2025ApJ...989..146Z

We present optical and near-infrared (NIR) observations of the outbursting, Halley-type comet 12P/Pons-Brooks (12P). Three NIR spectra were obtained during two outbursts in 2023 October and November, with the 3 m Infrared Telescope Facility and the Palomar 200 inch Telescope, respectively. The NIR spectra exhibited absorption features at 1.5 and 2.0 μm, consistent with the diagnostic absorption bands of water ice, superimposed on a red dust-scattering continuum. We find that the absorption bands and the red continuum can be well explained by micrometer-sized crystalline ice at 140-170 K, along with submicrometer-sized refractory grains (e.g., amorphous carbon). In addition, an optical spectrum was obtained with the Lijiang 2.4 m Telescope during the 2023 November outburst, which exhibited the emission bands of gaseous CN, C₃, C₂, and NH₂. The C₃/CN and C₂/CN ratios suggest that 12P was “typical” in C₃ abundance but somewhat depleted in C₂. The specific kinetic energy of the 2023 November outburst is estimated to be ∼8 × 10³ J kg^–1, suggesting a likely triggering mechanism similar to 332P/Ikeya-Murakami and 17P/Holmes, i.e., the crystallization of amorphous water ice. A refractory-to-ice ratio of ∼1.7-3.2 is derived from the total mass loss of dust and gas, aligning with the lower-end estimates for 67P/Churyumov-Gerasimenko and 1P/Halley. This suggests either a less evolved nucleus or an outburst region enriched in icy materials relative to the bulk nucleus.