1996A&A...307..761P

We present high resolution observations (FWHM=15 km/s) of the lithium region in 6 stars belonging to the old globular cluster NGC 6397. Three stars are located at the turn-off with T_eff on the Spite plateau. All three show the lithium line. Of the three other stars (all of which have evolved from the main sequence) two show depleted lithium and the third, most luminous one, has only an upper limit. Abundances are derived by using model atmospheres generated with the Atlas 9 code (Kurucz 1993) with α enhancement and no-overshooting. The average lithium abundance of the three turnoff stars is 2.28±0.10. For one of these we confirm the detection of Molaro and Pasquini (1994). This value is fully consistent with the Li plateau of field population II stars, obtained with the same set of models. Thus, the present results confirm that this 14Gyrs old globular cluster and the field population II share the same lithium abundance. Furthermore, within the errors, no spread is observed in the Li abundance of the turnoff stars. As NGC 6397 is one of the oldest clusters known, the observed Li abundance strongly supports a cosmological origin for lithium, leaving little room for alternative sources. In two subgiants with T_eff ∼5100K and ∼5600K that are respectively 2 and 0.5 magnitudes brighter than turnoff, we have measured Li abundance of 1.81 and 1.2. However, for the cooler and most luminous star of the sample only an upper limit could be derived. This drop in Li abundance is not predicted by any of the existing models.