Comet 21P/Giacobini-Zinner: Narrowband Photometry of the Prototype of Carbon-Chain Depleted Comets at Multiple Apparitions

TL;DR

This study presents 33 years of narrowband photometry data on Comet 21P/Giacobini-Zinner, confirming its consistent carbon-chain depletion and revealing long-term compositional and rotational axis changes.

Contribution

It provides a comprehensive, multi-apparition analysis of the comet's chemical composition and activity patterns, including re-reduction of historical data and new observations.

Findings

Giacobini-Zinner remains depleted in C2, C3, and NH compared to OH and CN.

Production rates peak 3-5 weeks before perihelion with asymmetrical activity.

Long-term data suggest a precession of the comet's rotation axis.

Abstract

We obtained extensive narrowband photoelectric photometry of Comet 21P/Giacobini-Zinner with observations spanning 33 years. The original data from 1985 (Schleicher et al. 1987) were re-reduced and are presented along with data from three additional apparitions including 2018/19. The original conclusion regarding Giacobini-Zinner's chemical composition remains unchanged, with it having a 4-6x depletion in the carbon-chain molecules C2 and C3, and in NH, as compared with both OH and CN. The comet continues to exhibit a large asymmetry in production rates as a function of time and heliocentric distance, with production reaching a peak 3-5 weeks prior to perihelion. All species, including dust, follow the same general production rate curve each apparition, and the carbon-bearing species are always very similar to one another. However, OH and NH each differ in detail from the carbon-bearing species, implying somewhat varied composition between source regions. Longer term, there are only small secular changes among the apparitions before and near perihelion, but larger changes are evident as the comet recedes from the Sun, suggestive of a progressive precession of the rotation axis.