ON THE EVOLUTIONARY LINK BETWEEN WHITE-DWARFS AND CENTRAL STARS OF PLANETARY-NEBULAE - NLTE ANALYSIS OF PG 1144+005

New high-S/N optical spectra of the hot hydrogen deficient pre-white dwarf PG 1144+005, a member of the PG 1159 class, are analyzed by means of line blanketed non-LTE model atmospheres. Its spectrum is unique amongst the PG 1159 stars since it exhibits strong nitrogen (N v) emission features not detected in any other PG 1159 star. The spectral analysis performed differentially with respect to the prototype PG 1159-035 results in T(eff) = 150 000 K, log g = 6.5, C/He = 0.5, O/He = 0.01 and N/He = 0.01 (number ratios). While its temperature, gravity, and carbon abundance are quite similar to PG 1159-035, the N/O ratio is more than hundred times larger than in the latter. The surface composition of the PG 1159 stars is interpreted as the helium buffer layer of a thermally pulsing double shell burning progenitor star. The "born-again post-AGB" scenario of Iben et al. (1983) can explain how these deep layers become visible at the stellar surface during the evolution. However, the high N abundance of PG 1144+005 seems to be at variance with this scenario, since N is rapidly destroyed by the 3-alpha process and should not be present in the helium buffer layer. Iben et al.(1983) have pointed at a possible mechanism how N might be preserved or even enriched if convection during a late thermal pulse mixes protons into deeper layers where they are burnt via the C-12(p,gamma)N-13(e+,nu)C-13 reaction. The PG 1159 stars allow the transition from the hydrogen deficient central stars of planetary nebulae (CSPN) to the DO white dwarfs to be studied. As amongst the PG 1159 stars, nitrogen-rich stars are rare amongst the H deficient CSPN. The link to the CSPN is discussed in detail from spectroscopic comparison, for which new optical spectra of the CSPN of K 1-16 and Abell 78 are presented. Two evolutionary sequences are proposed: The predominant (nitrogen-poor) sequence WC-CSPN - O(C)-CSPN - PG 1159 stars and a nitrogen-rich sequence which links the WCN-CSPN NGC 6751 and Abell 30, Abell 78 to PG 1144+005. These sequences might, however, not be separate since ongoing mass loss might remove the N-rich layers during the course of evolution. We also comment on the non-radial pulsations observed in many PG 1159 stars but not in PG 1144+005. The pulsations are believed to be driven by cyclic ionisation of C and O. The O abundance of PG 1144+005 is considerably lower than in PG 1159-035 and might be insufficient to drive pulsations.