N/O abundance ratio and the progenitor mass for the most luminous planetary nebulae of M 31

Abstract

Context. Plasma diagnostics form the basis of investigation into the physico-chemical properties of line-emitting gaseous systems. Aims. To perform plasma diagnostics properly, it is essential to accurately correct the input spectrum for extinction because determining the degree of extinction is dependent on the physical properties of the line-emitting gas. Hence, both extinction correction and plasma diagnostics have to be performed simultaneously and self-consistently. Methods. By comparing the results of analyses performed for a sample of nine bright planetary nebulae in M 31, both with and without the fully iterative self-consistent simultaneous extinction correction and plasma diagnostics, we demonstrate how a seemingly benign initial assumption of the physical conditions of the line-emitting gas in extinction correction could compromise the results of the entire analyses in terms of the extinction, electron density and temperature, and ionic and elemental abundances. Results. While the electron density and temperature are relatively immune to the imposed inconsistent assumptions, the compromised extinction would cause systematic offsets in the extinction-corrected spectrum, and hence, line strengths, which consequently would impose adverse effects on the resulting ionic and elemental abundances, and other inferences made from the incorrect results. Conclusions. We find that this M 31 PN sample simply represents those around the high-mass end of the mass range for low-mass planetary nebula progenitor stars as expected from the existing theoretical models. It appears that the suspicion raised in the previous study – that these PNe could be anomalously nitrogen-overabundant for the expected progenitor mass range – is simply caused by the apparent underestimation in extinction that originates from the imposed inconsistent assumptions in extinction correction. In a larger context, the results of plasma diagnostics in the literature that are done without seeking simultaneous self-consistency with extinction corrections have to be handled with caution. Ideally, such previous results should be re-evaluated by always seeking simultaneous self-consistency.