High resolution UV spectroscopy of two hot (pre-) white dwarfs with the Hubble Space Telescope

K. Werner1,2, S.Dreizler1,3, U.Heber3, T.Rauch1, T.A. Fleming4, E.M. Sion5, G.Vauclair6

1Institut für Astronomie und Astrophysik der Universität Kiel, D-24098 Kiel, Germany
2Institut für Theoretische Physik und Astrophysik der Universität Potsdam, Am Neuen Palais 10, D-14469 Potsdam
3Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, D-96049 Bamberg, Germany
4Steward Observatory, University of Arizona, Tucson, AZ 85721, U.S.A.
5Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085, U.S.A.
6Observatoire Midi-Pyrenees, Laboratorie d'Astrophysique CNRS/URA285, 14 avenue E. Belin, F-31400 Toulouse, France

High resolution UV spectra of KPD 0005+5106 and RXJ 2117+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 KPD 0005+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 +35kms-1 we find a distinct interstellar/circumstellar absorption line system shifted by -15kms-1.

Difficulties are encountered with line profile fits to the spectrum of RXJ 2117+3412, an extremely hot hydrogen-deficient (PG 1159-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 1371Å line is detected which suggests, that the effective temperature is higher than thought before (Teff=170000K). This means that RXJ 2117+3412 is the hottest known PG 1159 star together with H1504+65. Interstellar and photospheric line spectra are clearly separated, with shifts of -19 and +23 kms-1, respectively.

Both stars exhibit a strong O VIII emission line near 2977Å, which cannot be of photospheric origin but probably arises from shock fronts along a stellar wind. The line widths are of the order of 150kms-1. In addition, KPD 0005+5106 shows another, equally strong emission feature near 2982Å which either stems from photospheric N V or from non-photospheric C V.

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