ULTRAVIOLET AND OPTICAL-SPECTRA OF CENTRAL STARS OF HALO PLANETARY-NEBULAE

Abstract

Optical and UV spectrophotometric data of the central stars of eight Population II planetary nebulae are analyzed. From these data, we derive visual magnitudes, spectral classification, color temperatures and luminosities. All the stars in the sample show absorption type spectrum and most of them have normal H and He photospheric abundances, the only possible exceptions are M 2-29 and GJJC-1 which seem to be H-deficient. The visual magnitudes, derived from UV stellar continuum, are in good agreement with published fluxes at lambda5480. In the case of NGC 2242 we measured a value V = 17.1 mag (from the lambda5480 flux, wide band photometry and the UV extrapolation) instead of the 15.02 mag reported previously

this difference implies a distance of 6 +/- 1 kpc, 2 kpc larger than calculated by Torres-Peimbert et al. A&A 233, 540 (1990). The color temperatures derived for these objects range from less than 40 000 K to more than 80 000 K and, in general, they are consistent with effective temperatures calculated by other means. The UV continuum of the central star of NGC 4361 is steeper than expected from the effective temperature determined from modelling H and He absorption lines. From comparison with theoretical evolutionary tracks for nuclei of PNe we found that the stellar masses of these objects spread over a narrow range, from 0.55 to 0.57M.. The average mass is lower than the average masses derived for large samples of disk and bulge planetary nebulae (0.61M. and 0.59M. respectively).