Quasi-simultaneous observations of comet 41P/Tuttle-Giacobini-Kresak in 2017 apparition

Small celestial bodies are believed to contain primordial material from the epoch of Solar System formation. Long-period comets, which, due to their dynamical characteristics, spend most of their time far from the Sun, consist of material barely affected by solar radiation. Moreover, comets entering the inner part of the Solar System for the first time are the most prominent objects for study from this point of view. Research on short-period comets plays a crucial role in understanding the evolutionary processes of cometary matter, as the particles in their comas undergo significant changes due to multiple close encounters with the Sun. Unlike long-period comets, whose materials remain relatively unchanged, the material of short-period comets is transformed by solar radiation, allowing the study of changes in the properties of dust and gas with each perihelion passage. The colour variations observed in these comets serve as indicators of evolution, reflecting various physicochemical processes that affect the composition and structure of the particles.
Comet 41P/Tuttle-Giacobini-Kresák (hereafter 41P) is a short-period comet belonging to the Jupiter family, first discovered by Horace Tuttle in 1858. Then, Michel Giacobini observed the comet in 1907, but the link between the two apparitions was not established until 1928. The periodic nature of 41P was secured in 1951 by Lubor Kresák at Skalnaté Pleso observatory.

We present the results of observations obtained before the 2017 perihelion passage using quasi-simultaneous broadband photometric and polarimetric, and long-slit spectral methods. Such a complex analysis allows us to study the dust and gas components of the comet in greater detail.
The observations of comet 41P were carried out on January 22, 2017, when the heliocentric and geocentric distances of the comet were 1.471 au and 0.506 au, respectively, and the phase angle was 13 degrees.

The multimode focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA was used. The observations of the comet were obtained in packet mode, which allowed us to make a sequence of exposures to obtain direct CCD images, long-slit spectra, and imaging linear polarimetry. We used the CCD chip E2V-42-90 with 2K × 2K square pixels of 16 µm corresponding to 0.18ʹʹ on the sky plane without binning. Photometric observations of comet 41P were performed through the g-sdss (the central wavelength λ₀ and FWHM are λ4650/1300 Å) and r-sdss (λ6200/1200 Å) broadband filters, as well as in a medium-band filter SED500 (λ5019/246 Å). The dichroic polarization analyzer (POLAROID) was used to measure linear polarization in the r-sdss filter. We derived spectra of the comet with a long-slit mask using the transparent grism VPHG1200@540 as a disperser in the spectroscopic mode of the SCORPIO-2. The slit was placed at the nucleus position in the sky and oriented along the comet’s velocity vector. The obtained spectra covered the wavelength range 3600–7070 Å.

Photometry. We estimated the dust activity level using the Afρ parameter. It was about 50 cm in the red domain. The comet also demonstrated typical red colour, based on g-r calculated values. To reveal the low-contrast structures in the dust coma, we constructed an intensity map of comet 41P using digital filters. Based on the distribution map of intensity, we derived the radial profiles of the surface brightness for observed structures to describe the dust brightness as a function of the distance from the optocenter. 

Spectroscopy. We analyzed cometary spectra in the wavelength region of 3800–7000 Å. Significant gas emission lines were detected in the spectra. The strongest features belong to molecules of CN, C₂, C₃, and NH₂. Gas production ratios were calculated for all detected molecules using the Haser model. Also, we estimated the gas contribution in wavelength regions corresponding to broadband filters used for photometric studies. To compare with photometrical results, we also computed a colour slope value based on the spectral data. The results from both methods are in good agreement.

Polarimetry. We analyzed the distribution of linear polarization degree over the coma. This map did not reveal dramatic variation in the linear polarization with cometocentric distance. Such homogeneity could be caused by a steady coma without any prominent active processes.