Coma composition and profiles of comet 12P/Pons-Brooks using long-slit spectroscopy

TL;DR

This study uses long-slit spectroscopy to analyze the coma composition and profiles of comet 12P/Pons-Brooks during its 2023 outbursts, revealing consistent composition but complex coma morphology not fully explained by standard models.

Contribution

It provides detailed spectroscopic analysis of 12P/Pons-Brooks' coma, highlighting limitations of the Haser model and suggesting extended sources influence coma morphology.

Findings

Constant C2/CN ratio of about 90% during observations

Haser model inadequately reproduces coma shape

Extended sources likely influence coma morphology

Abstract

Comet 12P/Pons-Brook exhibited multiple large and minor outbursts in 2023 on its way to its 2024 perihelion, as it has done during its previous apparitions. We obtained long-slit optical spectra of the comet in 2023 August and 2023 November with the INT-IDS, and in 2023 December with NOT-ALFOSC. Using a standard Haser model in a 10000km-radius aperture and commonly used empirical parent and daughter scale-lengths, our calculated abundance ratios show a constant "typical" composition throughout the period with a C$_2$/CN ratio of about 90 per cent. Molecular density profiles of different species along the slit show asymmetries between opposite sides of the coma and that C$_2$ seems to behave differently than CN and C$_3$. Comparing the coma profiles to a standard Haser model shows that this model cannot accurately reproduce the shape of the coma, and therefore that the calculated production rates cannot be deemed as accurate. We show that an outburst Haser model is a {slightly} better match to the C$_3$ and CN profile shapes, but the model still does not explain the shape of the C$_2$ profiles and requires equal parent and daughter scale-lengths. Our results suggest that the coma morphology could be better explained by extended sources, and that the nature of 12P's activity introduces bias in the determination of its composition.