Optical and Near-infrared Spectroscopy of the Outbursting Comet 12P/Pons-Brooks

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 μ m, 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, C 3 , C 2 , and NH 2 . The C 3 /CN and C 2 /CN ratios suggest that 12P was “typical” in C 3 abundance but somewhat depleted in C 2 . The specific kinetic energy of the 2023 November outburst is estimated to be ∼8 × 10 3 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.