Distant activity of Comet C/2002 VQ94 (LINEAR): Optical spectrophotometric monitoring between 8.4 and 16.8 au from the Sun

Spectrophotometric monitoring of distant Comet C/2002 VQ94 (LINEAR) was performed with the 6-m telescope of SAO RAS (Special Astrophysical Observatory of Russian Academy of Sciences) and with the 2.5-m Nordic Optical Telescope (Observatory del Roque de los Muchachos, Canarias, Spain) between 2008 and 2013. During this period the comet was on the outbound segment of its orbit, between heliocentric distances of 8.36 au and 16.84 au. Analysis of the spectra revealed the presence of the CO+ and N 2 + emissions in the cometary coma at a distance of 8.36 au from the Sun. This distance is larger than ionic emissions have been detected in any previous objects. Only continuum, with no traces of emissions, was detected in the spectrum obtained in 2009 when the comet was at a distance of 9.86 au. From the spectra obtained in 2008, average column densities of 2.04 × 109 mol cm−2 for N 2 + and 3.26 × 1010 mol cm−2 for CO+ were measured in the cometary coma. The derived values correspond to N 2 + / CO + = 0.06 within the projected slit. Images obtained through a red continuum filter in 2008 showed a bright, dust coma, indicating a high level of physical activity. A considerably lower level of activity was observed in 2009 and 2011 at distances of 9.86 au and 13.40 au respectively. No noticeable activity was detected in 2013 at a heliocentric distance of 16.84 au. The Afρ parameter, which is used as an indicator of cometary activity, was measured as 2000 cm in 2008, and 800 cm in 2009 and 2011. The Afρ values correspond to dust production rates between 10–20 kg s−1, 4–6 kg s−1 and 3–5 kg s−1 at 8.36, 9.86, and 13.40 au respectively. There is an obvious correlation between the decrease of the dust production rate of the nucleus and the disappearance of the emissions in the spectrum of C/2002 VQ94 (LINEAR) at heliocentric distances greater than 9 au. The colors and size of the nucleus of C/2002 VQ94 (LINEAR) were estimated from the images obtained during the late stage at a heliocentric distance of 16.84 au, when the activity had probable ceased. The B–V and V–R colors were estimated to be 1.07 ± 0.05 and 0.54 ± 0.03 respectively. The effective nucleus radius of 48 ± 2 km is in agreement with the previously published results, obtained from the observations of the comet during its early inactive stage (Jewitt, D. [2005]. Astron. J. 129, 530–538; Weiler, M., Rauer, H., Sterken, C. [2011]. Icarus 212, 351–366).