Photometric and spectroscopic monitoring of the outbursting Halley-type comet 12P/Pons-Brooks

Context. Comet 12P/Pons-Brooks is a Halley-type comet known for its outbursting activity during its previous perihelia in 1883 and 1953. Its strong activity and favourable observing geometry gave a great opportunity to study this outburst-prone comet during its 2024 return with modern instruments. Aims. We aim to study the activity and composition of 12P/Pons-Brooks during its 2024 passage through long-slit spectroscopy and narrow- and broad-band photometry and imaging. Methods. The photometric survey contains over 130 nights of observation with TRAPPIST-North and -South from May 6, 2023 (rh = 4.62 au, inbound), to September 7, 2024 (rh = 2.06 au, outbound). We collected images with broad-band Johnson–Cousins filters (BVRcIc) and narrow-band HB filters (OH, CN, C2, C3, NH, and dust continuum). We obtained long-slit low-resolution spectra at the Observatoire de Haute Provence with the MISTRAL instrument on the night of March 23, 2024, at a heliocentric distance of 0.94 au. The spectra were used to analyse the emissions from the different molecular bands. Results. We detect nine strong outbursts, which show an approximate two-week periodicity; the OH production rate reaches values exceeds 1029 molecules/s and the A(0)fρ dust activity proxy above 105 cm. Dust images show the formation of a double horn-shaped coma and the expansion of a dust shell in the days following the strongest outbursts. The gas-to-dust ratio behaviour during these events differs from other observations of outbursting comets closer to the Sun, while outbursts rather share similarities with the centaur comet 29P/Schwassmann-Wachmann. The analysis of CN jets between 1.02 and 2.13 au suggests a nucleus rotation period of either 29.45 h or 58.90 h and a gas ejection velocity of 1.03 ± 0.09 km/s. From the MISTRAL spectra, emission bands from C2, NH2, and CN were observed, as well as H2O+ in the anti-sunward direction.