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

TL;DR

This study presents optical and near-infrared observations of comet 12P/Pons-Brooks during outbursts, revealing water ice features, gas emission ratios, and insights into the outburst mechanism and nucleus composition.

Contribution

First combined optical and NIR spectral analysis of 12P/Pons-Brooks during outbursts, identifying water ice and gas compositions, and proposing a crystallization-driven outburst mechanism.

Findings

NIR spectra show water ice absorption bands at 1.5 and 2.0 μm.

Optical spectrum reveals emission bands of CN, C3, C2, and NH2.

Estimated kinetic energy of the outburst suggests crystallization of amorphous water ice as trigger.

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 $\mu$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 $\sim8\times10^3$ 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.