On the evolutionary link between white dwarfs and central stars of planetary nebulae : NLTE analysis of PG 1144+005.

Abstract

Line-blanketed non-LTE (NLTE) model atmospheres are used to study the optical spectra of the hydrogen-deficient prewhite dwarf PG 1144 + 005 by means of a spectral analysis. The star is compared to a prototype PG 1159 star for the analysis, and the stellar parameters of the PG 1144 dwarf show that the N/O ratio is indicative of very strong nitrogen emission. The high N/O ratio suggests that N is not present in the helium-buffer layer, meaning that PG 1144 does not fit the scenario proposed by Iben et al. (1983). Based on a spectroscopic comparison of central stars of planetary nebulae (CSPN), the existence of N-rich stars is explained in relation to two evolutionary sequences. The sequences consider either N-rich or N-poor links, and it is suggested that the sequences can be linked because N-rich layers can be removed by continuing mass loss during the evolutionary process. Abundance determinations of N-rich objects are required for future clarification of evolutionary stages in these stars.