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Published online by Cambridge University Press: 30 March 2016
The abundance of the elements in the halo are mostly known from analysis of stellar atmospheres. Data from evolved stars and planetary nebulae have to be used with some caution. I will therefore rely primarily on unevolved cool dwarfs, using other information only as a complement.
I will designate these unevolved cool dwarfs indifferently by the words halo dwarfs or Population II dwarfs; such stars were formerly often called subdwarfs for historical reasons. Remaining in the field of vocabulary, I will call underdeficient an element which is deficient, but by a factor smaller than the general deficiency factor for the other elements. Finally, let us recall the classical notation: