Comet C/2017 K2 (PanStarrs) shows jet-shaped dust emissions modulated by extremely slow rotation at greater-than-usual distance from the Sun
ATel #14813; Federico Manzini (Stazione Astronomica di Sozzago), Virginio Oldani (Stazione Astronomica di Sozzago), Paolo Ochner (Padova University), Luigi R. Bedin (INAF-OAPd), Andrea Reguitti (UNAB; INAF-OAPd)
on 27 Jul 2021; 10:46 UT
Distributed as an Instant Email Notice Comets
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)
Comet C/2017 K2 (PanStarrs), coming from the Oort cloud, is approaching the Earth and the Sun towards
its perihelion, that will occur on 20 December 2022.
Following the observation of an asymmetric coma and of two jet-shaped structures on the nucleus of the
comet on 12 September 2020 with the INAF-OAPd 1.82-m Copernico telescope (Osservatorio Astrofisico di
Asiago, Italy) (ATEL #14026), we performed a deep analysis of publicly released images taken by the
Hubble Space Telescope (HST, PI David Jewitt) with the WFC3 camera and the F350LP broadband filter
during eight observation sessions between 17 March 2018 and 18 December 2020.
Each observing session was performed during one HST orbit and provided six images that were co-added
after being cleaned of cosmic rays and defects in order to obtain a uniform background. The image
resolution ranged between 232 and 417 km/pixel.
The analysis of the images, performed by applying a Larson-Sekanina filter (with alpha=45 deg) and a 1/rho
filter, showed a relatively stable activity of the comet during the entire 33-month period, characterized by
the presence of two radial structures (probably dust emissions) located roughly south-east and west of the
nucleus (Figure 1). The first of the two structures had always been more evident, so made it possible to
accurately measure its position on each image, which showed a cyclic oscillation, between PA 110 deg and PA
190 deg, with a period of about 156 days (Figure 2), suggesting an extremely slow rotation of the nucleus.
A computer modeling of the comet, performed by means of a proprietary software (Fase6), showed that the
structures observed on the images could be due to two different active emission sources, one located at mid
latitude and one near-polar on the nucleus, with a spin axis approximately directed towards PA 150 deg and with
an inclination of about 30-40 deg from the plane of the sky.
Both structures developing from the two active areas showed a wide curvature, which must have formed due
to the slow 156-day rotation of the nucleus, given the negligible effect of the solar radiation pressure at the
distance of the comet from the Sun in the analyzed period (between 14.33 and 7.55 AU).
The mechanism of a possible dust emission at this distance is unknown and deserves further investigations.
Figure captions.
Figure 1. The original HST images taken on 2020.10.06, with F350LP filter, were stacked on the optocenter
of the comet C/2017 K2. The stack was treated with a LS algorithm (alpha=45 deg) and a Gauss smoothing filter
(sigma=2). The structure actually in PA 170 deg about was measured as described in the text.
Figure 2. Plot of the measured position angles (measure error of 10 deg) of the southeast structure over the
observation period. The dotted line is a sinusoidal fit, showing a period of 156 days (R^2=0.981).
Figures are available at:
