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

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 November with the INT-IDS, and in 2023 December with NOT-ALFOSC. Using a standard Haser model in a 10 000-km-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.