Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4

¹ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: shuegelm@astro.physik.uni-goettingen.de
² Apache Point Observatory, New Mexico State University, 2001 Apache Point Road, PO Box 59, Sunspot, NM 88349, USA
³ Mt. Suhora Observatory, Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland
⁴ Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Gemini Observatory, 670 N. A'Ohoku Place, Hilo, HI 96720, USA
⁶ Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A'Ohoku Place, Hilo, HI 96720, USA

Received: 17 January 2006
Accepted: 10 April 2006

Abstract

Context.The Sloan Digital Sky Survey Data Release 4 has provided spectra of several new PG 1159 stars and DO white dwarfs. This increase in known hot H-deficient compact objects significantly improves the statistics and helps to investigate late stages of stellar evolution.

Aims.From the optical SDSS spectra, effective temperatures and surface gravities are derived in order to place the observed objects in an evolutionary context. Especially the connection between PG 1159 stars and DO white dwarfs shall be investigated.

Methods.Using our non-LTE model atmospheres and applying -fitting techniques, we determine stellar parameters and their errors. We derive total stellar masses for the DO white dwarfs using model evolutionary tracks.

Results.We confirm three PG 1159 stars, with one showing ultra-high excitation ion features, and one sdO which we originally classified as a PG 1159 star. Additionally, we re-analysed the known PG 1159 star, PG 1424+535, with our new models. Furthermore, we present the first spectral analyses of thirteen DO white dwarfs, three of which show M-star features in their spectra, while two display ultra-high excitation ion features.