Are He and N Abundances in Type I Planetary Nebulae as High as Empirically Derived?

Abstract

Type I planetary nebulae (PNs) are defined as those with high He and N abundances. These objects present in general bipolar geometries and have high stellar temperatures. In this paper we analyze the empirical methods for abundance determination in order to check if the He and N overabundances in type I PNs are a consequence of a geometrical effect due to the bipolarity or the ionization stratification due to the stellar temperature. For this, we obtain simulated, spherically symmetrical nebulae, as well as bipolar nebulae, using a three-dimensional photoionization code. From the projected emission line intensities for (a) the whole nebula, (b) for a slit crossing the nebula, as well as (c) for different positions in the nebula, we applied the formulae used in the literature to obtain empirical abundances. These empirical abundances are then compared with the adopted ones. We show that empirical abundances depend on the particular line of sight covered by the observation and can simulate an overabundance and/or the presence of abundance gradients of He and N in planetary nebulae with high stellar temperature. The geometrical effects are also discussed. Systematic errors in abundance determinations by empirical methods are higher for the N/H ratio than for N/O. Thus, it seems better to use the N/O value when discussing N-rich objects.