Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification.The 12C/13C ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. It is crucial, however, to verify whether internal mixing has modified their surface. The stellar N abundances and ratios are compatible with those found in Damped Lyman-alpha (DLA) systems.Įxtremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. At present it cannot be decided whether primary N is produced primarily in SNe II or in massive AGB stars, or in both. Alternatively, by analogy with the DLA's, the lower abundances (= -1.1, = -0.7) could reflect generally low yields from the first Sne II, the other stars being N enhanced by winds of massive Asymptotic Giant Branch (AGB) stars. The larger values of these ratios define a flat upper plateau (= 0.0, = +0.1), which could reflect higher values within a wide range of yields of zero-metal Sne II. These 'unmixed' stars reflect the abundances in the early Galaxy: the ratio is constant (about +0.2 dex) and the ratio is close to solar at low metallicity, favouring a high C production by massive zero-metal supernovae. The second group shows no evidence for C to N conversion, and Li is only moderately diluted, and we conclude that their C and N abundances are very close to those of the gas from which they formed in the early Galaxy. The first group shows evidence of C to N conversion through CN cycling and strong Li dilution, a signature of mixing. Our sample shows a clear dichotomy between two groups of stars. Because any dredge-up of CNO processed material to the surface may complicate the interpretation of CNO abundances in giants, we have also measured the surface abundance of lithium. We have investigated the poorly-understood origin of nitrogen in the early Galaxy by determining N abundances in 35 extremely metal-poor halo giants (22 stars have <-3.0) using the C and O abundances determined in Paper V.

CN CNO equilibrium abundance dredge up