CRIRES+ investigations of the volatile composition of Halley-type comet 12P/Pons-Brooks during its exceptional passage in 2024

We present the results obtained from VLT/CRIRES⁺ observations of the Halley-type comet 12P/Pons-Brooks, during its exceptional apparition in 2024. 

Comets are considered among the least processed bodies of the solar system. Stored in a frozen environment from their formation to the present day, they are expected to retain most of the chemical and mineralogical properties associated with their formation site. Thus, chemical differences among comets coming from different reservoirs (e.g., Jupiter Family vs Oort Cloud comets) can provide important insights into the chemical and physical conditions prevalent in our protoplanetary disc about 4.6 billion years ago, and test a possible radial gradient in the chemistry of forming icy planetesimals, along with the dynamical models that predict their dissemination [1,2,3].

Infrared (IR, 2 - 5 μm) spectroscopic studies of comets provide key information on the relative abundances of many primary (parent) molecules thought to be released directly from the nucleus (e.g., H₂O, CH₃OH, CH₄, C₂H₆, CO, NH₃, H₂CO, …), allowing for chemical characterisation. The most recent IR statistics show chemical variability within the comet population but no distinct chemical groups [4,5]. Nonetheless, several dynamical classes of comets remain underrepresented in terms of compositional studies, with Halley-type comets (HTCs) being the least investigated. HTCs are linked to the Oort cloud, but have dynamically evolved into shorter (decades long) orbital periods. 

In 2024, 12P/Pons-Brooks (hereafter 12P) reached perihelion on April 20, at 0.78 au, becoming one of the brightest comets in the last decades, and giving the opportunity to easily measure its chemical composition from ground-based observatories. 

We will show 12P spectra obtained in July 2024 using the cross-dispersed spectrograph CRIRES⁺ at ESO/VLT, and report retrieved rotational temperatures, production rates and relative abundances for H₂O, CO, CH₃OH, CH₄, C₂H₆, NH₃, H₂CO, HCN, C₂H₂. In addition, we will explore for significant upper limits for species characterised by fainter spectral emission lines such as HDO, HCl and C₂H₄. 

Results for 12P will be put in the overall picture of comet composition, taking into account results from CRIRES⁺ observations of comets in the last four years (e.g., C/2021 A1 (Leonard) [6], C/2023 H2 (Lemmon) [7], and C/2017 K2 (PANSTARRS) [8]), and more generally considering the existing databases [4,5], to determine whether there are statistically significant differences in composition among the various dynamical types (Halley-type vs Jupiter Family vs long-period and dynamically new Oort Cloud comets).

Following [9], we will finally compare abundance ratios measured in 12P and other comets to those found in planet-forming disks surrounding young solar analogues (10⁴ — 10⁶ years old, see for example [10,11]), with the final aim of understanding the degree of chemical reprocessing in the disk (inherited vs reset scenario) and/or the possibility of material evolution in comets after their formation. 

References: [1] Mumma, M. J., & Charnley, S. B., 2011, ARA&A, 49, 471; [2] Eistrup, C., et al., 2019, A&A, 629, A84; [3] Morbidelli, A., & Rickman, H., 2015, A&A, 583, A43; [4] Dello Russo, N., et al., 2016, Icarus, 278, 301; [5]  Lippi, M., et al., 2021, AJ, 162, 74; [6] Lippi et al., 2023, A&A, 676, A105; [7] Lippi et al., 2024, A&A, 689, A77; [8] S. Hmiddouch et al., in preparation; [9] Lippi, M., et al., 2024, ApJ, 970, L5; [10]​​ Podio L., et al., 2020, A&A, 644, A119; [11] Yang et al., 2021, APJ, 910, 1.