High resolution UV spectroscopy of two hot (pre-) white dwarfs with the Hubble Space Telescope. KPD0005+5106 and RXJ2117+3412.

Abstract

High resolution UV spectra of KPD0005+5106 and RXJ2117+3412 have been obtained with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. The data were subject to a non-LTE model atmosphere analysis. For KPD0005+5106, the hottest known helium-rich white dwarf, the C and N abundances as well as an upper limit for O have been determined (log(C/He)=-3, log(N/He)=-6, log(O/He)<-5.5, by number), improving results from previous studies. These abundances are severely at odds with diffusion/radiative forces theory. They are possible primordial relics of their ancestral progenitor in the post-AGB phase. Besides the photospheric lines with a velocity shift of +35km/s we find a distinct interstellar/circumstellar absorption line system shifted by -15km/s. Difficulties are encountered with line profile fits to the spectrum of RXJ2117+3412, an extremely hot hydrogen-deficient (PG1159-type) central star. Some of the strong C IV and O VI lines are too narrow to match the observations. Either the neglect of wind-effects in our models or the lack of reliable line broadening data may be responsible for this. A weak O V 1371A line is detected which suggests, that the effective temperature is higher than thought before (T_eff_=170000K). This means that RXJ2117+3412 is the hottest known PG1159 star together with H1504+65. Interstellar and photospheric line spectra are clearly separated, with shifts of -19 and +23km/s, respectively. Both stars exhibit a strong O VIII emission line near 2977A, which cannot be of photospheric origin but probably arises from shock fronts along a stellar wind. The line widths are of the order of 150km/s. In addition, KPD0005+5106 shows another, equally strong emission feature near 2982A which either stems from photospheric N V or from non-photospheric C V.