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

Abstract

We present optical and near-infrared (NIR) observations of the outbursting, Halley-type comet 12P/Pons-Brooks. Three NIR spectra were obtained during two outbursts in October and November 2023, with the 3-meter Infrared Telescope Facility and the Palomar 200-inch Telescope, respectively. The NIR spectra exhibited absorption features at 1.5 and 2.0 $\hat{I}¼$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 sub-micrometer-sized refractory grains (e.g., amorphous carbon). In addition, an optical spectrum was obtained with the Lijiang 2.4-meter Telescope during the November 2023 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/Pons-Brooks 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 $\sim 8\times 10^3$ 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 $\sim$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.

18 pages, 4 figures, 1 table, accepted for publication in ApJ