LITHIUM ABUNDANCE IN THE GLOBULAR-CLUSTER NGC-6397

We present high resolution observations (FWHM=15 km s(-1)) of the lith ium 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 platea u. All three show the lithium line. Of the three other stars (all of w hich have evolved from the main sequence) two show depleted lithium an d the third, most luminous one, has only an upper limit. Abundances ar e derived by using model atmospheres generated with the Atlas 9 code ( Kurucz 1993) with alpha enhancement and no-overshooting. The average l ithium abundance of the three turnoff stars is 2.28+/-0.10. For one of these we con firm the detection of Molaro and Pasquini (1994). This v alue is fully consistent with the Li plateau of field population II st ars, obtained with the same set of models. Thus, the present results c onfirm that this 14 Gyrs 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 s trongly supports a cosmological origin for lithium, leaving little roo m for alternative sources. In two subgiants with T-eff similar to 5100 K and similar to 5600 K that are respectively 2 and 0.5 magnitudes br ighter than turnoff, we have measured Li abundance of 1.81 and 1.2. Ho wever, for the cooler and most luminous star of the sample only an upp er limit could be derived. This drop in Li abundance is not predicted by any of the existing models.