What planetary nebulae tell us about helium and the CNO elements in Galactic bulge stars

Abstract

Thermally pulsing asymptotic giant branch (TP-AGB) models of bulge stars are calculated using a synthetic model. The goal is to infer typical progenitor masses and compositions by reproducing the typical chemical composition and central star masses of planetary nebulae (PNe) in the Galactic bulge. The AGB tip luminosity and the observation that the observed lack of bright carbon stars in the bulge are matched by the models. Five sets of galactic bulge PNe were analyzed to find typical abundances and central star of planetary nebulae (CSPN) masses. These global parameters were matched by the AGB models. These sets are shown to be consistent with the most massive CSPN having the largest abundances of helium and heavy elements. The CSPN masses of the most helium rich (He/H$\ga$0.130 or $Y\ga0.34$) PNe are estimated to be between 0.58 and 0.62$ {\rm M}_{\sun}$. The oxygen abundance in form $\log{\rm (O/H)}+12$ of these highest mass CSPN is estimated to be $\approx$8.85. TP-AGB models with ZAMS masses between 1.2 and 1.8$ {\rm M}_{\sun}$ with $Y_{\rm ZAMS}\approx0.31-0.33$ and $Z_{\rm ZAMS}\approx0.19-0.22$ fit the typical global parameters, mass, and abundances of the highest mass CSPN. The inferred ZAMS helium abundance of the most metal enriched stars implies $dY/dZ\sim4$ for the Galactic bulge. These models produce no bright carbon stars in agreement with observations of the bulge. These models produce an AGB tip luminosity for the bulge in agreement with the observations. These models suggest the youngest main sequence stars in the Galactic bulge have enhanced helium abundance ($Y\approx0.32$) on the main sequence and their ages are between 2 and 4 Gyrs.