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

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 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 , a mass of 0.4 ± 0.1 M , 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.

A new period determination for the close PG1159 binary SDSS J212531.92–010745.9

“…2, we compare the folded profile to the sinusoidal amplitude of 0.299(33) relative intensity change. Clearly, the observed peak-to-peak variation is not fully reproduced by the fit, owing to the broader trough and the narrow maximum already described by Nagel et al (2006), who fitted magnitude changes, while we now fit intensity changes. For a higher fit amplitude and/or raised zero line, the observed Nagel et al (2006) still suffers from a degeneracy in mass and radius for the PG 1159 component, this light curve shape emphasizes the superiority of their more realistic model over a plain sine fit and validates their interpretation of the observed brightening as a reflection effect.…”

“…Following up the light curve of SDSS J212531.92-010745.9 presented by Nagel et al (2006), we have obtained new observations with the Tübingen 80 cm and the Göttingen 50 cm telescopes during 19 nights between September and November 2006. The full data set now available is listed in Table 1 with details of the set-up.…”

“…These methods however rely on stellar structure and evolution modelling, and can so far not be tested independently. Nagel et al (2006) have recently announced the unique discovery of a PG 1159 star in a close binary system, which might for the first time allow a dynamical mass determination for a PG 1159 star. SDSS J212531.92-010745.9 was discovered to show Hα emission during a systematic search of SDSS archival data for white dwarf + main sequence companion candidates.…”

Radial velocity variable, hot post-AGB stars from the MUCHFUSS project

“…Should the RV variations in J1556 be confirmed as originating from a close companion, then J1556 would be just the second known PG 1159 close binary system besides SDSS J212531.92−010745.9 (Nagel et al 2006;Schuh et al 2009;Shimansky et al 2015). The companion in the latter system is a low-mass main-sequence star as it is obvious from the reflection effect in the light curve and from the hydrogen Balmer series, which appears in emission owing to the radiation of the PG 1159 star (Nagel et al 2006).…”

“…The parameter fit was then performed by means of a χ 2 minimization technique with SPAS (Spectrum Plotting and Analysing Suite, Hirsch 2009), which is based on the FITSB2 routine (Napiwotzki 1999). We fitted all Balmer and He ii lines detected in the SDSS spectra of these stars.…”