Refining the period of the cyclic positions of a morphological structure in the inner coma of Comet C/2017 K2 (PanStarrs)
ATel #15151; Federico Manzini (Stazione Astronomica di Sozzago), Paolo Ochner (Padova University), Virginio Oldani (Stazione Astronomica di Sozzago), Luigi R. Bedin (INAF-OAPd), Andrea Reguitti (UNAB; INAF-OAPd)
on 4 Jan 2022; 12:36 UT
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
perihelion that will occur on 20 December 2022 at 1.8 AU from the Sun.
Following the observation of an asymmetrical 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), and of a cyclic angular movement of the south-east structure (ATEL #14813),
we analyzed the publicly released images taken by the Hubble Space Telescope (PI David Jewitt) with the
WFC3 camera (F350LP broadband filter) during two additional observation sessions on 24 March and 13
June 2021, for a total of eleven sessions. During the whole observation period, from 17 March 2018 to 13
June 2021, the comet's distance from the Sun decreased from 14.33 AU to 6.13 AU, and the resolution of
the images increased from 417 to 164 km/pixel.
All images were cleaned of cosmic rays and defects. Spatial filters (Larson-Sekanina with alpha = 30 - 45 deg,
1/rho, and division by azimuthal median) centered on the optocenter of the comet were then applied to
highlight and confirm the presence of morphological structures in the inner coma.
The inner coma was characterized during the entire 39-month period by the presence of two radial structures
(probably dust emissions; Jewitt 2017) located roughly south-east and west of the nucleus (Fig. 1, 2). The
first of the two structures was constantly well visible in the processed images of all the observing dates,
therefore it was always possible to measure its position. The western jet-shaped structure showed a more
variable intensity and on some dates it appeared almost faded.
The measured position angles of the SE structure showed a cyclic oscillation between PA 105 deg and PA 190 deg,
with respect to its origin on the nucleus, with an estimated period of 155.5 ± 10 days (R^2 = 0.803; Figure 3).
Assuming that the SE structure is related to the presence of an active source on the cometary nucleus
(Sekanina, 1987), if the observed oscillation of its position is not resulting from a precession of the spin axis,
its estimated period could suggest a very slow rotation of the nucleus. In the hypothesis that the nucleus is
spherical, the active area would be located at mid latitude. Furthermore, if the amplitude of the observed
oscillation corresponds to the extreme positions assumed by the spin axis during the rotation of the nucleus,
it would be oriented approximately towards PA 145 deg.
Figures captions.
Figures 1 and 2. The original HST images taken on 2021.03.24 and 2021.06.13, were stacked on the optocenter of the
comet C/2017 K2. The stacks were treated with a 1/rho + LS algorithm (alpha = 30 deg). The PA of the south-eastern
structure was measured as described in the text.
Figure 3. Plot of the measured position angles (± standard deviation) of the south-east structure over the
observation period. The red dotted line is the sinusoidal best fit, showing a period of 155.5 ± 10 days (R^2=0.803).
Figures are available at:
https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2017K2_III/00_C2017_K2_ATEL_3_figures.pdf
