2013ApJ...771L..21F

On 2013 March 21, high-resolution slit spectrographs of the comet C/2011 L4 (Panstarrs), at a heliocentric distance r = 0.46 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 potassium lines were also detected. These have rarely been observed in comets since the apparition of the 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 the abundance ratios Na/K = 54±14 and Na/Li ≥ 810³. In addition to Mercury's exosphere (Leblanc & Doressoundiram), we show that photoionization at the beginning of the alkali tails may increase the solar ratio Na/K = 15.5 (Asplund et al.) by a factor three, close to that required to match the observed value. In the same tail position, the Na/Li ratio increases only by a factor two, very far from the factor ≥ 8 required to match an original meteoritic ratio. We apply the same model to similar alkali data (Preston) of the comet C/1965 S1 (Ikeya-Seki) and obtain 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.