The orbits of subdwarf-B + main-sequence binaries

Abstract

Context. The predicted orbital-period distribution of the subdwarf-B (sdB) population is bi-modal with a peak at short ( < 10 days) and long ( > 250 days) periods. Observationally, many short-period sdB systems are known, but the predicted long period peak is missing as orbits have only been determined for a few long-period systems. As these predictions are based on poorly understood binary-interaction processes, it is of prime importance to confront the predictions with reliable observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars. Aims. In this paper we aim to determine the orbital parameters of the three long-period sdB+MS binaries BD+29 • 3070, BD+34 • 1543 and Feige 87, to constrain their absolute dimensions and the physical parameters of the components. Methods. High-resolution spectroscopic time series were obtained with HERMES at the Mercator telescope on 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 binaries. Atmosphere models were used to fit these SEDs and to determine the surface gravities and temperatures of both components of all systems. Spectral analysis was used to check the results of the SEDs.Results. An orbital period of 1283 ± 63 d, a mass ratio of q = 0.39 ± 0.04 and a significant non-zero eccentricity of e = 0.15 ± 0.01 were found for BD+29 • 3070. For BD+34 • 1543 we determined P orb = 972 ± 2 d, q = 0.57 ± 0.01 and again a clear non-zero eccentricity of e = 0.16 ± 0.01. Last, for Feige 87 we found P orb = 936 ± 2 d, q = 0.55 ± 0.01 and e = 0.11 ± 0.01. Conclusions. BD+29 • 3070, BD+34 • 1543 and Feige 87 are long period sdB + MS binaries on clearly eccentric orbits. These results are in conflict with the predictions of stable Roche-lobe overflow models.

An Extremely Fast Halo Hot Subdwarf Star in a Wide Binary System

“…Similar degeneracies have been reported in other composite spectra binaries (Vos et al 2013). It is not possible to determine the orbital parameters of the binary system with the available data.…”

“…About 20%-30% of sdB stars are in composite spectrum binaries with F-G-K type companions. The orbital periods for about a dozen of these systems could be determined only recently (e.g., Vos et al 2013). They turned out to lie between ∼700 and ∼1300 days, which is consistent with the Roche-lobe overflow evolution channel (Han et al 2002).…”

Pulsating red giant stars in eccentric binary systems discovered fromKeplerspace-based photometry

“…Otherwise, the system is likely to become a CV. Currently a majority of the observed long period sdB binaries have an eccentric orbit (Vos et al 2013), indicating that their progenitor must have had a high eccentricity as well, as is the case with hearthbeat stars. We therefore expect to find nearly exclusively stars in the H-shell burning phase in these eccentric systems and hardly any stars which have experienced He-core ignition and now settled on the red clump.…”

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

“…We also include in Fig. 10 the sdB binaries with long periods (black crosses) from Østensen & Van Winckel (2011, 2012, Vos et al (2012Vos et al ( , 2013, Deca et al (2012), and Barlow et al (2012Barlow et al ( , 2013.…”