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 Afρ technique. Heliocentric dependence of the Afρ 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 Afρ reaching a maximal value of 380 kg s-1 at a distance of 3.13 au before perihelion passage