ΔY/ΔZ — No Real Controversy Between Theory and Observations

Abstract

For several years a controversy has been claimed to exist between observational determinations of ΔY/ΔZ and theoretically computed values from nucleo-synthesis in stellar models. More specifically, since neither observational nor theoretical estimates are very numerous, the essentially controversial results are the value ΔY/ΔZ ~ 6 obtained by Pagel et al. (1992) from observations of emission lines in HII galaxies and the theoretical results following from the extensive stellar model grids by Maeder et al.: ΔY/ΔZ = 1.1 ÷ 1.8 for metal-deficient systems Z = 0.001 or ΔY/ΔZ = 1.5 ÷ 2.2 when Z = 0.02 (Maeder 1992). Still another series of observational ΔY/ΔZ determinations, published by Peimbert et al. over the years, converges to a value ΔY/ΔZ = 2.4 ÷ 2.5 (e.g. the latest publications: Peimbert (1993) — ΔY/ΔZ =2.5, Torres-Peimbert et al. (1994) — ΔY/ΔZ =2.41). This value is based on selected uncontaminated HII regions in the Galaxy and nearby dwarfs with the correction of abundances from collisionally excited lines for temperature variations inside nebulae. It also agrees well with the Sun’s value of ΔY/ΔZ ~ 2.3.