Production of Lithium in the Galactic Disk

Abstract

The abundance of Li in stars formed within the past 5 Gyr is logN(Li) = 3.2(±0.2),while the corresponding value for the oldest stars in the Galaxy is logN(Li) = 2.2(±0.2).The global evidence suggests that the latter represents the full, or the major part of the primordial abundance, so that the difference of an order of magnitude is due to Li produced in the Galaxy. It is well known that spallation of insterstellar CNO by 4He and protons in galactic cosmic rays (GCR) can produce Li,but models yield a shortfall of almost an order of magnitude compared with the current observed abundance range.Another GCR reaction, α+α fusion has been invoked to explain some Li production in the early Galaxy,but application of this to the disk yielded too much early Li or too little current Li.These failures led to a search for alternative mechanisms,essentially stellar, at particular phases of evolution:the helium flash phase in AGB stars,in novae,and during supernova. Here we stress the importance of the observed upper envelope in the plot of Li v. Fe in stars as a constraint on any mechanism