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

Cometary outbursts are sudden increases in brightness caused by various mechanisms, including cliff collapse, pressure pockets, or impacts etc. The most plausible explanation for recurring and large-scale outbursts may be the crystallization of amorphous water ice. 12P/Pons-Brooks, a Halley-type comet with recurring outbursts, presents a valuable case study for understanding the nature of cometary activity. This work aims to analyze the near-infrared (NIR) and optical spectra of 12P during two major outbursts in October and November 2023, characterize its dust and gas properties, and explore the underlying triggering mechanisms.

We obtained three NIR spectra during two outbursts in October and November 2023, using the 3-m Infrared Telescope Facility (IRTF) and the Palomar 200-inch Telescope (P200), respectively. As shown in Figure 1, all three NIR spectra exhibited absorption bands at 1.5 and 2.0 μm, consistent with the diagnostic absorption features of water ice, superimposed on a red dust-scattering continuum. Through a single-scattering dust model, we found that the November 2 spectrum can be well explained by micrometer-sized crystalline ice at 140-170 K, along with sub-micrometer-sized amorphous carbon. Analysis of the NIR spectra reveals no significant weakening in the depth of the water ice absorption bands from October to November, despite stronger sublimation being expected at smaller heliocentric distances. Possible explanations include: a slight shift in size distribution towards larger grains; continuous replenishment of icy grains from the nucleus; or the presence of exceptionally pure ice.

An optical spectrum was obtained with the Lijiang 2.40-m Telescope during the November outburst.  As shown in Figure 2, emission bands of CN, C₂, C₃ and NH₂ were detected. The C₃/CN and C₂/CN ratios suggest that 12P was “typical” in C₃ abundance but near the limit of C₂-depletion at the time of observation. In terms of the Afρ value, 12P is a dusty comet when compared to Halley-type and Jupiter-family comets at similar heliocentric distances.

Combining NIR and optical observations, we found that the specific kinetic energy of the November outburst was about 8 × 10³ J kg^-¹, suggesting a triggering mechanism similar to 332P/Ikeya--Murakami and 17P/Holmes, likely the crystallization of amorphous water ice, though other mechanisms remain possible due to limited evidence. In addition, a refractory-to-ice ratio of >1.7 is derived from the total mass loss of dust and gas.

This study provides new insights into the physical properties of water ice grains in outbursting comets. The findings reinforce the hypothesis that crystallization of amorphous water ice is a dominant mechanism driving such events. The results have broader implications for understanding cometary evolution and outburst dynamics, particularly for Halley-type comets.

Figure 1: NIR spectra of 12P taken with SpeX and TSpec. The date and instrument are labeled below the respective spectrum. The red-system band of CN and the two water ice absorption bands are labeled. Spectral regions heavily contaminated by telluric absorption are masked in gray. The “emission” feature near 1.3 μm in the Nov. 3 spectrum is due to imperfect telluric correction. The Nov. 2 spectrum is well reproduced by a dust model (red line), with the best-fit parameters labeled at the bottom: δ-the mass ratio of amorphous carbon to water ice in the coma, α-power-law index of the grain size distribution, a_p-the peak grain size.

Figure 2: (a) Optical spectrum of 12P taken on Nov. 2. Also shown are the spectrum of a G2V star (grey line) and the reproduced dust continuum (red dashed lines). (b) Emission component in the spectrum of 12P. Emission bands of CN, C₃, C₂, and NH₂ are labeled.