The Helium and Heavy Elements Enrichment of the Galactic Disk

Abstract

We present chemical evolution models for the Galactic disk. We also present a new determination of X, Y, and Z for M17 a Galactic metal-rich Hii region. We compare our models for the Galactic disk with the Galactic Hii regions abundances. The $\Delta Y/\Delta O$ ratio predicted from the galactic chemical evolution model is in very good agreement with the $\Delta Y/\Delta O$ value derived from M17 and the primordial helium abundance, Yp, taking into account the presence of temperature variations in this Hii region. From the M17 observations we obtain that $\Delta Y/\Delta Z = 1.97 \pm 0.41$, in excellent agreement with two $\Delta Y/\Delta Z$ determinations derived from K dwarf stars of the solar vicinity that amount to $2.1 \pm 0.4$ and $2.1 \pm 0.9$ respectively. We also compare our models with the solar abundances. The solar and Orion nebula O/H values are in good agreement with our chemical evolution model.