Potassium detection and Lithium depletion in comets C/2011 L4 (Panstarrs) and C/1965 S1 (Ikeya-Seki)

TL;DR

This study analyzes alkali metal emissions in comets C/2011 L4 and C/1965 S1, revealing potassium detection, lithium depletion, and the effects of photoionization on alkali ratios in comet tails.

Contribution

It provides the first detection of potassium in comet C/2011 L4 and models the influence of photoionization on alkali metal ratios, highlighting lithium depletion in comets.

Findings

Potassium was detected in comet C/2011 L4, rare in comets.

Lithium was not detected, with strong upper limits on its abundance.

Photoionization can explain the observed Na/K ratio increase.

Abstract

On 21 March 2013 high-resolution slit spectrographs of comet C/2011 L4 (Panstarrs), at the heliocentric distance r = 0.46 Astronomical Units (AU), were obtained at the Osservatorio Astronomico Campo dei Fiori, Italy. Emission lines of sodium were the strongest in the spectrum as is common in comets, but also potassium lines were detected. These have been rarely observed in comets since the apparition of brightest comet C/1965 S1 (Ikeya-Seki). Lithium was not detected, and stringent upper limits of its abundance compared to other alkali were derived. We obtain abundance ratios Na/K = 54 +/-14 and Na/Li > 8000. As well as in Mercury exosphere (Leblanc & Douressoundiram 2011)}, we show that photoionization at the beginning of the alkali tails may increase the solar ratio Na/K = 15.5 (Asplund et al 2009) by a factor 3, close to that required to match the observed value. In the same tail position the Na/Li ratio increases by a factor 2 only, very far from the factor > 8 required to match an original meteoritic ratio. We apply the same model to similar alkali data (Preston 1967) of comet C/1965 S1 (Ikeya-Seki), obtaining consistent results. An original solar Na/K ratio fits the observed value at the beginning of the alkali tails within the slit size, whereas Li is depleted by a factor > 8.