Monitoring Comet 12P/Pons-Brooks: Spectral Analysis and Outburst Phenomenology

Cometary nuclei are small bodies within the Solar System, believed to be remnants of the original agglomerates of dust grains formed approximately 4.6 billion years ago during the birth of the Solar System. Despite the presumed common origin of comet formation, their different dynamical evolutions have separated them into two primary reservoirs: the Kuiper Belt and the Oort Cloud. Oort Cloud comets, often referred to as "long-period" comets, are currently at the forefront of scientific interest, particularly with the upcoming launch of ESA’s Comet Interceptor mission in 2029. Comets from both dynamical groups are intrinsically valuable, as they contain the most pristine materials from the early Solar System.

While short-period comets have been extensively studied over the past decade, notably through in-situ analyses by spacecraft such as Giotto, Deep Impact, and Rosetta, many aspects of periodic comets remain unresolved. One such unresolved aspect is the recurrent outburst phenomenon observed in certain comets. Halley-type comet 12P/Pons-Brooks is notable for its recurrent outbursts, which have been observed during each of its recent perihelion passages, occurring approximately every 71 years. This behaviour is not unique to 12P/Pons-Brooks; other comets such as 9P/Tempel 1, which exhibited
mini-outbursts before the Deep Impact collision, and 29P/Schwassmann-Wachmann 1, known for its recurring outbursts, display similar phenomena.

Outbursts in comets are characterized by a sudden increase in the comet’s brightness, typically ranging from 2 to 5 magnitudes, with a higher likelihood of occurring around heliocentric distances of 1 AU. Despite these observations, the mechanisms driving these outbursts remain poorly understood.

From July to December 2023, a detailed follow-up study of comet 12P/Pons-Brooks was conducted using the 1.22m Galileo Telescope at the Asiago Astrophysical Observatory. The spectral data were obtained using the telescope’s Cassegrain focus equipped with a Boller & Chievens spectrograph. This spectrograph features a long-slit aperture with a fixed length of 28 mm and a variable width, reaching a maximum aperture of 1 mm. The available gratings provide spectral coverage from 3300 Å to beyond 7800 Å, with a dispersion reaching 0.6 Å/px. This configuration enables a clear detection of all main
emission features in the optical spectrum of comets, including the CN violet system (3880 Å), the C3 band (3920-4100 Å), several NH2 lines, C2 Swan bands (4500-4745 Å, 5000-5174 Å, and 5410-5640 Å), the CH band (4300-4312 Å), and [OI] lines (6300, 6364, 5577 Å).

Using the Haser coma distribution model, the production rates (Q) for each molecular species were calculated from these emission bands. This method effectively monitors the changes in overall cometary activity during an outburst. Preliminary analysis revealed a rapid change in the production rate of the CN molecule as a direct consequence of the outburst. The analysis was expanded to include other important molecular components, such as C2, C3, and H2O.

During the observation period, the comet was on its inbound orbit from 4.0 AU to 2.5 AU, covering two significant outbursts on October 5 and November 14, 2023. The November event was particularly notable, with the Q(CN) production rate varying by approximately an order of magnitude. The study highlights significant variations in the production rates of main molecular species within a few days before and after the onset of the outburst.

The rapid nature of cometary outbursts, which can last from hours to several days, emphasizes the importance of continuous monitoring. Observations from easily accessible facilities, such as the 1.22m Galileo Telescope, can provide critical insights into these unpredictable phenomena. Our analysis demonstrates the excellent performance achievable with 1-meter class telescopes, underscoring their value in the study of transient and fast cometary events.

In conclusion, this research provides a temporal analysis of the spectral changes in comet 12P/Pons-Brooks during its recent outbursts. The results underline the importance of continuous spectral monitoring and the use of accessible telescopes to study the complex and transient behaviours of comets. This work not only enhances our understanding of cometary outbursts but also contributes to the broader knowledge of the physical processes governing these ancient Solar System bodies.