Constraints on Comet 332P/Ikeya-Murakami

Encke-type comet 332P/Ikeya-Murakami is experiencing cascading fragmentation events during its 2016 apparition. It is likely the first splitting Encke-type comet ever being observed. A nongravitational solution to the astrometry reveals a statistical detection of the radial and transverse nongravitational parameters, $A_{1} = \left(1.54 \pm 0.39\right) \times 10^{-8}$ AU day$^{-2}$, and $A_{2} = \left(7.19 \pm 1.92\right) \times 10^{-9}$ AU day$^{-2}$, respectively, which implies a nucleus erosion rate of $\left(0.91 \pm 0.17\right)$% per orbital revolution. The mass-loss rate likely has to be supported by a much larger fraction of an active surface area than known cases of short-period comets; it may be relevant to the ongoing fragmentation. We failed to detect any serendipitous pre-discovery observations of the comet in archival data from major sky surveys, whereby we infer that 332P used to be largely inactive, and is perhaps among the few short-period comets which have been reactivated from weakly active or dormant states. We therefore constrain an upper limit to the nucleus size as $2.0 \pm 0.2$ km in radius. A search for small bodies in similar orbits to that of 332P reveals comet P/2010 B2 (WISE) as the best candidate. From an empirical generalised Jupiter-family (Encke-type included) comet population model, we estimate the likelihood of chance alignment of the 332P--P/2010 B2 pair to be 1 in 33, a small number indicative of a genetic linkage between the two comets on a statistical basis. The pair possibly originated from a common progenitor which underwent a disintegration event well before the twentieth century.

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