Interstellar Lithium: Differential Depletion in Diffuse Clouds

Echelle-CCD observations of interstellar Li I (6708-A) have yielded detections toward seven stars and limits toward 15 others. Examination of all existing data on interstellar Li absorption reveals little variations in the Li/K abundance ratio, but significant variation in the Li/Na ratio. Individual depletion values derived from lines of the trace neutral atoms Li, C, Na, and K correlate with one another and with electron density. These elements exhibit differential depletion in the densest, most shielded regions of diffuse clouds, with Li and K showing a smaller range of variation than C or Na. Considered as diagnostics of the grain-mantle accretion process, the observed correlations yield ratios of the effective sticking probabilities for the different atoms. The expectation that N and K should behave similarly then leads to the conclusion that the atoms accrete as ions rather than as neutrals. The resulting effective sticking coefficient for C is slightly smaller than for Na and K; that for Li must be several times smaller. The correlations further suggest that the background depletions for Li and K may be about one-half those for Na and C, and that C depletes roughly as n to the -2/3, where n is the density of H nuclei. The extreme depletions and low H2 fraction in the Rho Oph cloud imply a low H2 formation rate, significantly smaller than 10 to the -17th cu cm.