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We present new measurements of the abundances of carbon and oxygen derivedfrom high-excitation C I and O I absorption lines in metal-poor halo stars,with the aim of clarifying the main sources of these two elements in the earlystages of the chemical enrichment of the Galaxy. We target 15 new starscompared to our previous study, with an emphasis on additional C/Odeterminations in the crucial metallicity range -3<[Fe/H]<-2. Departures fromlocal thermodynamic equilibrium were accounted for in the line formation forboth carbon and oxygen. The non-LTE effects are very strong at the lowestmetallicities but, contrary to what has sometimes been assumed in the past dueto a simplified assessment, of different degrees for the two elements. Inaddition, for the 28 stars with [Fe/H]<-1 previously analysed, stellarparameters were re-derived and non-LTE corrections applied in the same fashionas for the rest of our sample, giving consistent abundances for 43 halo starsin total. The new observations and non-LTE calculations strengthen previoussuggestions of an upturn in C/O towards lower metallicity (particularly for[O/H]<-2). Adopting the H collisional cross-sections estimated from theclassical Drawin formula leads to [C/O]~0 at [O/H]~-3. To remove the upturn inC/O, near-LTE formation for O I lines would be required, which could onlyhappen if the H collisional efficiency with the Drawin recipe is underestimatedby factors of up to several tens of times, which we consider unlikely. The highC/O values derived at the lowest metallicities may be revealing thefingerprints of Population III stars or may signal rotationally-aidednucleosynthesis in more normal Population II stars.

The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars