Narrowband Observations of Comet 46P/Wirtanen During its Exceptional Apparition of 2018/19 II: Photometry, Jet Morphology, and Modeling Results

TL;DR

This paper presents extensive photometry and imaging of Comet 46P/Wirtanen during its 2018/19 apparition, analyzing its activity, morphology, and rotation, and developing a Monte Carlo model to explain observed coma features.

Contribution

It provides new detailed observations and a self-consistent Monte Carlo model of Wirtanen's activity, morphology, and rotation during its 2018/19 apparition.

Findings

Secular decrease in activity since 1991.

Symmetric production rates around perihelion for certain species.

Model estimates of the comet's rotation pole and active sources.

Abstract

We report on our extensive photometry and imaging of Comet 46P/Wirtanen during its 2018/19 apparition and use these data to constrain modeling of Wirtanen's activity. Narrowband photometry was obtained on nine epochs from 2018 October through 2019 March as well as 10 epochs during the 1991, 1997, and 2008 apparitions. The ensemble photometry reveals a typical composition and a secular decrease in activity since 1991. Production rates were roughly symmetric around perihelion for the carbon-bearing species (CN, C$_3$, and C$_2$), but steeper for OH and NH outbound. Our imaging program emphasized CN, whose coma morphology and lightcurve yielded rotation periods reported in a companion paper (Farnham et al., PSJ, 2, 7). Here, we compare the gas and dust morphology on the 18 nights for which observations of additional species were obtained. The carbon-bearing species exhibited similar morphology that varied with rotation. OH and NH had broad, hemispheric brightness enhancements in the tailward direction that did not change significantly with rotation, which we attribute to their originating from a substantial icy grain component. We constructed a Monte Carlo model that replicates the shape, motion, and brightness distribution of the CN coma throughout the apparition with a single, self-consistent solution in principal axis rotation. Our model yields a pole having (R.A., Decl.) = 319$^\circ$, $-$5$^\circ$ (pole obliquity of 70$^\circ$) and two large sources (radii of 50$^\circ$ and 40$^\circ$) centered at near-equatorial latitudes and separated in longitude by $\sim$160$^\circ$. Applications of the model to explain observed behaviors are discussed.