The Abundance of Lithium in Early M-Type Stars

A spectroscopic survey has been made of early M-type stars at 38 and 88 A/mm with the grating spectrograph of the Lick 36-inch refractor, to search for stars possessing a Li I line at 6707. For 58 early M-type giants and supergiants high-dispersion spectra (4, 8, and 16 A/mm) were obtained with the coude' spectrograph of the 120-inch reflector The abundance of Li relative to Ca was determined using a fairly standard curve-of-growth technique. This abundance ratio, Li/Ca, ranges in M stars from about 2 to 10 times the Greenstein-Richardson solar value On the basis of these results and Iben's theory of post-main-sequence dilution of Li, it is not necessary to postulate that Li is formed on the surfaces of evolved stars. Li abundances are now available for giant stars over the spectral type interval F3-M4; there is a steady decrease of Li abundance with advancing spectral type for these giants, which is thought to be due to increasing convective dilution as a star evolves to the right in the H-R diagram. The concentrations of the molecules LiH, LiO, and LiOH were evaluated and found to be negligible relative to atomic Li. A comparison of the resonance lines of Li I and Na I showed no correlation, but the equivalent widths of the D-lines were found to range over a factor of 2 for a given spectral type and luminosity class A tendency was found for the Li-rich stars to be high-velocity objects. Only four stars show more Li than the Sun; there is no difficulty understanding an excess over the solar value on the basis of an initial T Tauri-like abundance and post-main-sequence convective dilution of Li.