143P/Kowal-Mrkos and the Shapes of Cometary Nuclei

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Abstract
We add 143P/Kowal-Mrkos to the small but growing sample of well-observed cometary nuclei. Photometric observations from 3.4 to 4.0 AU heliocentric distance reveal a pointlike object with no detect- able outgassing. Periodic modulation of the scattered light (DmR = 0.45 � 0.05) is attributed to rotation of the bare nucleus with a double-peaked period 17.21 � 0.10 hr and a projected ratio of the shortest to longest axis of about 0.67/1. We also measured the phase coefficient (0.043 � 0.014 mag deg � 1 ), the BVRI colors (VR = 0.58 � 0.02), and the absolute red magnitude (mR(1, 1, 0) = 13.49 � 0.20). The effective circular radius is 5.7 � 0.6 km (geometric albedo 0.04 assumed). We study the properties of 11 well-observed Jupiter- family comet (JFC) nuclei. On average, the nuclei are systematically more elongated (average photometric range DmR = 0.54 � 0.07) than main-belt asteroids of comparable size (DmR = 0.32 � 0.05) and more elon- gated than fragments produced in laboratory impact experiments. We attribute the elongation of the nuclei to an evolutionary effect, most likely driven by sublimation-induced mass loss. However, we find no evidence for any relation between the nucleus shape and the sublimation timescale. This may be because the timescale for evolution of the nucleus shape is very short compared with the dynamical timescale for the JFCs, meaning that most nuclei in our sample are already highly physically evolved.