Multi-Wavelength Observations of Comet C/2011 L4 (Pan-Starrs)

TL;DR

This study presents multi-wavelength observations of comet C/2011 L4 (Pan-Starrs), revealing its dust-rich nature, water ice features, and low gas emissions, providing insights into its composition and activity during its approach to the Sun.

Contribution

First multi-wavelength observational analysis of C/2011 L4, identifying water ice features and its unusually high dust-to-gas ratio, advancing understanding of comet composition.

Findings

Detected water ice absorption at 2.0 μm but not at 1.5 μm, explained by sub-micron ice grains.

No gas emissions (CN, HCN, CO) detected pre-perihelion, with upper limits established.

Comet exhibited a redder optical slope as it approached the Sun, indicating dust-rich composition.

Abstract

Dynamically new comet C/2011 L4 (PanSTARRS) is one of the brightest comets since the great comet C/1995 O1 (Hale-Bopp). Here, we present our multi-wavelength observations of C/2011 L4 during its in-bound passage to the inner Solar system. A strong absorption band of water ice at 2.0 $\mu$m was detected in the near infrared spectra, taken with the 8-m Gemini-North and 3-m IRTF telescopes. The companion 1.5 $\mu$m band of water ice, however, was not observed. Spectral modeling show that the absence of the 1.5 $\mu$m feature can be explained by the presence of sub-micron-sized fine ice grains. No gas lines (i.e. CN, HCN or CO) were observed pre-perihelion either in the optical or in the sub-millimeter. 3-$\sigma$ upper limits to the CN and CO production rates were derived. The comet exhibited a very strong continuum in the optical and its slope seemed to become redder as the comet approached the Sun. Our observations suggest that C/2011 L4 is an unusually dust-rich comet with a dust-to-gas mass ratio $>$ 4.