Helium contamination from the progenitor stars of planetary nebulae: The He/H radial gradient and the ΔY / ΔZ enrichment ratio

In this work, two aspects of the chemical evolution of ⁴He in the Galaxy are considered on the basis of a sample of disk planetary nebulae (PN). First, an application of corrections owing to the contamination of ⁴He from the evolution of the progenitor stars shows that the He/H abundance by number of atoms is reduced by 0.012 to 0.015 in average, leading to an essentially flat He/H radial distribution. Second, a determination of the helium to heavy element enrichment ratio using the same corrections leads to values in the range 2.8 < ΔY / ΔZ < 3.6 for Y _p = 0.23 and 2.0 < ΔY / ΔZ < 2.8 for Y _p = 0.24, in good agreement with recent independent determinations and theoretical models. This revised version was published online in July 2006 with corrections to the Cover Date.