Rotation period and Morphological Structures in the inner coma of comet C/2022 E3 (ZTF)
ATel #15909; Federico Manzini (SAS - Stazione Astronomica di Sozzago, Novara, Italy), Virginio Oldani (SAS), Paolo Ochner (UniPD, Padua University, Italy), Luigi R. Bedin (INAF-OAPd, Padua Astronomical Observatory - National Astrophysics Institute, Italy), Andrea Reguitti (INAF-OAPd)
on 18 Feb 2023; 13:09 UT
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)
We observed the comet C/2022 E3 for twelve nights from January 31 to February 17, 2023, with the 0.4-m
Savonarola Cassegrain telescope of the Stazione Astronomica di Sozzago (Italy-MPC A12), with a
resolution of 154 to 255 km/pixel on the plane of the sky at the distance of the comet, using a filter
centered on the emission of CN (388 nm) with a FWHM of 25 nm.
Our CCD images (automatically tracked on the optocenter of the comet's coma) always showed a
prominent morphological structure in the inner coma in the solar direction, suggesting a strong activity
deriving from at least two large areas on the cometary nucleus and modulated by rotation. This
morphology, easily visible by applying numerical filters to the original images, changed over time due to the
geometric conditions between the positions of the Earth and the comet along their respective orbits. In the
dates around the perigee passage, the morphology in the inner coma was identifiable with certainty only
on immediately subsequent days, while from February 8 the geometric orbital conditions of observation
allowed the comparison of images taken even over several days.
The morphological structures show shell fragments or incomplete arcs originating directly from the
nucleus. Their distance increases over time, with a mean expansion velocity of 0.950 km/s on the plane of
the sky at the distance of the comet. From the analysis of the morphology of the inner coma of comet
C/2022 E3, shown by 16 pairs of images obtained on different nights, the structures appear to be rotating,
and it was possible to identify the regular recurrence of the same appearance. This allowed to determine a
rotation period of the cometary nucleus of 8.49 h ± 0.12 h, with no obvious temporal changes or deviations
from a simple rotational state over the reported time interval. This value is lower than that of
8.7 ± 0.10 h recently published by Knight et al. (ATel #15879).
The strong emission of gases and dust from the active sources on the nucleus may have caused an increase
in the rotation speed. However, from our analyses on a 15 days time-span, no variation of the rotation
period was detected.
Enhanced dust and C2 images (Rj- and C2-filtered, respectively) showed a significantly different morphology
from CN, but the active sources on the nucleus appear always on the same spatial position. The structures
related to the emission of dust are clearly affected by solar radiation pressure, and numerical models of the
dust expansion created with our proprietary software Fase 6 are in agreement with the period of 8.49 h.
The morphology of the inner coma suggests that during our observation period the spin axis was directed
towards PA 250°-280° with a possible inclination (around 30°) from the plane of the sky.
Figures captions.
Figure 1. 2023.02.02T17:54 to 2023.02.03T02:00 UT. Animation of the motion of the CN emissions from the
nucleus of comet C/2022 E3 (ZTF).
Stazione Astronomica di Sozzago (IAU-MOC A12). Refl. 0.4m, filter CN, resolution: 150
km/pixel. Each frame is a stack of 10x300s images treated with a LS algorithm (alpha=40°) to
enhance the morphology of the inner coma.
Figure 2. 2023.02.10. Enhancing of the shape of the inner coma and the direction of the tail with a 1/R spatial
filter applied to the comet's optocenter/nucleus. In the animation, the images are from CN, CII,
Clear filters, respectively.
Stazione Astronomica di Sozzago (IAU-MPC A12). Refl. 0.4m, filters CN+CII+Clear, resolution:
175 km/pixel. North is up, East left.
Figures and extra material are available at:
Figures and animations
