions in the (0–0) band of the CN molecule violet system, and a number of emission band heads of the C3 molecule. The gas production rate of the molecules is determined using the Haser model and amounts to 1.41 × 1024 and 4.20 × 1023 molecules per second for CN and C3, respectively. The ratio of gas production rates log[Q(C3)/Q(CN)] is equal to −0.85, which is close to the mean value, determined for a significant number of comets. A normalized gradient of the cometary dust reflectivity, calculated for the 4430–6840 Å spectral range amounts to 14.3 ± 1.2%.

Physical activity of the selected nearly isotropic comets with perihelia at large heliocentric distance

Context. The systematic investigation of comets in a wide range of heliocentric distances can contribute to a better understanding of the physical mechanisms that trigger activity at large distances from the Sun and reveals possible differences in the composition of outer solar system bodies belonging to various dynamical groups. Aims. We seek to analyze the dust environment of the selected nearly isotropic comets with a perihelion distance between 4.5 and 9.1 au, where sublimation of water ice is considered to be negligible. Methods. We present results of multicolor broadband photometric observations for 14 distant active objects conducted between 2008 and 2015 with various telescopes. Images obtained with broadband filters were used to investigate optical colors of the cometary comae and to quantify physical activity of the comet nuclei. Results. The activity level was estimated with Afρ parameters ranging between 95 ± 10 cm and 9600 ± 300 cm. Three returning comets were less active than the dynamically new comets. Dust production rates of the comet nuclei were estimated between 1 and 100 kg s −1 based on some assumptions about the physical properties of dust particles populating comae. The measured colors point out reddening of the continuum for all the comets. The mean values of a normalized reflectivity gradient within the group of the comets amount to 14 ± 2% per 1000 Å and 3 ± 2% per 1000 Å in the BV and VR spectral domains, respectively. The comae of the dynamically new comets, which were observed on their inbound legs, may be slightly redder in the blue spectral interval than comae of the comets observed after the perihelion passages. The dynamically new comets observed both pre- and post-perihelion, seem to have higher production rates post-perihelion than pre-perihelion for similar heliocentric distances.