Observations of distant comet C/2011 KP36 (Spacewatch): photometry, spectroscopy, and polarimetry

Abstract

Aims. The main objective of our study is to obtain new observational results for the active long-period comet C/2011 KP36 (Spacewatch). This comet has unusual orbital properties and moves at distances larger than 5 au from the Sun. Methods. We carried out extensive observations of comet C/2011 KP36 (Spacewatch) at the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the focal reducer SCORPIO-2. We obtained quasi-simultaneous long-slit spectra in the visible, as well as photometric and linear polarimetric images with the g-sdss and r-sdss filters on November 25, 2016 when the heliocentric and geocentric distances of the comet were 5.06 au and 4.47 au, respectively. We modeled the behavior of the color and polarization in the coma, considering the dust as an ensemble of polydisperse nonspherical particles. Results. Two strong jet-like structures in solar and antisolar directions and two short and narrow jet features in the perpendicular direction were revealed in the coma. Our simulations showed that the latter two jets originated from the same active area. We determined the orientation of the rotation axis of the nucleus and the position of three active areas. High activity of the comet was characterized by Afρ values of 1065 ± 11 cm in the g-sdss filter and 1264 ± 17 cm in the r-sdss filter. The comet was found to be rich in CO+, while there was no clear detection of CN, C3, C2, and N2+. The dust color g–r varies over the coma from about 0.2m to 0.7m, and the linear polarization degree from about −1% to −6% at the phase angle 9.6°. The color of the nucleus of comet C/2011 KP36 (Spacewatch) is ultrared, B – R = 1.9m ± 0.3m. Conclusions. The high variability of the observed characteristics over the coma of comet C/2011 KP36 (Spacewatch) indicates significant and variable activity of the nucleus with, probably, numerous small active areas. Together with the three identified large active areas, they are characterized by different combinations of water ice, CO2 ice, and refractory dust and sizes of their particles, which are in the micron-size range.