Production of Lithium in the Galactic Disk

Abstract

We examine the evolution of Li in the Galactic disk,basing our model on the compilation of Li v.Fe observations in stars of iron abundance ranging between halo values,which give a baseline for the disk Li abundance,and 0.2 dex above solar.The upper envelope of these observations is taken to represent the locus of minimum stellar Li depletion,and thus to follow the Galactic IS Li abundance.The most striking feature of this envelope is its steep rise between Fe/H ~ -0.4 and -0.2,corresponding to a relatively late epoch.The Li abundance increases by an order of magnitude,from its halo value of logLi ~2.2 to the "current" value of ~3.2 within this narrow range of Fe abundances.It is well known that spallation reactions between CNO and 4He in the ISM produce Li,but models give a nearly tenfold shortfall for this process.The alpha+alpha fusion reaction appeared to yield too much early Li or too little current Li.These failures led to the exploration of stellar sources:supernovae,novae,and the He flash phase in AGB stars.The Li-Fe envelope is a strong constraint on any process.Our models can account for this in the context of disk chemical evolution with a near constant or growing rate of low metallicity gas infall,and a purely interstellar Li source: alpha-alpha fusion.The Li production rate is found to be proportional to the gas expulsion rate from intermediate and low mass stars.Low-energy alphas,emitted by these stars,but accelerated in more energetic processes associated with SNe,or the bow-shocks of stellar winds,can yield the observed abundance and its variation with Fe or O.Our model is consistent with the cosmic ray spectrum in the few MeV range,corrected for solar modulation and with new 7Li/6Li ratios found by Knauth et al.(2000) in the local ISM.