Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany
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Preprint 04/06

SDSS J212531.92-010745.9 - the first definite PG1159 close binary system

T. Nagel (1), S. Schuh (2), D.-J. Kusterer (1), T. Stahn (2), S.D. Hügelmeyer (2), S. Dreizler (2), B.T. Gänsicke (3), M.R. Schreiber (4)

(1) Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany
(2) Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
(3) Department of Physics, University of Warwick, Coventry, CV4 7AL, Great Britain
(4) Departamento de Fisica y Meteorologia, Facultad de Ciencias, Universidad de Valparaiso, Valparaiso, Chile

A&A 448, L25 (2006)

Abstract. The archival spectrum of SDSS J212531.92-010745.9 shows not only the typical signature of a PG 1159 star, but also indicates the presence of a companion. Our aim was the proof of the binary nature of this object and the determination of its orbital period.
We performed time-series photometry of SDSS J212531.92-010745.9. We observed the object during 10 nights, spread over one month, with the Tübingen 80 cm and the Göttingen 50 cm telescopes. We fitted the observed light curve with a sine and simulated the light curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSS J212531.92-010745.9 with NLTE models, the results of which also constrain the light curve solution.
An orbital period of 6.95616(33) h with an amplitude of 0.354(3) mag is derived from our observations. A pulsation period could not be detected. For the PG 1159 star we found, as preliminary results from comparison with our NLTE models, T_eff ~ 90 000 K, log g ~ 7.60, and the abundance ratio C/He ~ 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4±0.1 R_sun, a mass of 0.4±0.1 M_sun, and a temperature of 8 200 K on the irradiated side, good agreement between the observed light curve and the nightfall simulation, but we do not regard those values as final.
 
Key words: white dwarfs -- PG1159 stars -- binary systems

Preprint (415 kb PDF file including figures)
Astrophysics (astro-ph): astro-ph/0601512

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