The computation of interstellar extinction in photoionized nebulae

Abstract

Context. The proper plasma analysis practice (PPAP) method has been proposed to analyze spectroscopic data of ionized nebulae. The method is based on a coherent and simultaneous determination of the reddening correction and physical conditions in nebulae. The PPAP method’s creators reanalyzed the results of nine of the brightest planetary nebulae in M31. They claim that, if standard values of the physical conditions are used to compute the extinction instead of their proposed method, extinction correction is underestimated by more than 50%, and hence, ionic and elemental abundance determinations, especially the N/O ratio, are incorrect. Aims. The discrepancies between the two methods are investigated. Methods. Several tests were performed to assess the accuracy of the non-PPAP results when determining: (i) the interstellar extinction coefficient, (ii) the plasma electron temperature and density, and (iii) the ionic abundances, in particular of singly ionized nitrogen. In the last case, the ionic N+/H+ abundance was recalculated using both Hα and Hβ as the reference H I emissivity. Results. The analysis shows that the errors introduced by adopting standard values of the plasma conditions are small, within the uncertainties. On the other hand, the interstellar extinction determined using the PPAP method (uncritically making use of all available Paschen and Balmer lines, without considering observational, random and/or systematic effects) is found to be overestimated for five of the nine nebulae considered. This has consequences for the subsequent analysis of the physical and chemical properties of the nebulae and their progenitors. The python notebook used to generate all the results presented in this Letter is publicly available at a GitHub repository. Conclusions. The initial, non-PPAP results and conclusions are proven valid. Although the PPAP method is, in principle, a recommended practice, we insist that it is equally important to critically assess which H I lines are included in the determination of the interstellar extinction coefficient, and to make sure that physical results are obtained for the non-de-reddened line ratios.