Post-perihelion observations of comet 67P/Churyumov-Gerasimenko at the 6 m BTA telescope: optical spectroscopy

Abstract

In the framework of the worldwide ground-based campaign, long-slit spectroscopy of comet 67P/Churyumov-Gerasimenko was performed at the 6 m BTA telescope of the Special Astrophysical Observatory RAS in the post-perihelion period from 2015 November to 2016 April at a heliocentric distance from 1.61 to 2.72 au. Strong CN and relatively weak C₂, C₃, and CO⁺ emissions were identified in the spectra of the comet on 2015 November 8 and December 9, while NH₂ emissions were only observed on November 8. In the spectrum derived on 2016 April 3, only the CN emission was found. The production rates of CN, C₃, and C₂ are in good agreement with data obtained by other authors. The value log[Q(C₂)/Q(CN)] = -0.43 corresponds to the carbon-chain depleted class of comets. The spectra of the comet show a high level of the continuum referring to dust-rich comets. The dust production Afρ decreased from 172 cm at r = 1.62 au to 55 cm at r = 2.72 au, which is close to data derived from photometry at this telescope. The normalized gradient of the cometary dust reflectivity in the 3600-7070 Å spectral range varied from 12.5 ± 2 per cent 1000 Å^-1 to 11.3 ± 2 per cent 1000 Å^-1, which is close to a mean spectral reddening of cometary dust of 11 ± 2 per cent 1000 Å^-1. In general, the photometric properties of comet 67P/Churyumov-Gerasimenko show that it is a typical Jupiter-family comet.