Comet 2P/Encke in apparitions of 2013 and 2017: I. Imaging photometry and long-slit spectroscopy

Abstract

We present the results of imaging photometric and long-slit spectroscopic observations of comet 2P/Encke performed at the heliocentric distance r = 0.56 au, geocentric distance Δ = 0.65 au, and phase angle α = 109.2° on November 4, 2013 and at r = 1.05 au, Δ = 1.34 au, and α = 46.8° on January 23, 2017. Observations were carried out at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the multimode focal reducer SCORPIO-2. In 2013, the direct images of comet Encke were obtained with the broad-band V filters, whereas in 2017 the narrow-band cometary BC, RC, and NH2 filters as well as the medium-band SED500 and broad-band r-sdss filters were used for observations. About 60 emissions belonging to the CN, C2, C3, NH2, CH, and CO+ molecules were identified within the range λ3750–7100 Å. The ratios of the production rates C2/CN and C3/CN correspond to the typical comets, not depleted in the carbon-chain. A complex structure of the coma was detected in both observational periods. In January 2017, the dust was in general concentrated near the nucleus, the dust/gas ratio was 2.9 in the r-sdss filter, however, this ratio was larger than 1 at distances 3000–40,000 km from the nucleus. We found that about 75% of the flux of the reflected light in the central pixel was due to the nucleus, whereas the nucleus's flux contributed 48% in the total intensity of the 2000 km area of the coma. We found that after correction for the dust coma contamination the nucleus magnitude is 18.8m ± 0.2m. Corrected for emissions penetrating into the BC and RC filters and nucleus contribution, color index BC–RC decreased sharply from about 1.43m in the innermost near-nucleus coma to ~0.4m at the distance ~2500 km. Color BC–RC of the cometary nucleus was about 1.39m. We performed a dynamical simulation of dust particles to characterize the morphology and found that visible jets in both observational periods were formed by a single active source located in the north hemisphere at the cometocentric latitude +55°. The behavior of the dust coma near the nucleus indicates a strong dust transformation at the distances below ~2000 km.