The orbits of subdwarf B + main-sequence binaries

I. The sdB+G0 system PG 1104+243

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: jorisv@ster.kuleuven.be
² Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
³ Dr. Remeis-Sternwarte Astronomisches Institut, Universität Erlangen-Nürnberg, 96049 Bamberg, Germany
⁴ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁵ Université Libre de Bruxelles, C.P. 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
⁶ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany

Received: 31 May 2012
Accepted: 5 October 2012

Abstract

Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with a peak at short (<10 days) and long (>250 days) periods. Observationally, however, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to well constrained observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars.

Aims. In this contribution we aim to determine the absolute dimensions of the long-period binary system PG 1104+243 consisting of an sdB and a main-sequence (MS) component, and determine its evolution history.

Methods. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to determine the radial velocities of both the sdB and MS components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component.

Results. An orbital period of 753  ±  3 d and a mass ratio of q = 0.637 ± 0.015 were found for PG 1104+243 from the radial velocity curves. The sdB component has an effective temperature of T_eff = 33   500 ± 1200 K and a surface gravity of log g = 5.84  ±  0.08 dex, while the cool companion is found to be a G-type star with T_eff = 5930 ± 160 K and log g = 4.29  ±  0.05 dex. When a canonical mass of M_sdB = 0.47 M_⊙ is assumed, the MS component has a mass of M_MS = 0.74  ±  0.07 M_⊙, and its temperature corresponds to what is expected for a terminal age main-sequence star with sub-solar metalicity.

Conclusions. PG 1104+243 is the first long-period sdB binary in which accurate and consistent physical parameters of both components could be determined, and the first sdB binary in which the gravitational redshift is measured. Furthermore, PG 1104+243 is the first sdB+MS system that shows consistent evidence for being formed through stable Roche-lobe overflow. An analysis of a larger sample of long-period sdB binaries will allow for the refinement of several essential parameters in the current formation channels.