Narrowband Observations of Comet 46P/Wirtanen During Its Exceptional Apparition of 2018/19 I: Apparent Rotation Period and Outbursts

TL;DR

This study presents extensive observations of comet 46P/Wirtanen during its 2018/19 apparition, revealing its rotation period changes, active regions, and outbursts, providing insights into its rotational dynamics and activity patterns.

Contribution

It provides the first detailed analysis of the comet's apparent rotation period variation and outburst activity during its close approach, using extensive imaging data.

Findings

Rotation period increased from 8.98 hr to 9.14 hr near perihelion and then decreased to 8.94 hr.

Detected two outbursts with velocities of 68 and 162 m/s, respectively.

Identified two primary active regions producing spiral structures in the coma.

Abstract

We obtained broadband and narrowband images of the hyperactive comet 46P/Wirtanen on 33~nights during its 2018/2019 apparition, when the comet made an historic close approach to the Earth. With our extensive coverage, we investigated the temporal behavior of the comet on both seasonal and rotational timescales. CN observations were used to explore the coma morphology, revealing that there are two primary active areas that produce spiral structures. The direction of rotation of these structures changes from pre- to post-perihelion, indicating that the Earth crossed the comet's equatorial plane sometime around perihelion. We also used the CN images to create photometric lightcurves that consistently show two peaks in the activity, confirming the two source regions. We measured the nucleus' apparent rotation period at a number of epochs using both the morphology and the lightcurves. These results all show that the rotation period is continuously changing throughout our observation window, increasing from 8.98~hr in early November to 9.14~hr around perihelion and then decreasing again to 8.94~hr in February. Although the geometry changes rapidly around perihelion, the period changes cannot primarily be due to synodic effects. The repetition of structures in the coma, both within a night and from night-to-night, strongly suggests the nucleus is in a near-simple rotation state. We also detected two outbursts, one on December~12 and the other on January~28. Using apparent velocities of the ejecta in these events, 68$\pm$5~m~s$^{-1}$ and 162$\pm$15~m~s$^{-1}$, respectively, we derived start times of 2018~December~12 at 00:13~UT~$\pm$7~min and 2019~January~27 at 20:01~UT~$\pm$30~min.