3. LITHIUM 7

3.1. Lithium in the galaxy

⁷Li is readily detected spectroscopically from the resonance doublet of LiI at 6707.8 Å and has been found in hundreds of stars cooler than effective temperature 7500 K (spectral type F0) and in the ISM as well as in meteorites where its abundance is equivalent to Li/H = 2 × 10^-9. A similar abundance is found in the photospheres of young stars (if hotter than about 5500 K, spectral type G6), but in older stars lithium is depleted due to mixing or dilution with deeper layers where ⁷Li is destroyed by the (p, ) reaction at temperatures above 2.5 × 10⁶ K. Because cooler stars have deeper subphotospheric convection zones, they deplete their surface lithium faster so that, in the young Pleiades cluster (age ~ 10⁸ yrs), Li has its normal abundance down to effective temperature 5500 K, whereas in the older Hyades cluster (~ 10⁹ yrs) depletion sets in and intensifies steadily below 6300 K (type F7). In the Hyades and NGC 752, furthermore, there is also a high degree of depletion in a narrow range of effective temperatures around 6600 K (Boesgaard & Trippico 1986; Boesgaard & Budge 1988; Hobbs & Pilachowski 1988) near the region of pulsational instability, possibly caused by mass loss; the amount of mass actually lost has to be quite small because beryllium is not significantly depleted in these stars (Schramm, Steigman & Dearborn 1990). In cooler stars, there is evidence that the depletion depends on other factors besides mass and age, since the degree of depletion in the solar photosphere, which is a factor 100 or so, is greater than in some stars of similar effective temperature in the still older galactic cluster NGC 188 (Hobbs & Pilachowski 1988; Hobbs, Iben & Pilachowski 1989); such factors may include rotationally driven meridional circulation, magnetic effects etc. Thus ⁷Li is subject to destruction by astration, but it is certainly synthesised in a subset of carbon stars where it is observed to be superabundant (Smith & Lambert 1991) and possibly in novae and there is a minor contribution due to cosmic-ray spallation; the net effect of all these processes is quite likely to lead to an increase in ⁷Li abundance in the ISM as a result of stellar activity (see fig. 2).