The orbits of subdwarf B + main-sequence binaries

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.

IP Eridani: A surprising long-period binary system hosting a He white dwarf

“…Several long-period eccentric systems (with P ∼ 10 3 d) were recently discovered among sdB stars (Østensen & Van Winckel 2011, 2012Vos et al 2012Vos et al , 2013Deca et al 2012;Barlow et al 2012Barlow et al , 2013. Using new radial-velocity data collected with the HERMES/Mercator spectrograph (Raskin et al 2011), we show in this paper (Sect.…”

The orbits of subdwarf-B + main-sequence binaries

“…These observed short-period sdB binaries correspond very well with the results of BPS studies. However, only a few long period sdB binaries are known (Green et al 2001;Østensen & Van Winckel 2011Deca et al 2012;Barlow et al 2012;Vos et al 2012), and the current studies show that there are still large discrepancies between theory and observations (Geier 2013). In a recent response to these discoveries Chen et al (2013) have revisited the RLOF models of Han et al (2003) with more sophisticated treatment of angular momentum loss.…”

“…In this paper we present the orbital and atmospheric parameters of the three long-period sdB + MS binaries BD+29 • 3070, BD+34 • 1543 and Feige 87, using the methods described in Vos et al (2012), hereafter Paper I. In Sect.…”

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“…It is very unlikely that this variation is caused by the main-sequence companion. The solved orbits of sdB+MS binaries have long periods of the order of 1000 d (Vos et al 2012(Vos et al , 2013Barlow et al 2012Barlow et al , 2013. We therefore conclude that J205101.72+011259.7 is another candidate for a hierarchical triple system consisting of an sdB in a short-period binary with unseen companion and a main sequence star orbiting this inner binary with a long period (e.g.…”