SDSS J212531.92–010745.9 – the first definite PG 1159 close binary system

Free access article

Issue		A&A Volume 448, Number 2, March III 2006


Page(s)		L25 - L28
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200600009

A&A 448, L25-L28 (2006)
DOI: 10.1051/0004-6361:200600009

Letter

SDSS J212531.92-010745.9 - the first definite PG 1159 close binary system

T. Nagel¹, S. Schuh², D.-J. Kusterer¹, T. Stahn², S. D. Hügelmeyer², S. Dreizler², B. T. Gänsicke³ and M. R. Schreiber⁴

¹ Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: nagel@astro.uni-tuebingen.de
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

(Received 14 December 2005 / Accepted 19 January 2006 )

Abstract
Aims.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.
Methods.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.
Results.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, $T_{\rm eff}$ ~ 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 $\pm$ $0.1~R_\odot$ , a mass of 0.4 $\pm$ $0.1~M_\odot$ , and a temperature of 8200 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: stars: AGB and post-AGB -- white dwarfs -- binaries: close

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