Hello all,
This recent ATEL about the rotation of Comet C/2022E3 is an intersting
read.

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ATEL #15909      ATEL #15909

Title: Rotation period and Morphological Structures in the inner coma
of comet C/2022 E3 (ZTF)
Author: 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)
Queries: manzini.ff@aruba.it
Posted: 18 Feb 2023; 13:09 UT
Subjects:Optical, Comet

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:
https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2022E3
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Password Certification: Andrea Reguitti (andreareguitti@gmail.com)
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