Rotation period of comet 12P/Pons-Brooks from CN coma morphology
ATel #16508; M. M. Knight (U. S. Naval Academy), B. A. Skiff (Lowell Observatory), D. G. Schleicher (Lowell Observatory), L. G. Spiro (U. S. Naval Academy), I. C. Fernald (U. S. Naval Academy), B. Y. Guan (U. S. Naval Academy), L. J. Lininger (U. S. Naval Academy), J. A. Larsen (U. S. Naval Academy)
on 6 Mar 2024; 21:10 UT
Credential Certification: Matthew Knight (astromatthew@gmail.com)
We report on imaging of comet 12P/Pons-Brooks obtained on nine epochs between 2024 Jan. 31 and 2024 Feb. 16 using the Lowell Observatory Hall 42in (1.1.-m) telescope and the U.S. Naval Academy Hopper Hall (0.5-m) telescope. Narrowband CN images enhanced by azimuthal median subtraction reveal two apparently face-on spiral features approximately 180 degrees apart. Although only a single snapshot was acquired at each epoch, phasing of the data to various potential rotation periods produced a smooth progression of the features only for periods between 56 hr and 58 hr. Nearly matching morphology, offset by a relatively small rotation, was seen in numerous pairs of images. On the assumption that the morphology varied at a constant rate over 360 degrees, only a period near 57 hours was consistent with all pairs of images. From these two independent assessments and assuming a constant rotation period, we conclude that the apparent rotation period is 57 +/- 1 hr. Integer aliases (e.g., 28.5 hr, 19 hr, etc.) were excluded due to differences in morphology between the two spirals or because they did not produce a smoothly varying sequence when phased. Furthermore, observations acquired from both facilities ~2 hours apart showed only a small rotation, consistent with a period near 55 hr. This assessment does not account for changing viewing geometry and any associated synodic or sidereal effects, but these are expected to be relatively small over the observing interval.
